Insulin-to-Carbohydrate Ratio: A Reference
Understand how insulin-to-carbohydrate ratios work, how the 500 rule provides a starting estimate, and why ratios must be established and adjusted with a diabetes care team rather than a calculator.
This page explains how insulin-to-carbohydrate ratios work. It does not calculate an insulin dose. Ratios must be established and adjusted by your diabetes care team using your own logged glucose data — never changed based on an online calculator. If your glucose is consistently missing target, contact your care team.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Insulin-to-Carbohydrate Ratio: A Reference | — | 500 rule: ICR ≈ 500 / total daily insulin dose | grams per unit |
Step-by-Step Examples
A person using 50 total units of insulin daily.
- 500 / 50 = 10
- Starting estimate: 1 unit per 10 g of carbohydrate
- This is where titration begins, not where it ends
The same person may need different ratios at different times of day.
- Insulin sensitivity is typically lowest in the morning
- A breakfast ratio of 1:8 alongside a dinner ratio of 1:12 is common
- The dawn phenomenon drives much of this pattern
- A single ratio applied all day often overdoses one meal and underdoses another
Ratios are not fixed once set.
- Weight change, activity level, and illness all shift insulin needs
- Pregnancy alters requirements substantially and progressively
- Some medications change insulin sensitivity
- Ratios need periodic review against logged glucose data
Real-World Applications
Common Mistakes to Avoid
It is a textbook starting estimate used by clinicians as a point of departure. Actual ratios are determined through titration against real glucose responses.
Insulin sensitivity varies through the day. Most people on intensive insulin therapy end up with different ratios for breakfast, lunch, and dinner.
If basal insulin or correction factors are wrong, changing the carbohydrate ratio chases the wrong problem and can make control worse.