How-to · Educational, not a prescription · Updated October 2, 2026
How do you use the 1800 rule for insulin?
Direct answer
For rapid-acting insulin, correction factor = 1,800 ÷ total daily dose. At 40 units, 1 unit lowers about 45 mg/dL. A reading of 250 mg/dL with a target of 120 is (250 − 120) ÷ 45 = 2.89, which rounds to 3 units. Regular insulin uses 1,500 instead of 1,800.
Correction factor by total daily dose
| Total daily dose | 1,800 rule | 1,500 rule | 100 rule |
|---|---|---|---|
| 20 units | 90 mg/dL | 75 mg/dL | 5 mmol/L |
| 30 units | 60 mg/dL | 50 mg/dL | 3.3 mmol/L |
| 40 units | 45 mg/dL | 37.5 mg/dL | 2.5 mmol/L |
| 50 units | 36 mg/dL | 30 mg/dL | 2 mmol/L |
| 60 units | 30 mg/dL | 25 mg/dL | 1.7 mmol/L |
| 80 units | 22.5 mg/dL | 18.8 mg/dL | 1.3 mmol/L |
The 1,800 and 100 rules are for rapid-acting (lispro, aspart, glulisine). The 1,500 rule is for regular (short-acting) human insulin. Each cell is “1 unit lowers about this much.”
Two worked corrections
- Rapid-acting, 40 units a day, 250 mg/dL, target 120: 1800 rule: 1800 ÷ 40 units = 45 mg/dL per unit. Correction 2.89 units, rounded to 3.
- Regular insulin, 30 units a day, 300 mg/dL, target 150: 1500 rule: 1500 ÷ 30 units = 50 mg/dL per unit. Correction 3 units, rounded to 3.
- Same 40-unit rapid-acting plan in mmol/L: 16 mmol/L with a target of 6 is 4 units, rounded to 4. 100 rule: 100 ÷ 40 units = 2.5 mmol/L per unit.
Limits
- These rules are starting estimates. A correction factor set from your own readings, with your diabetes team, replaces them.
- Subtract insulin still working from a recent dose before adding another correction. Do not stack doses.
- Below 70 mg/dL (3.9 mmol/L), the correction is zero. This page does not replace a written insulin plan.
Use the calculator
Band charts: how to read a sliding scale. Run a total daily dose in the insulin sliding scale calculator.