450 Rule Insulin Calculator
Estimate your insulin-to-carb ratio for Regular (short-acting) insulin using the 450 Rule. Using a rapid-acting analog like Humalog or NovoLog? Use the 500 Rule calculator instead, or compare all three rules.
🔢 450 Rule Calculator
Estimated Results
Insulin-to-Carb Ratio (Regular insulin, 450 Rule)
(rapid-acting)
(alternative)
60 g-carb meal
How the 450 Rule Works
The 450 Rule Formula
Worked example: TDD of 45 units → 450 ÷ 45 = 1 unit per 10 g of carbohydrate. A meal with 60 g carbs → 60 ÷ 10 = 6 units of Regular insulin, taken ~30 minutes before eating.
The 450 Rule is the Regular-insulin counterpart of the better-known 500 Rule. Regular (short-acting) insulin — Humulin R, Novolin R — has a slower onset (30–60 min), later peak (2–4 h), and longer tail (up to 6–8 h) than rapid-acting analogs. Because more of its action overlaps between meals, a smaller divisor (450 instead of 500) produces a slightly stronger ratio that accounts for that overlap.
450 vs 500 vs 550 — which rule applies?
| Rule | Insulin type | Examples | ICR at TDD 45 u |
|---|---|---|---|
| 450 | Regular (short-acting) | Humulin R, Novolin R | 1 : 10 g |
| 500 | Rapid-acting analog | Humalog, NovoLog, Apidra, Fiasp | 1 : 11 g |
| 550 | Alternative rapid-acting variant used by some clinicians | — | 1 : 12 g |
Compare all three side-by-side with the Carb Factor Calculator.
Limitations & Safety Notes
- Rules of thumb assume a fairly stable TDD; recent dose changes make the estimate unreliable.
- Many people need different ratios per meal — breakfast often requires a stronger ratio.
- Ratios are refined from 2-hour post-meal readings under care-team supervision, not from formulas alone.
- Regular insulin requires a 30-minute pre-meal lead time; timing errors change outcomes as much as dose errors.
Sources & References
- Walsh J, Roberts R. Using Insulin: Everything You Need for Success with Insulin. (Origin of the 450/500 Rules.)
- American Diabetes Association. Standards of Medical Care in Diabetes — 2024. Section 9. Link
- Davidson PC, et al. Analysis of guidelines for basal-bolus insulin dosing. Endocr Pract. 2008.
Last reviewed: July 2026 · See our methodology
Frequently Asked Questions
What is the 450 Rule for insulin?
Divide 450 by your total daily insulin dose to estimate how many grams of carbohydrate 1 unit of Regular (short-acting) insulin covers. TDD 45 u → 450 ÷ 45 = 1 unit per 10 g carbs. It is a starting estimate to refine with your care team.
When do I use the 450 Rule vs the 500 Rule?
450 for Regular insulin (Humulin R, Novolin R); 500 for rapid-acting analogs (Humalog, NovoLog, Apidra). Regular insulin's longer action overlap justifies the more conservative divisor. Compare both — plus the 550 variant — on the Carb Factor Calculator.
Is the 450 Rule accurate?
It gets most people into a workable starting range, but the real ratio comes from checking 2-hour post-meal glucose and adjusting under supervision. Ratios frequently differ by meal and change with activity, illness, and weight.
What TDD should I use?
Your true average daily total — basal plus all boluses — over several typical days. No usage history yet? Estimate TDD from weight with the TDD calculator and refine later.