Carb-to-Insulin Ratio (ICR) Calculator
Estimates how many grams of carbohydrate one unit of rapid-acting insulin covers, using the 500 Rule.
All insulin in 24 hours: basal + all bolus doses combined.
If entered, calculates your estimated meal bolus dose.
How the 500 Rule Works
ICR = number of grams of carbohydrate covered by 1 unit of rapid-acting insulin. Developed by Walsh & Roberts (Pumping Insulin, 5th ed.) as a starting-point formula for carb counting with insulin. The 500 Rule assumes the total daily insulin dose reflects overall insulin requirement.
Meal bolus: Carbs (g) ÷ ICR = meal dose (units)
Typical ICR Values by TDD
| TDD (units/day) | 500 Rule ICR | Example |
|---|---|---|
| 20 | 1:25 | 1 unit per 25 g carbs |
| 30 | 1:17 | 1 unit per 17 g carbs |
| 40 | 1:12.5 | 1 unit per 12–13 g carbs |
| 50 | 1:10 | 1 unit per 10 g carbs |
| 70 | 1:7 | 1 unit per 7 g carbs |
Carb-to-Insulin Ratio vs Insulin-to-Carb Ratio
These two terms describe the same number from opposite directions, which is why they're easy to mix up. The insulin-to-carb ratio (ICR) is usually written "1 unit : X grams" — one unit of insulin covers X grams of carbohydrate. "Carb-to-insulin ratio" is simply the everyday way of saying the same thing: how many grams of carbs each unit handles. The working number is identical, and both come from the 500 Rule.
This page focuses on the practical side — turning a plate of food into a bolus. For the full derivation of the 500 Rule and how it ties into your sensitivity factor, see the Insulin-to-Carb Ratio Calculator.
Turning Carbs Into a Meal Bolus
Once you know your ratio, the mealtime math is a single step: meal bolus = grams of carbohydrate ÷ ICR. The accuracy of that bolus depends almost entirely on how accurately you count the carbs.
- Read the label, then weigh the portion. Labels give carbs per serving — but your serving is rarely the label's serving. A kitchen scale removes most of the guesswork.
- Count total carbohydrate, not just "sugar." Starches raise glucose too; the number that matters is total carbohydrate grams.
- Subtract fiber only if your team taught you to. Some people subtract fiber (and some sugar alcohols) from the total — check what your care team recommends before doing this.
Example: a meal has 72 g of carbs and your ratio is 1:12. Meal bolus = 72 ÷ 12 = 6 units. Run your own numbers with the calculator above.
Signs Your Ratio Needs Adjusting
The 500 Rule gives a starting ratio; your real ratio is found through observation with your care team. Watch the 3–4 hour window after meals when no correction dose or exercise is involved:
| What you notice after meals | What it suggests |
|---|---|
| Consistently high 3–4 hours later | Ratio may be too weak (too many grams per unit) — your team may lower the grams figure |
| Consistently low 3–4 hours later | Ratio may be too strong — your team may raise the grams figure |
| Fine at lunch but high after breakfast | You may need a separate, stronger morning ratio (dawn insulin resistance) |
Frequently Asked Questions
What is a carb-to-insulin ratio?
It is how many grams of carbohydrate one unit of rapid-acting insulin covers. A 1:12 ratio means one unit handles 12 grams. It is the same number as the insulin-to-carb ratio (ICR), and a common starting estimate is 500 ÷ your total daily insulin dose.
How do I calculate my meal insulin from carbs?
Divide the grams of carbohydrate in your meal by your ratio. For a 60 g meal and a 1:12 ratio: 60 ÷ 12 = 5 units. Add a correction dose separately if your blood glucose is above target before the meal.
Is the carb-to-insulin ratio the same as the 500 Rule?
The 500 Rule is how the ratio is first estimated: ICR = 500 ÷ TDD. The carb-to-insulin ratio is the result you then use at meals. Regular (human) insulin sometimes uses the 450 Rule instead of 500.
Can my ratio be different at each meal?
Yes. Many people are more insulin-resistant in the morning and need a stronger (lower-gram) breakfast ratio. Your care team can set separate ratios for breakfast, lunch, and dinner.
Source
- Walsh J, Roberts R. Pumping Insulin. 5th ed. 2012. (500 Rule)
Last reviewed: June 2025