Why 1800 for rapid-acting insulin and 1500 for regular?

The constant tracks the duration of action. Rapid analogues (lispro, aspart, glulisine) are assigned 1800, while regular human insulin uses 1500 because its longer tail means each unit acts over more hours. Using the wrong constant for the insulin in the pen is a frequent source of over-correction.

How is the correction dose worked out?

Subtract the target glucose from the current glucose and divide by the sensitivity factor. With a factor of 50, a glucose of 250 mg/dL and a target of 100 mg/dL, the correction is (250 - 100) / 50 = 3 units. The result is rounded to the nearest half or whole unit the pen can deliver.

When is a correction dose unsafe to give?

When active insulin from a recent bolus is still on board, when exercise is planned within a few hours, when the patient is ill or eating poorly, or when the sensitivity factor has not been validated for that individual. Stacking corrections is the usual route to a severe low.

The 1800 Rule: Insulin Sensitivity Factor

The 1800 rule estimates how far one unit of rapid-acting insulin will drop blood glucose: divide 1800 by the total daily dose. This page computes the sensitivity factor and the correction dose for a current and target glucose.

ISF = rule constant ÷ total daily insulin dose. Source: Walsh & Roberts, Using Insulin (2003); ADA Standards of Care. Educational only.

Insulin Rule

Total Daily Insulin Dose (units)

Current Glucose (mg/dL)

Target Glucose (mg/dL)

45.0mg/dL per unit

Insulin sensitivity factor — Correction dose 2.2 units of insulin to return glucose to target.

Insulin Sensitivity Factor45.0 mg/dL / unit1 unit lowers glucose by ≈ 45.0 mg/dL

Correction Dose2.22 units(current − target) ÷ ISF

Rule	Insulin type
1800	Rapid-acting analogue (lispro, aspart)
1500	Regular human insulin
2000	Very insulin-sensitive patients

Clinical Guidance

The insulin sensitivity factor (ISF, or correction factor) estimates how much 1 unit of insulin lowers blood glucose. The 1800 rule is used for rapid-acting analogues and the 1500 rule for regular insulin: divide the rule constant by the total daily insulin dose. A person on 40 units/day therefore has an ISF of 1800 ÷ 40 = 45 mg/dL per unit.

The correction dose is (current glucose − target glucose) ÷ ISF. ISF is a starting estimate only: insulin sensitivity varies through the day (often 20–30% greater overnight and in the morning) and with exercise, illness and renal function. Verify against actual glucose response and adjust with the diabetes care team.

This calculator is for education and reference. It is not a substitute for an individualised insulin plan, and correction dosing should follow the regimen prescribed by a clinician.

How to Use

Enter the values for the scenario you are assessing. The 1800 rule estimates how far one unit of rapid-acting insulin will drop blood glucose: divide 1800 by the total daily dose. This page computes the sensitivity factor and the correction dose for a current and target glucose. Use the insulin correction result to inform your calculation.

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Frequently Asked Questions

Why 1800 for rapid-acting insulin and 1500 for regular?

How is the correction dose worked out?

When is a correction dose unsafe to give?