Insulin Sensitivity Factor Calculator
Estimate your correction factor — how far one unit of insulin drops blood glucose — from your total daily insulin dose using the 1800/1500 rule.
How it works
ISF (mg/dL per unit) = rule constant ÷ total daily dose · mmol/L = ISF ÷ 18
The insulin sensitivity factor (correction factor) estimates how many points one unit of rapid-acting insulin will lower blood glucose. The common shortcut divides a constant by your total daily insulin dose: 1800 (or 1700) for rapid-acting analogs in mg/dL, or 1500 for older regular insulin. A higher total daily dose means a lower sensitivity factor — each unit does less. This is only an initial estimate; real correction factors are individualised and refined from glucose data by a clinician.
Worked example
For someone using 40 units of a rapid-acting insulin per day, the rule of 1800 gives 1800 ÷ 40 = 45 mg/dL per unit. So one unit is expected to drop glucose by roughly 45 mg/dL, or about 2.5 mmol/L (45 ÷ 18).
Frequently asked questions
What is the difference between the 1800 and 1500 rules?
The rule of 1800 (some clinicians use 1700) applies to rapid-acting insulin analogs; the rule of 1500 applies to older short-acting regular insulin, which is less potent per unit for corrections. Using the wrong constant gives a misleading factor, so match it to your insulin type.
Is total daily dose basal plus bolus?
Yes — the total daily dose is all the insulin you take in a day, both long-acting (basal) and mealtime plus correction (bolus). An accurate total is essential because the sensitivity factor is inversely proportional to it.
Can I set my own correction factor from this?
No. This is a textbook starting point only. Your true correction factor depends on time of day, activity, illness and many other factors, and must be set and adjusted by your diabetes care team from your own glucose readings — never change insulin dosing based on this estimate alone.
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Educational estimate only — not medical advice, and not a substitute for a clinician, pharmacist or product label. Dosing, fluid and kidney decisions must be confirmed by a qualified professional using the actual drug, patient and situation. Do not use this tool to self-treat.