Insulin Sensitivity Factor (Correction Factor) Explained
Your insulin sensitivity factor (ISF), or correction factor, is how much one unit of rapid-acting insulin is expected to lower your blood glucose. To estimate a correction dose, subtract your target from your current reading and divide by your ISF. Your ISF is set with your healthcare provider, is personal to you, and can change.
What is an insulin sensitivity factor?
An insulin sensitivity factor (ISF) — also called a correction factor — is how far one unit of rapid-acting insulin is expected to lower your blood glucose. It is written as an amount of glucose per unit: for example, 3 mmol/L per unit, or 50 mg/dL per unit. The larger the number, the more sensitive you are to insulin, because one unit moves your glucose further.
The ISF is not the same as the carb-to-insulin ratio. The carb ratio decides how much insulin covers the food you are about to eat; the ISF decides how much insulin brings an already-high reading back toward your target. A full mealtime dose can use both.
How a correction dose uses it
To estimate a correction dose, subtract your target glucose from your current reading, then divide by your ISF. The result is the units needed to bring you back toward target.
In mmol/L: if you are at 12, your target is 6, and your ISF is 3 mmol/L per unit, that is (12 − 6) ÷ 3 = 2 units. In mg/dL: if you are at 220, your target is 110, and your ISF is 50 mg/dL per unit, that is (220 − 110) ÷ 50 = 2.2 units. Keep your reading, target, and ISF all in the same unit.
Where your ISF comes from
Your ISF is personal and is set with your healthcare team. Clinicians sometimes start from an estimate based on your total daily insulin — for instance the "1800 rule" for mg/dL or the "100 rule" for mmol/L, where the number is divided by your total daily dose — and then fine-tune it by reviewing how your glucose actually responds. These rules are a starting point for clinicians, not a number to set for yourself.
Your ISF can change with weight, activity, illness, stress, pregnancy, and time of day, so your care team may adjust it over time. Never change your correction factor on your own.
How CarbTrack uses it
You enter the correction factor and target your provider gives you, and CarbTrack uses them only to calculate an estimated correction dose — adding it to the carb dose where you choose. It does not measure your glucose, recommend a factor, or change the numbers you enter. The result is arithmetic based entirely on your inputs; always confirm it against your care plan before dosing.
Step by step
- Check your current glucose Take a reading and note whether it is above your target range.
- Subtract your target Subtract your target glucose from your current reading to find how far above target you are.
- Divide by your ISF Divide that difference by your insulin sensitivity factor to estimate the correction units, keeping everything in the same unit (mmol/L or mg/dL).
- Confirm before dosing Account for any insulin already on board, compare the result with your care plan, and follow your healthcare provider’s instructions.
Frequently asked questions
What is an insulin sensitivity factor?
It is how much one unit of rapid-acting insulin is expected to lower your blood glucose — for example 3 mmol/L per unit or 50 mg/dL per unit. It is used to size a correction dose for a high reading.
How do I calculate a correction dose?
Subtract your target glucose from your current reading and divide by your insulin sensitivity factor. With a reading of 12 mmol/L, a target of 6, and an ISF of 3: (12 − 6) ÷ 3 = 2 units.
Is the correction factor the same as the carb ratio?
No. The carb-to-insulin ratio covers the carbs in a meal; the correction factor (ISF) brings a high glucose back toward target. A mealtime dose may use both, added together.
Sources
CarbTrack does this math for you — instantly, from a photo or a food search.
Download CarbTrackCarbTrack is a calculator tool only. Not a medical device. Not medical advice. Insulin dose calculations are based solely on the carb ratio you provide. Always consult your healthcare provider before making changes to your insulin regimen or diabetes management plan.