This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
Two numbers for flexible insulin dosing
People who take mealtime insulin often use two personal numbers. The correction factor (insulin sensitivity factor) tells you how much one unit of rapid-acting insulin lowers your blood glucose. The insulin-to-carb ratio tells you how many grams of carbohydrate one unit covers. Simple rules based on the total daily dose give starting values that the care team then fine-tunes.
The rules
Correction factor = 1,800 ÷ total daily dose (mg/dL per unit, rapid-acting analogs)
Correction factor = 1,500 ÷ total daily dose (regular human insulin)
Insulin-to-carb ratio = 500 ÷ total daily dose (grams per unit; 450 for regular insulin)
In mmol/L, the 1800 rule becomes 100 ÷ total daily dose. The total daily dose includes both basal (long-acting) and mealtime insulin, averaged over several typical days.
Example
With a total of 50 units a day of rapid-acting analog plus basal insulin:
- Correction factor: 1,800 ÷ 50 = 36 mg/dL per unit (2 mmol/L)
- Carb ratio: 500 ÷ 50 = 10 g of carbs per unit
- Correction for a reading of 250 mg/dL with a target of 120: (250 − 120) ÷ 36 ≈ 3.6 units
A meal with 60 g of carbs at that moment would need about 6 units for the food plus the correction, minus any insulin still active from an earlier dose.
Insulin on board
Rapid-acting insulin works for about 3 to 5 hours. If you took a dose two hours ago, part of it is still lowering your glucose. Correcting again without subtracting it is called stacking and is a common cause of low blood sugar. Insulin pumps and smart pens calculate insulin on board automatically.
Why the numbers vary
- Sensitivity is often lower in the morning (the dawn phenomenon) and higher after exercise.
- Illness, stress, steroids and menstrual cycles can raise insulin needs.
- Some clinicians use 1,700 or 2,000 instead of 1,800, or 450 to 550 instead of 500.
Safety
If your glucose is below 70 mg/dL (3.9 mmol/L), treat the low with 15 g of fast-acting sugar, recheck after 15 minutes and never inject insulin. Do not change your insulin plan based on this calculator alone: test any new factor with your diabetes team, and keep records so you can adjust it together.