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A Small Revolution in Diabetes Management

Whats New in Version 2

There are some subtle, but significant changes in this release based largely on valuable feedback from our users.

If you are upgrading or restoring data from a previous version of RapidCalc you must review and, where necessary, update your settings before use.

  • Customised time periods: The day is now divided into 6 time periods: Breakfast, Morning, Lunch, Afternoon, Evening and Overnight. You can specify the start time for each period, which then runs until the start of the next time period.
  • Carbohydrate ratios: When measuring carbs in grams, carbohydrate ratios are now expressed in terms of "grams of carbohydrate covered by 1 unit of insulin" (g/unit) rather than "units of insulin needed to cover 1 carb portion" (u/portion). If you are upgrading or restoring a backup from a previous version of RapidCalc where you were measuring carbs in grams, then your carbohydrate ratios will be converted automatically from u/portion to g/unit.
  • Basal dose reminders: Basal doses can now be scheduled at specific times of day with optional reminders.
  • Post-meal Blood Glucose check reminders: You can now schedule reminders to test your blood glucose at a configurable time after meals.
  • Maximum safe RAI dose limit: The suggested RAI dose can now be limited to a maiumum safe value for pediatric use.
  • Additional statistics: Additional statistics are now available via drill-downs from the main statistics page.
  • Improved settings interface: The settings interface has been completely redesigned to make it easier to use.
  • Extended BG input range: The maximum blood glucose value you can enter has been increased to 33 mmol/L (600 mg/dL).
  • Support for iPhone: The screen layout has been extended on the iPhone 5 to make maximum use of the extra screen height.
  • Support for iOS 7: The application has be updated to fully support iOS 7.
  • Minimum iOS 5 required: The application now requires at least iOS 5 to to run. Don't upgrade or delete the app if you have an old device that does not support at least iOS 5.

Quick Carb ratio Conversion table

RapidCalc follows the convention of expressing carb ratios as "units of RAI needed to cover 1 portion of carbohydrate". Pump users however sometimes express the ratio instead as "grams of carbohydrate covered by 1 unit of RAI". The table below allows a quick conversion of these ratios into a form suitable for entry into RapidCalc.

For example, if 1 unit of RAI covers 12g of carbohydrate and we measure carb units in 10g portions, then scanning down the left hand column to 12g and then across to the 10g portion column gives us a value of 0.833 units RAI needed per 10g portion.

Grams of Carbohydrate covered by 1 unit of RAI Units of RAI needed to cover 1 portion of carb, for different standard portion sizes
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Getting your Basal Dose Right

Basal Insulin doses form the foundation upon which good control of your BGLs rest. It is essential to get these levels right as it should minimise the need for constant adjustments to your rapid acting insulin doses. You probably will know the feeling best expressed as "If I wake up and my BGL is good, I know I am going to have a good day..".

In the ideal world Basal Insulin doses are the first things that need to be set correctly. Carb and correction doses can only really be fine tuned once your correct basal insulin doses have been established. Unfortunately in the real world, the effects of basal insulin doses and carb/correction doses tend to blur the picture and it becomes necessary to take a bit of time to refine your doses.

There are many different ways of calculating and fine tuning basal insulin doses. It is important to remember however that the only correct basal insulin dose is what works for you and keeps your BGL at the desired level both overnight and between meals whilst reducing hypoglycaemia to the lowest acceptable rate.

One approach is to calculate the basal insulin requirement based on the Total Daily Insulin Dose (TDD), body weight and level of activity. Such an approach is followed by Gary Scheiner in "Think Like a Pancreas a practical guide to managing diabetes with insulin" (the 2011 edition published by De Capo). It uses your age, weight and activity level as follows:-

Daily Basal Insulin Requirement (units per Kg body weight)

Activity Level Basal Requirement per Kg body weight by Age
Young ChildrenAdolescentsAdultsOlder Adults
Mostly Inactive0.25 - 0.600.30 - 1.000.25 - 0.600.20 - 0.50
Moderately Active0.20 - 0.500.30 - 0.750.20 - 0.500.15 - 0.40
Very Active0.15 - 0.400.25 - 0.600.15 - 0.400.15 - 0.40

In his book, Gary suggests that as basal insulin accounts for 40-50% of the body’s insulin needs, basal insulin doses should be within these ranges but that these should also be modified dependent also on the degree of carbohydrate intake. A low carb eater would have a greater proportion of their TDD as basal insulin whilst a high carb eater would have a lesser proportion of their TDD as basal insulin and a greater proportion as meal time doses.

