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Insulin-to-Carb Ratio & Correction Factor Calculator

Insulin-to-Carb Ratio & Correction Factor Calculator

Starting estimates for carbohydrate coverage and correction dosing from the total daily insulin dose (TDD).

Formulas

  • 500 rule: insulin-to-carb ratio = 500 ÷ TDD → grams of carbohydrate covered by 1 unit of rapid-acting insulin.
  • 1800 rule: correction (sensitivity) factor = 1800 ÷ TDD → mg/dL lowered by 1 unit.
  • 100 rule: 100 ÷ TDD → mmol/L lowered by 1 unit.

Worked example

TDD 50 units → ICR = 500/50 = 1 unit per 10 g; correction factor = 1800/50 = 36 mg/dL per unit (2.0 mmol/L per unit).

Notes and limitations

  • These rules give starting estimates only; ratios are refined from glucose logs and differ by time of day.
  • The 1800 rule assumes rapid-acting insulin; the older 1500 rule applies to regular insulin.
  • Not for insulin-naive dose initiation – see the total-daily-dose guidance from the treating clinician.

References

Walsh J, Roberts R. Pumping Insulin; ADA Standards of Medical Care in Diabetes.

Educational use only. This tool does not replace clinical judgement or the prescriber’s instructions. Always confirm doses and supply quantities with the treating clinician or pharmacist. Reviewed 2026-08-23.

For clinics and DME suppliers

Turning calculations like this into a complete, compliant supply order – e-prescribe intake, documentation checks and prescriber signatures for diabetes supplies – is what Chatty ePrescribe does. This calculator stores nothing; the order workflow lives on the secure platform.