What is the 500 Rule?

The 500 Rule states that 500 divided by your Total Daily Insulin (TDI) gives an approximate number of grams of carbohydrate covered by one unit of rapid-acting insulin. It is a widely used starting estimate, not a precise prescription — your care team may adjust the ratio based on real-world glucose data.

What counts as Total Daily Insulin?

TDI is the sum of all insulin you take in a typical 24-hour period: long-acting (basal) units plus all rapid-acting (bolus) units for meals and corrections. Use an average from several recent days for the most accurate estimate.

Why does my ratio differ from the 500 Rule estimate?

Individual insulin sensitivity varies with activity level, body weight, stress, illness, and other factors. The 500 Rule is a population-based estimate. Your actual ICR — verified through glucose logs — may be higher or lower.

Is this calculator a substitute for medical advice?

No. Insulin dosing is a medical decision. Use this tool to understand the concept and as a starting reference, then confirm any dosing changes with your endocrinologist or diabetes educator.

Does the ICR change throughout the day?

Yes. Many people have different ICRs at breakfast versus dinner due to hormonal variation (the dawn phenomenon, for example). This calculator gives a single ratio; your care team may prescribe time-of-day-specific ratios.

Insulin-to-Carb Ratio Calculator

Find your insulin-to-carb ratio and estimate the bolus insulin dose needed for any meal.

Step 1 — Set Your Insulin-to-Carb Ratio

Total Daily Insulin (TDI) — units/day

Add all basal + bolus units from a typical day.

Step 2 — Enter Meal Carbohydrates

Meal Carbs — grams

Check the nutrition label or use a carb-counting app.

Your Insulin-to-Carb Ratio

—

grams of carb per 1 unit of insulin

Estimated Meal Bolus Dose

—

units of rapid-acting insulin

Medical disclaimer: This tool provides an educational estimate only. Insulin dosing decisions must be made with your endocrinologist or certified diabetes educator. Never adjust your insulin regimen based solely on a calculator.

Summary

The insulin-to-carbohydrate ratio (ICR) tells you how many grams of carbohydrate one unit of rapid-acting insulin covers. Knowing your ICR helps you calculate the correct bolus dose before meals so post-meal glucose stays in range. This calculator lets you derive your ICR from your total daily insulin (TDI) using the 500 Rule, or enter a known ratio directly, and then estimate the insulin units needed for any planned meal.

How it works

Enter your Total Daily Insulin (TDI) — all basal plus bolus units taken on a typical day.
The 500 Rule divides 500 by your TDI to estimate your ICR (grams of carb per 1 unit of insulin).
Alternatively, enter a known ICR provided by your care team to skip the estimation step.
Enter the carbohydrate grams in your planned meal to see the estimated bolus insulin dose.
Adjust meal carbs to see how the dose changes in real time.
Always confirm doses with your diabetes care provider before making treatment decisions.

Use cases

Estimating a starting ICR when no previous ratio is documented.
Calculating bolus insulin before a carb-heavy meal.
Cross-checking a care-team prescribed ICR against the 500 Rule.
Teaching newly diagnosed patients the relationship between carbs and insulin.
Quickly re-estimating after a total daily insulin change.
Meal planning to stay within a target glucose range.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu