Diabetes Insulin to Carb Ratio (ICR) & Correction Dose Calculator
The 500 rule and 1800/100 rule are classic starting-point formulas clinicians use to estimate insulin-to-carb ratios and correction factors from total daily insulin dose. This calculator runs those formulas instantly so you can see how TDD, blood glucose readings and meal carbs interact — purely as an educational reference, not a prescription.
Estimated total bolus is 6.4 units (4.8 for carbs + 1.6 for correction). This is an educational estimate only, not medical advice — always follow your clinician's prescribed ratios and never adjust insulin doses based solely on this tool.
| Component | Value | Notes |
|---|---|---|
| ICR (1 unit covers) | 12.5 g carbs | 500 rule |
| ISF | 45.0 mg/dL | 1800 rule |
| BG above target | 70.0 | 180 - 110 |
| Meal bolus | 4.8 units | Carb coverage |
| Correction bolus | 1.6 units | BG correction |
| Total estimated bolus | 6.4 units | Sum of both |
Formula & step-by-step maths
- TDD
- Total daily dose of insulin in units
- ICR
- Insulin to carb ratio
- ISF
- Insulin sensitivity/correction factor
What the 500 rule estimates
The 500 rule divides 500 by your total daily dose (TDD) of insulin to estimate how many grams of carbohydrate one unit of rapid-acting insulin covers. It is a commonly taught starting point in diabetes education, but individual ratios often differ meal to meal and are refined over time with a clinician using real glucose logs.
Insulin sensitivity factor (ISF)
The 1800 rule (for mg/dL) or 100 rule (for mmol/L) estimates how much one unit of rapid-acting insulin will lower blood glucose. Dividing 1800 or 100 by TDD gives this factor, which is then used to calculate a correction dose when current glucose is above target.
Combining meal and correction boluses
A full bolus dose typically combines carb coverage (meal bolus) with a correction for any current out-of-range glucose. Meal bolus is carbs divided by ICR; correction bolus is the difference between current and target glucose divided by ISF. Adding them gives a single total dose estimate.
This is an estimator, not a prescription
These formulas are educational starting points used in some clinical settings but are not personalized medical advice. Real-world insulin ratios depend on activity, illness, hormones, and other factors your doctor or diabetes educator tracks over time. Always use the insulin ratios and doses prescribed by your healthcare provider, and consult them before changing your regimen.
Reference ICR and ISF by total daily dose
| TDD (units) | ICR (1 unit : g) | ISF mg/dL | ISF mmol/L |
|---|---|---|---|
| 20 | 25.0 | 90.0 | 5.0 |
| 30 | 16.7 | 60.0 | 3.3 |
| 40 | 12.5 | 45.0 | 2.5 |
| 50 | 10.0 | 36.0 | 2.0 |
| 60 | 8.3 | 30.0 | 1.7 |
| 80 | 6.3 | 22.5 | 1.3 |
| 100 | 5.0 | 18.0 | 1.0 |
People also ask
Is this calculator a substitute for medical advice?
No. This is an educational estimator based on general formulas and must never replace insulin ratios or doses prescribed by your doctor or diabetes care team.
What is the 500 rule?
The 500 rule estimates your insulin-to-carb ratio by dividing 500 by your total daily insulin dose, giving grams of carbohydrate covered by one unit of rapid-acting insulin.
What is the difference between the 1800 and 100 rules?
The 1800 rule is used with mg/dL blood glucose readings and the 100 rule with mmol/L readings; both estimate how much one unit of insulin lowers blood glucose.
What counts as total daily dose (TDD)?
TDD is the sum of all insulin you take in a typical day, including both background (basal) and mealtime (bolus) insulin.
Why did my correction bolus show as zero?
If your current blood glucose is at or below your target, no correction is needed, so the calculator returns zero for that component.
Can these ratios change over time?
Yes, insulin sensitivity and carb ratios shift with weight changes, activity levels, illness, stress and hormonal cycles, which is why clinicians periodically re-evaluate them.
Should I use this for children with diabetes?
These general formulas are less reliable for children and should only ever be used as discussed with a pediatric endocrinologist or diabetes team.
What if my meal bolus seems too high?
Double-check your carb count and ICR; if it still seems off, discuss adjusting your prescribed ratio with your healthcare provider rather than changing doses independently.
Does this account for insulin on board?
No, this simple calculator does not subtract active insulin still working in your body; a full bolus calculator or insulin pump would factor that in separately.
Can I use this instead of my insulin pump's bolus calculator?
No — your pump or prescribed regimen is individually calibrated by your care team and should always take priority over this general educational tool.
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: total daily dose (tdd) of insulin 31.2 units, blood glucose unit "mg/dL"
On the lower / more conservative end. Estimated total bolus is 5.0 units (3.7 for carbs + 1.2 for correction). This is an educational estimate only, not medical advice — always follow your clinician's prescribed ratios and never adjust insulin doses based solely on this tool.
Example 2: total daily dose (tdd) of insulin 40 units, blood glucose unit "mg/dL"
A typical middle-of-the-road setup. Estimated total bolus is 6.4 units (4.8 for carbs + 1.6 for correction). This is an educational estimate only, not medical advice — always follow your clinician's prescribed ratios and never adjust insulin doses based solely on this tool.
Example 3: total daily dose (tdd) of insulin 52 units, blood glucose unit "mmol/L"
On the higher / more demanding end. Estimated total bolus is 42.6 units (6.2 for carbs + 36.4 for correction). This is an educational estimate only, not medical advice — always follow your clinician's prescribed ratios and never adjust insulin doses based solely on this tool.
Quick answers about the Insulin to Carb Ratio Calculator
What exactly does the Insulin to Carb Ratio Calculator work out?
The 500 rule and 1800/100 rule are classic starting-point formulas clinicians use to estimate insulin-to-carb ratios and correction factors from total daily insulin dose. You enter total daily dose (TDD) of insulin, blood glucose unit, current blood glucose and target blood glucose (plus 1 more optional details) and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 5 fields: total daily dose (TDD) of insulin, blood glucose unit, current blood glucose, target blood glucose and meal carbohydrates. Nothing else is needed and nothing is stored.
What the 500 rule estimates?
The 500 rule divides 500 by your total daily dose (TDD) of insulin to estimate how many grams of carbohydrate one unit of rapid-acting insulin covers. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for insulin to Carb Ratio?
The 1800 rule (for mg/dL) or 100 rule (for mmol/L) estimates how much one unit of rapid-acting insulin will lower blood glucose. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "Reference ICR and ISF by total daily dose" table on this page tell me?
It is the reference range this tool works against — 7 rows from "20" (25.0) up to "100" (5.0). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Which blood glucose unit should I pick?
The dropdown offers 2 choices — mg/dL and mmol/L. Pick the one that matches your real situation rather than the one you would like to be true; blood glucose unit usually moves the final figure more than any other single input, so it is worth running it twice with the option above and below your guess.
Does it work in both metric and imperial (or another currency)?
Yes. The "Blood glucose unit" control converts every field and every result, so you never have to convert anything by hand before typing it in. Switch it after entering your numbers and the output re-renders instantly in the new system.
Do I have to press a button or reload the page to see the result?
No. Insulin to Carb Ratio Calculator runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into Insulin to Carb Ratio Calculator leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
No. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
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