The insulin correction factor (also called the insulin sensitivity factor, or ISF) estimates how far one unit of insulin is expected to lower blood glucose. This calculator derives it from total daily insulin dose using two published starting-point conventions — the 1800 Rule and the 1500 Rule — and uses it to estimate a correction dose from a current and target glucose reading.

How the correction factor is estimated from TDD

Diabetes educators commonly estimate a starting correction factor by dividing a constant by total daily insulin dose (TDD). Rapid-acting analog insulins — lispro (Humalog), aspart (Novolog), and glulisine (Apidra) — use the 1800 Rule: Correction Factor = 1800 / TDD. Regular (short-acting) insulin uses the 1500 Rule: Correction Factor = 1500 / TDD, reflecting its slower onset and longer duration of action.

A companion formula, the Rule of 500, estimates the carb ratio — grams of carbohydrate covered by one unit of insulin — as 500 / TDD. Both formulas depend entirely on TDD staying accurate and current.

From correction factor to correction dose

Once a correction factor is known, it can estimate a correction dose: how many units are needed to bring a current glucose down to a target glucose. The formula is Correction Dose = (Current Glucose − Target Glucose) / Correction Factor. If current glucose is already at or below target, the estimated correction dose is zero — there is nothing to correct.

The correction factor used here does not have to come from the 1800/1500 Rule calculation on this page — many people already have a correction factor prescribed by their care team, and the Correction Dose tab lets you type that value in directly instead of using the auto-calculated one.

Limits: these are starting points, not prescriptions

The 1800/1500/500 Rules are population-based empirical formulas, not individualized measurements. They do not account for insulin-on-board (insulin stacking) from a previous dose, illness, exercise, stress, alcohol, menstrual cycle, or kidney function — all of which can meaningfully shift a person's real insulin sensitivity. TDD itself must also be a stable, accurate reflection of current insulin use; a recent dose change makes these estimates less reliable until TDD stabilizes.

This tool is for education and clinical reference only. It is not a substitute for individualized titration by a diabetes care team, and correction-factor or target adjustments should always be reviewed with a prescriber before being put into practice.