Insulin Carb Ratio, Correction & Sensitivity Calculators
Work out the numbers behind every mealtime and correction dose — your insulin-to-carb ratio (ICR), correction dose, and insulin sensitivity factor (ISF) — using the 450, 500, 1500 and 1800 rules.
🥗 Carb Ratio Tools
How many grams of carbohydrate one unit of insulin covers — in both directions.
Insulin-to-Carb Ratio
500 Rule: ICR = 500 ÷ TDD. How many carbs does one unit of insulin cover?
Carb-to-Insulin Ratio
The reverse view: grams of carbohydrate per unit of insulin for meal planning.
450 / 500 Rule Calculator
Compare both carb-ratio rules side-by-side to see which fits your insulin.
🎯 Correction & Sensitivity Tools
How far one unit lowers glucose, and how much to correct a high reading.
Insulin Sensitivity Factor (ISF)
1800 Rule (rapid-acting) or 1500 Rule (regular): mg/dL drop per unit of insulin.
Correction Factor Calculator
Derive your correction factor from TDD using the 1800 or 1500 Rule.
Correction Dose Calculator
(Current BG − Target BG) ÷ ISF. Estimate how much to correct a high glucose.
The formulas behind your ratios
Both your carb ratio and your correction factor are usually estimated from your total daily dose (TDD) using simple "magic number" rules. A higher TDD (more insulin-resistant) gives a smaller ratio; a lower TDD gives a larger one.
Key formulas
Example (TDD 50): ICR = 500 ÷ 50 = 1 unit per 10 g; ISF = 1800 ÷ 50 = 36 mg/dL per unit. To correct glucose of 200 to a target of 110: (200 − 110) ÷ 36 ≈ 2.5 units.
How the ratios combine in a mealtime bolus
A full mealtime dose usually adds two parts together: a carb dose (meal carbs ÷ ICR) and, when glucose is above target, a correction dose (using the ISF).
Always subtract any active insulin on board before adding a correction, to avoid stacking and lows. The 450 (carb) and 1500 (correction) rules are more conservative options often used with regular insulin or for greater insulin sensitivity.
These ratios are starting points. They commonly differ by time of day — many people need a stronger ratio at breakfast because of morning insulin resistance — and are refined with post-meal glucose checks and your care team.
Frequently Asked Questions
What is the insulin-to-carb ratio (ICR)?
The insulin-to-carb ratio is the grams of carbohydrate that one unit of rapid-acting insulin covers. A 1:10 ratio means one unit handles 10 grams of carbs. It is commonly estimated with the 500 Rule: ICR = 500 ÷ total daily dose.
What is the insulin sensitivity factor (ISF)?
The insulin sensitivity factor, or correction factor, is how far one unit of insulin lowers blood glucose. It is usually estimated with the 1800 Rule for rapid-acting insulin (1800 ÷ TDD) or the 1500 Rule for regular insulin, giving a value in mg/dL per unit.
How do I calculate a correction dose?
Correction dose = (current glucose − target glucose) ÷ insulin sensitivity factor. For example, with glucose 220 mg/dL, a target of 120 and an ISF of 50, the correction is (220 − 120) ÷ 50 = 2 units. Always subtract any active insulin on board first.
What are the 450, 500, 1500 and 1800 rules?
They estimate your ratios from total daily dose. The 500 Rule (and the more conservative 450 Rule) sets the carb ratio: 500 ÷ TDD. The 1800 Rule (rapid-acting) and 1500 Rule (regular insulin) set the sensitivity factor: 1800 ÷ TDD or 1500 ÷ TDD.
What's the difference between the carb ratio and the sensitivity factor?
The carb ratio (ICR) covers the carbohydrate in a meal, while the sensitivity factor (ISF) corrects a high glucose back toward target. A full mealtime bolus often combines both — a carb dose plus a correction dose.
Are these ratios fixed or do they change?
They are starting estimates and often change over time, with activity, illness, weight and time of day — many people need a stronger ratio at breakfast. Confirm and refine your ratios with post-meal glucose checks and your care team.
Sources
- Walsh J, Roberts R. Pumping Insulin. The 500 Rule (carbohydrate factor) and the 1800/1500 Rule (correction factor).
- American Diabetes Association. Standards of Care in Diabetes — 2024. Carbohydrate counting and insulin dosing.
Last reviewed: June 2025