DAFNE guidelines suggest that most adult Type 1's have a TDD requirement of between 0.5 - 0.8 units of insulin per kilogram of body weight each day and that roughly half of this should be basal insulin. Bearing in mind that the DAFNE program is currently only recommended for people aged 18 years and over, there is broad agreement on TDD requirements and the split between BI and RAI.

Another approach is that followed by John Walsh in his paper "Guidelines for Optimal Bolus Calculator Settings in Adults" published in the Journal of Diabetes Science and Technology, Vol 5, Issue 1, January 2011 which is based purely on the Total Daily Dose of insulin. The TDD is the sum of the basal insulin injected per day plus an average of the total rapid acting insulin injected for both carbohydrates and corrections. This paper advocates the basal insulin should ideally be 48% of the TDD with this being modified depending on whether the person is on a low or high carbohydrate diet.

As we cannot give medical advice on what doses and titration methods to use, we would urge you to either attend a DAFNE course which is very highly recommended and/or read "Think Like a Pancreas", which we also highly recommend as it is written in an easy to understand manner and is most informative in its coverage of the use of insulin in all situations. Importantly always agree your doses with your healthcare professional. They know you and your diabetes best.

We would however offer the general advice that when starting out, start at the lower end of any basal dose ranges suggested to minimise the risk of hypos and titrate your basal insulin up from there until you find the appropriate basal insulin dose for yourself. Always consult with your healthcare professional before changing any doses recommended by them and follow their titration recommendations.

So you have now agreed a basal insulin dose with your healthcare professional and after a week or so, you probably want to know whether your basal insulin dose is now correct for you and your lifestyle.

A correct basal insulin dose should keep your BGLs fairly steady throughout the night and within an acceptable range with no or minimal risk of low BGLs/Hypos. "Think Like a Pancreas" states that a change in BGL of more than 30mg/dl or 1.7mmol/L overnight would indicate that a change in basal insulin is needed. An overnight Hypo would definitely suggest a reduction in BI, unless you can clearly identify a reason other than BI. Gary then goes into detail as to how to fine tune your basal insulin. DAFNE advise that this change in overnight BGLs should be no more than 1.5 mmol/L.

We must add some additional comments to this:-

  • Make sure that your food has been fully digested and that all rapid acting insulin action has taken place before measuring your BGL before bed. Remember Rapid Acting Insulin (RAI) acts for 5-6 hours and that low GI foods such as pasta are absorbed over a long period of time.
  • Any time you change your basal insulin, it is worth doing a 3am BGL check for safety reasons. This is recommended in the DAFNE program for safety reasons. It is also good to do this as the before breakfast BGL is likely to be impacted by the "Dawn Phenomena" where BGLs rise as the body becomes less sensitive to insulin through the action of hormones secreted as the body gets ready for the day ahead. Indeed some physicians believe that titration of basal insulin should be done based on 3 AM BGLs!
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Calculating your Bolus Dosing Ratios

Bolus doses of Rapid Acting Insulin (RAI) generally comprise 2 elements: a carb dose to cover the planned carbohydrate intake and a correction dose to bring an elevated Blood Glucose Level back to target. To work out the carb dose you need to know your Carb ratio and to work out the correction dose you need to know your Correction Factor. Guidelines for determining both of these ratios are given below.

Carb Ratios : Insulin to Carbohydrate ratios

The most common approach to calculating the carb ratio initially is to divide the total daily dose (TDD) into 500 to give the amount of carbohydrate in grams covered by 1 unit of RAI. This approach is often referred to as the "rule of 500" This will then need to be converted into units of RAI per carbohydrate portion or serve.

