Insulin Sensitivity Factor: A Reference
Understand the insulin sensitivity factor — also called the correction factor — how the 1800 and 1500 rules estimate it, and why correction dosing must be set by a clinician.
This page explains how the insulin sensitivity factor works. It does not calculate a correction dose. Correction dosing must be set by your diabetes care team and accounts for insulin still active from previous doses — something no standalone calculator can know. If your glucose is persistently high or you have ketones, contact your care team.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Insulin Sensitivity Factor: A Reference | — | 1800 rule: ISF ≈ 1800 / total daily insulin dose | mg/dL per unit |
Step-by-Step Examples
A person using 45 total units of insulin daily.
- 1800 / 45 = 40
- Starting estimate: 1 unit lowers glucose by about 40 mg/dL
- Equivalent to about 2.2 mmol/L per unit
The rule uses different numerators for different insulin types.
- 1800 is used with rapid-acting analogues
- 1500 is the older figure used with regular human insulin
- Some clinicians use 2000 for very insulin-sensitive individuals
- The choice reflects insulin pharmacokinetics, not arbitrary preference
Correction dosing is not just arithmetic on current glucose.
- Rapid-acting insulin remains active for several hours after injection
- Correcting without accounting for insulin still working causes stacking
- Stacking is a common route to severe hypoglycaemia
- Pump bolus calculators track this automatically; manual dosing requires the user to
Real-World Applications
Common Mistakes to Avoid
Giving another correction before the previous dose has finished acting is one of the most common causes of severe hypoglycaemia in insulin users.
Exercise increases insulin sensitivity for hours afterward, while illness and stress reduce it. A fixed factor does not capture this.
Persistent high glucose with ketones is a different situation requiring urgent management, not routine correction dosing.