Insulin-to-Carb Ratio and Correction Factor Calculator

MEDICAL DISCLAIMER This calculator is for educational purposes only. Always follow your doctor’s or certified diabetes educator’s prescribed ratios and correction factors. Never adjust insulin without medical supervision.

Insulin-to-Carb Ratio (ICR) ICR tells you how many grams of carbohydrate 1 unit of rapid-acting insulin covers. ICR = 500 / Total Daily Dose (TDD) of insulin (the “500 Rule”) Example: TDD = 40 units → ICR = 500/40 = 12.5 → 1 unit covers 12.5g carbs. More sensitive patients (low TDD): higher ICR (fewer units for same carbs). More resistant patients (high TDD): lower ICR (more units per gram of carbs). Children often have higher ICR (1:20–1:30); adults often 1:10–1:15.

Correction Factor (Insulin Sensitivity Factor, ISF) ISF tells you how much 1 unit of rapid-acting insulin lowers blood sugar. ISF = 1700 / TDD (mg/dL scale) or ISF = 94 / TDD (mmol/L scale) Example: TDD = 40 → ISF = 1700/40 = 42.5 mg/dL per unit. 1 unit lowers blood glucose by ~42.5 mg/dL (or 2.4 mmol/L). Also called “correction factor” or “sensitivity factor.”

Meal Bolus (Mealtime Insulin Dose) Carb dose = Carbs eaten / ICR Correction dose = (Current BG − Target BG) / ISF Total meal bolus = Carb dose + Correction dose (or − correction if BG is low)

Target Blood Glucose ADA recommends pre-meal target: 80–130 mg/dL (4.4–7.2 mmol/L). 2-hour post-meal: < 180 mg/dL (< 10 mmol/L). Individual targets vary, and yours are set by your endocrinologist.

Below 70 mg/dL, stop calculating

A blood glucose under 70 mg/dL (3.9 mmol/L) is hypoglycemia, and it is not a dosing problem. The answer is fast-acting carbohydrate, not a smaller bolus:

• 15 grams of fast carbs, then retest in 15 minutes. Four glucose tablets, half a cup of juice or regular soda, or a tablespoon of sugar or honey.
• Repeat until you are above 70, then eat a meal or a snack with protein.
• Below 54 mg/dL (3.0 mmol/L) is severe. Treat immediately, and if the person cannot swallow safely, that is glucagon and emergency services, not food.

This calculator refuses to return a bolus below 70 mg/dL on purpose. Correcting a meal dose downward is not the same thing as treating a low, and the difference matters most when you are least able to think clearly.

Insulin on board

The other thing this calculator does not know is how much insulin is still working from your last dose. Rapid-acting insulin acts for roughly 3 to 5 hours, so a correction given two hours after the last one stacks on top of insulin that has not finished, and stacking is one of the most common routes to a severe low. Pumps and most bolus-calculator apps subtract insulin on board automatically. If you are dosing from a formula, subtract it yourself, or wait.