For example if a person is on a TDD of 50 units then 1 unit of RAI should cover 500/50 = 10 grams of carbohydrate. If the person uses 15 gram portions then the ratio will be 1.5 units of RAI to every 15 gram serve. If the person uses 1 gram portions the ratio would be 1/10 or 0.1 units of RAI per gram of carbohydrate.

We must stress that these starting ratios are rough as once again each person is different and they do not take into account the natural variation in insulin resistance that occurs throughout the day. This is most noticeable during the morning period when insulin resistance increases in response to hormones released by the body.

Fine tuning your carb doses is where the science of diabetes becomes more of an art. Remember that the RAIs generally last 5 to 6 hours whereas most food, apart from low GI carbs, will be mostly digested and converted into glucose within 2 hours. The first step in getting the correct carb doses is doing the following for each meal of the day:-

  • Get a fairly typical meal for which you know the carb content.
  • Ensure your BGLs are within range and stable (not rising/falling or being influenced by exercise, stress, recent hypos etc).
  • Test your BGL.
  • Inject your current calculated dose of RAI for the known carb content.
  • Do not eat again or do anything that will significant affect your BGL.
  • Test your BGL every hour for the next six hours.

This exercise will give you the opportunity to see if the selected dose returns your BGL to your premeal level before your next meal as well as help you find the RAI profile that works for you. If your BGL returns to the original premeal level within the next 5-6 hours then you have the carb ratio for that meal. Carb ratios for meals should generally be the same at different times of day except for breakfast where you are likely to need more insulin per unit of carbohydrate due to increased insulin resistance at that time of day. The DAFNE program advise, as general guidance for adults, that they will need between 1-3 units of RAI per 10 gram carbohydrate portion for breakfast and between 1-2 units of RAI per 10 gram portion for other meals. The more sensitive you are to insulin, the less RAI you need to cover cab intake.

Once again the general advice is to start low and titrate upwards.

Correction Factor : Blood Glucose Drop per unit of insulin

The most common approach is to use the "rule of 100" which entails dividing the Total Daily Dose (TDD) into 100 to give the blood glucose drop in mmol/L. For those who use mg/dL to measure BGL, the rule is expressed as the "rule of 1800".

To give an example: If a person is on a TDD of 50 units then 1 unit of RAI should drop their BGL by 100/50 = 2 mmol/L or using mg/dL this would be expressed as 1800/50 = 36 mg/dL.

For most Type 1 adults 1 unit of RAI will drop BGLs by between 2-3 mmol/L (36-54 mg/dL) but for people on lower TDDs, 1 unit of RAI will achieve even greater drops of BGLs. Using the “rule of 100” a person on a TDD of 10 units should find that 1 unit of RAI will drop their BGL by 10 mmol/L (180 mg/dL). Paediatric patients tend to be more sensitive to insulin and the BGL drop per unit of insulin can be significantly greater than the ranges shown above.

Just as carb ratios can vary throughout the day, you should not be surprised if your correction factor varies, as the same factors are at play.

To test your correction factor:-

  • Ensure your BGLs are within range and stable (not rising/falling or being influenced by exercise, stress, recent hypos, food, IOB etc)
  • Test your BGL and if it is above your target BGL.
  • Inject your calculated correction dose of RAI.
  • Do not eat again or do anything that will significant affect your BGL.
  • Test your BGL every hour for the next six hours.
  • The drop in BGLs to your target BGL within this time period should verify whether you have the right correction dose.

General Comments

Remember that all days are not the same, we all have days where things just don’t work out the way they have in the past and we cannot establish just why we suddenly have an unexpected high or low BGLs. The aim is to get insulin doses correct as many times as possible, as safely as possible. You will need to constantly analyse your RapidCalc history file to try to identify clues as to why these days do not deliver the BGLs you expect and take corrective action under the guidance of your Diabetes Educator.

