Insulin to Carb Ratio Calculator

units
Health Updated 14 Jun 2026

Estimate your insulin-to-carb ratio using the 500 rule — how many grams of carbohydrate one unit of rapid-acting insulin covers. Enter your total daily insulin dose and this calculator divides 500 by it to give a starting I:C ratio.

Rates and fees change frequently. Always confirm the current figures with the official authority before relying on this result.
  • Starting estimate only. Dosing must be supervised by your diabetes care team.

Formula

Carb ratio = 500 ÷ total daily insulin dose (grams of carb per unit)
  • The 500 rule estimates how many grams of carbohydrate one unit of rapid-acting insulin will cover.
  • Carb ratio = 500 ÷ total daily dose (TDD) of insulin (basal + bolus).
  • A result of 1:10 means one unit covers 10 g of carbs.
  • A higher TDD gives a smaller ratio (each unit covers fewer carbs), reflecting greater insulin needs.
  • Some clinicians use the 450 rule (for regular insulin) instead of 500 — confirm which applies to you.
  • This is a starting estimate only; your ratio must be set and adjusted with your diabetes team.

Total daily dose of 50 units

Inputs
  • Total daily insulin dose: 50 units

Carb ratio = 500 ÷ 50 = 10. So roughly one unit of insulin covers 10 g of carbohydrate. A 60 g meal would need about 60 ÷ 10 = 6 units.

Frequently asked questions

What is the 500 rule?
It's a quick way to estimate your insulin-to-carb ratio: divide 500 by your total daily insulin dose. The result is the grams of carbohydrate covered by one unit of rapid-acting insulin.
What counts as total daily dose?
Your TDD is all the insulin you use in a typical day — long-acting (basal) plus mealtime and correction (bolus) doses combined.
Is 500 always the right number?
For rapid-acting analogues, 500 is common. For older regular insulin, some use 450. Your clinician may individualise it based on your response.
Can I rely on this for dosing?
No — treat it as a starting point. Insulin dosing must be reviewed and adjusted by your diabetes care team using your real glucose data.

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