Insulin Sensitivity Factor Calculator
The insulin sensitivity factor (ISF), or correction factor, estimates how many points one unit of rapid-acting insulin is expected to lower blood glucose. A common starting estimate divides a constant by your total daily insulin dose: the 1800 rule for rapid-acting analog insulin (or 1500 for regular insulin) in mg/dL, and 100 (or 83) in mmol/L. This calculator returns the estimated ISF for your chosen rule and unit. It is only a starting point: real sensitivity varies and is refined with monitoring. This tool is educational and not a substitute for guidance from your diabetes care team.
ISF formula
ISF = constant / total daily dose
mg/dL: 1800 (rapid) or 1500 (regular)
mmol/L: 100 (rapid) or 83 (regular)
Result is glucose drop per unit of insulin
For example, the 1800 rule with a 45-unit total daily dose gives an ISF of 40 mg/dL per unit. The result is an estimate to be confirmed and adjusted with monitoring.
Using the ISF
- The total daily dose is all basal plus bolus insulin per day.
- Higher total daily doses give a smaller ISF (less drop per unit).
- The 1500 rule is often used for older regular insulin.
- Sensitivity changes with activity, illness, stress and time of day.
- Always confirm correction doses with your care team.
Insulin sensitivity factor: frequently asked questions
What is the insulin sensitivity factor?
The insulin sensitivity factor (ISF), also called the correction factor, estimates how far one unit of rapid-acting insulin is expected to lower blood glucose. It is a starting estimate based on your total daily insulin dose, not a fixed value.
What is the 1800 rule?
The 1800 rule divides 1800 by your total daily insulin dose to estimate the ISF in mg/dL per unit for rapid-acting analog insulin. For example, 1800 divided by 45 units gives an ISF of 40 mg/dL per unit. A 1500 rule is sometimes used for regular insulin.
Which rule applies for mmol/L?
For glucose measured in mmol/L, the equivalent of the 1800 rule divides 100 by the total daily dose, and 83 is the analog of the 1500 rule. The calculator lets you choose units and rule.
Is this an exact dose?
No. The rules give a starting estimate only. Real insulin sensitivity varies by person, time of day, activity, illness and other factors, and is refined through monitoring with a clinician or diabetes team.
Is this medical advice?
No. This is an educational estimate. Never change insulin doses based on a calculator alone. Insulin therapy must be guided by your healthcare team.
Official sources
- National Institute of Diabetes and Digestive and Kidney Diseases: Insulin and diabetes management.
- Centers for Disease Control and Prevention: Diabetes and insulin information.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.