As a general rule if you find that you are having to constantly correct your BGL at every meal then it is likely that either your basal insulin doses are wrong, your carb estimation/ratios are wrong or your correction doses are wrong. Your diabetes educator or physician should then be able to help you identify the changes you need to make

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Calculating Insulin on Board

Surprisingly insulin pump manufacturers all seem to have different methods of calculating IOB and accounting for it when calculating bolus insulin doses, some of which are significantly different. The question then is - which is best?

Clinica Diabetologica at summarises the different approaches succinctly but I have yet to find detailed data on IOB calculation directly from the major insulin pump manufacturers.

It is clear that IOB is crucial to getting dosing of rapid acting insulins (RAI) correct as often the problems of Type 1 diabetes revolve around assessing carbohydrate content of a meal correctly, assessing the Glycaemic Index (GI) of the combined foods (and not their component parts) and accurately predicting the glucose lowering ability and duration of action of the RAI. Clearly it is too complex to try and track glycaemic index of mixed foods so we are left with estimating carbohydrate content of our food, rapid acting insulin, activity and duration of action of the rapid acting insulin.

Carbohydrate content is always an approximation but it appears that 10 grams either way is an acceptable margin with no significant impact on blood glucose which is great as it makes life a little easier. We can't always weigh food or read labels hence many diabetes programs for Type 1 patients use serves or portions ranging through 10, 12 and 15 grams and give users rough guides e.g. slice of bread or size of palm of your hand is 15 grams. People get used to estimating these portion sizes and can then assess food served to them with greater accuracy, although still less than perfect. Many patients make mistakes with carb estimation.

Insulin manufacturers publish insulin profiles that give a fair guide to the time action profiles of their rapid acting insulin. These are averages and are often done in people who have fasted and who are hence more sensitive to insulin action as well as on a limited range of dose sizes. Individuals also differ considerably in a multitude of ways - renal function, injection depth, blood flow around the injection site, insulin storage prior to injecting, injection site (subcutaneous, intramuscular, lipodystrophy) and injection technique etc. all of which impact on the RAI profile. So once again we have approximations and individual differences.

When we started developing RapidCalc the questions we faced with IOB were:-

  • Should IOB be calculated from the total dose of RAI (meal + correction dose) or only the correction dose given previously?
  • At what rate is the IOB used up?
  • Should IOB, once calculated, only be deducted from any correction dose due at the next meal?
  • or
  • Should it be deducted from the total (meal+correction dose) of the next dose?

Because of errors in estimating carbohydrate content of meals and GI, we have taken the approach with RapidCalc of using the full dose (meal + correction) to calculate the IOB. Having entered the current BGL and planned carb intake, RapidCalc calculates the residual IOB from previous doses, based on the users own usage profile, and subtracts it from the total (meal + correction) dose to arrive at a suggested dose

We believe that this approach is the safest as it allows for errors in carb estimation, individual variation in insulin usage and duration of activity. Also, any errors in RAI dosing are more likely result in under dosing of RAI and hence less likely to cause hypoglycaemia. Actual IOB is therefore, we believe, more accurately and safely calculated.

This approach does however present one problem - that of eating an unplanned desert or another helping of food in close proximity (within 1.5-2 hrs) to the previous RAI dose. In this scenario BGLs are likely to be high as much of the RAI is yet to act. To get around this, we added a "BG not measured/Extra Carbs" check box. In this specific situation you are unlikely to test your BGLs again. Checking this option forces the "Measured Blood Glucose" slider to your target level and disregards any IOB. The suggested RAI dose thus matches the RAI dose needed to only cover the new carbohydrate intake.

The "BG not measured/Extra Carbs" option also safely handles those situations where you are unable to test blood glucose levels. With Type 1 diabetes, we do however recommend that you always test before every meal, where practical, as it gives a fixed point from which to make decisions. Not testing your blood glucose level, is a bit like trying to navigate your car to an address without knowing your current location!

