450 Rule Insulin Calculator
Estimates the carb-to-insulin ratio (ICR) for patients using Regular (short-acting) insulin, where the slower action profile requires a more conservative ratio than the 500 Rule.
All insulin in 24 hours: basal + all bolus doses.
Calculates estimated meal bolus if entered.
450 Rule vs 500 Rule
Regular insulin has a peak around 2–4 hours and a tail lasting 6–8 hours — longer than rapid-acting analogues (~3–5 hours). Because Regular covers more time per unit, it takes fewer units to cover the same carbs, meaning each unit covers fewer grams — hence the smaller divisor (450 vs 500).
| TDD | 450 Rule (Regular) | 500 Rule (rapid-acting) |
|---|---|---|
| 20 | 1:22.5 | 1:25 |
| 30 | 1:15 | 1:16.7 |
| 40 | 1:11.3 | 1:12.5 |
| 50 | 1:9 | 1:10 |
Why Regular Insulin Uses 450 Instead of 500
The constant in these "magic number" rules reflects how much total work a unit of insulin does across its action curve. Regular (human) insulin has a slower onset and a longer tail than rapid-acting analogs — it keeps lowering glucose for 5–8 hours, versus roughly 3–5 hours for lispro or aspart. Because each unit of Regular insulin covers more time, established practice uses a smaller divisor (450), which produces a slightly stronger carb ratio — fewer grams covered per unit.
In practice the difference is modest. At a TDD of 40, the 450 Rule gives 1:11.3 while the 500 Rule gives 1:12.5 — about one extra unit on a large meal. What matters most is that the rule you start with matches the insulin you actually inject.
Who Should Use the 450 Rule?
The 450 Rule is intended for people whose mealtime insulin is Regular (short-acting human) insulin — brands such as Humulin R and Novolin R. Regular insulin is still widely used where cost or access favors human insulin over analogs, and in some hospital and enteral-feeding protocols.
- Using Regular insulin? Start with the 450 Rule on this page.
- Using a rapid-acting analog (Humalog, NovoLog, Apidra, Fiasp, Lyumjev)? Use the 500 Rule instead.
- Regular insulin is usually injected about 30 minutes before eating, unlike analogs taken at the start of a meal — timing matters as much as the ratio.
Frequently Asked Questions
What is the 450 Rule for insulin?
The 450 Rule estimates the insulin-to-carb ratio for Regular (human) insulin: ICR = 450 ÷ total daily dose. The result is how many grams of carbohydrate one unit covers. For a TDD of 45, ICR = 450 ÷ 45 = 1:10 (one unit per 10 g of carbs).
What is the difference between the 450 and 500 Rule?
Both estimate the carb ratio (divisor ÷ TDD). The 450 Rule is used for Regular short-acting insulin; the 500 Rule is used for rapid-acting analogs. The 450 Rule yields a slightly stronger ratio because Regular insulin acts over a longer period.
Which insulins use the 450 Rule?
Regular human insulin — for example Humulin R and Novolin R. Rapid-acting analogs (lispro, aspart, glulisine, Fiasp, Lyumjev) use the 500 Rule instead.
Is the 450 Rule accurate for everyone?
No — it is only a starting estimate. Your true ratio is found through real-world titration with your care team, and it can differ by meal and by time of day.
Source
- Walsh J, Roberts R. Pumping Insulin. 5th ed. 2012.
Last reviewed: June 2025