The next question is - what is the effective duration of action of a rapid acting insulin dose. Insulin manufacturers publish stylised graphs that often show averages, without the 95% confidence intervals, so you can never be exactly sure how a particular insulin is likely to work for you. One is also seldom sure whether these graphs display the glucose lowering ability of the insulin or whether they display the levels of the insulin in the blood - which can result in very different profiles. There is also some variation between each of the three RAI currently available RAI analogues (insulin lispro, aspart and glulisine). Insulin glulisine appears to have a greater glucose lowering effect initially and hence less of a "tail" or residual action, but there is still debate on the clinical significance of this.

Some Health Care Providers use 2 hour profiles for RAIs in pumps but it appears to be a more accepted view that, on average, all the new analogue rapid acting insulins are active for at least 5 hours. We have looked at the insulin profiles and, by calculating the area under the glucose disposal curves, we have arrived at the 6 hour profiles shown in the help section of RapidCalc i.e. 15%, 30%, 25%, 15%, 10% and 5% for each of the 6 hours. The reason for so little of RAIs action occurring in the first hour is the 15 minute lag time from injecting before it starts working and the shape of the glucose disposal curve. Many people use RapidCalc very successfully using a 4 hr. profile with 30% of the action occurring in each of the first 3 hours and 10% in the 4th hour.

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Calculations and Formulae used in RapidCalc

Calculation of Suggested RAI Dose

The total suggested bolus RAI dose is made up of meal, correction and IOB components as follows:

Total Suggested RAI Dose = Meal Dose + Correction Dose - Insulin on Board

Each of these components is described below.

Meal Dose

The meal component of the RAI dose is designed to offset the planned carbohydrate intake. It is calculated as follows:

Meal Dose = active_carb_ratio * planned_carb_intake * (1 - exercise_percentage_reduction)


  • active_carb_ratio is the carb ratio applicable to the current time of day.
  • planned_carb_intake is the planned carb intake value you entered.
  • exercise_percentage_reduction is the percentage dose reduction for any exercise you selected.

Correction Dose

The correction component of the RAI dose designed to reduce an elevated blood glucose level to the your current target. It is calculated as follows:

Correction Dose = ((measured_bg - active_target_bg) / active_correction_factor) * (1 - exercise_percentage_reduction)


  • measured_bg is the BG reading you entered.
  • active_target_bg is the target BG level for the current time of day.
  • active_correction_factor is the correction factor for the current time of day.
  • exercise_percentage_reduction is the percentage dose reduction for any exercise you selected.

Insulin on Board

It takes on average 15 minutes from the time a rapid acting insulin dose is injected before it becomes active in the bloodstream and it can remain active in the bloodstream for up to 6 hours. The amount of insulin still active in the bloodstream is known as Insulin on Board (IOB). The rate at which the insulin is used depends on several factors including the type of insulin and your metabolism. You set your personal insulin usage profile in the Insulin Usage Profile settings.

IOB must be considered when calculating a bolus dose to avoid the risk of overdosing or ‘dose stacking’ which could cause a hypo. The IOB is calculated by adding up the insulin remaining from each of the doses taken in the last 6 hours. (Since the maximum life of a rapid insulin is effectively 6 hours, older records can be ignored). For each of these doses, the insulin remaining is calculated from the actual dose taken as follows: Each dose is assumed to have a maximum effective life of 6 hours with a user-configurable amounts of insulin used at a linear rate within each of the six 1-hour periods following the initial dose. The calculation assumes a 15 minute onset delay before injected insulin begins to act. For example if the amount used is 30% in hour 1 and 30% in hour 2 then after 1.5 hours the amount used will be 45% (i.e. 55% remaining).

Calculating Estimated HbA1c

HbA1c is a measure of the long-term blood glucose level in the blood stream. The measurement is carried out in a laboratory from a blood sample, but the value can be estimated from your average blood glucose readings. HbA1c can be measured as a percentage or in units of mmol/mol. The formula for estimated HbA1c as a percentage, from average blood glucose measured in mmol/L is as follows.

HbA1cpercent = ((18 * average_blood_glucosemmol/L) + 46.7) / 28.7


  • average_blood_glucose is the average of all the your BG readings (excluding estimates) expressed in mmol/L.
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