Insulin Sensitivity Factor Calculator
1800-rule ISF, carb ratio, correction, and meal bolus from total daily insulin.
What it does
Insulin sensitivity factor (ISF) answers one question: how far will one unit of insulin drop my blood glucose? The carb ratio (ICR) answers the mealtime twin: how many grams of carbohydrate does one unit cover? Together they turn a glucose reading and a plate of food into a starting dose.
The 1800 rule estimates ISF as 1800 divided by the total daily insulin (TDD, all basal plus bolus units in a day). An older sibling, the 1500 rule, was written for regular human insulin; rapid-acting analogs fit the 1800 constant better in common practice handouts.
The 500 rule does the same for carbohydrates: ICR = 500 / TDD. Adjustments such as 450 or 400 appear for very insulin-sensitive people; this calculator uses the standard 1800 and 500 constants.
Both are starter estimates, not prescriptions. Real sensitivity swings with illness, hormones, activity, and site absorption, so calibration with a clinician and real glucose data matters more than any single formula.
How to use
- Enter total daily insulin: all long and short acting units in a typical day.
- Enter the carbohydrate grams for the meal, or 0 if you only correct.
- Enter the current blood glucose.
- Enter the target blood glucose.
- Read the ISF and carb ratio.
- Read the correction, meal, and total doses.
Formula
- TDD
- Total daily insulin: every basal and bolus unit in a typical day.
- ISF
- Milligrams per deciliter one unit of rapid-acting insulin is expected to lower glucose.
- ICR
- Grams of carbohydrate covered by one unit.
- BG, target
- Current and target blood glucose in mg/dL.
- carbs
- Carbohydrate grams in the meal.
The 1800 rule fits rapid-acting insulin; regular insulin practice often uses 1500. The 500 rule is a starting carb ratio, tuned toward 450 or 400 for very sensitive people. The correction clamps at zero when glucose is under target, because insulin is never the answer to a low. Rounding to half units matches half-unit pens; pumps deliver finer steps.
Worked example
Example: TDD 40, 50 g meal, correcting 240 toward 120
Total daily insulin 40 units, meal 50 g of carbs, current 240 mg/dL, target 120 mg/dL.
- ISF = 1800 / 40 = 45 mg/dL per unit.
- ICR = 500 / 40 = 12.5 g per unit.
- Correction = (240 - 120) / 45 = 2.7 units.
- Meal = 50 / 12.5 = 4.0 units.
- Total = 6.7, rounded to 6.5 units for a half-unit pen.
- Tool shows ISF 45, ICR 12.5, correction 2.7, meal 4, total 6.5 units.
Reference table
| Total daily insulin | ISF (1800 rule) | ICR (500 rule) |
|---|---|---|
| 20 units | 90 mg/dL per unit | 25 g per unit |
| 40 units | 45 mg/dL per unit | 12.5 g per unit |
| 60 units | 30 mg/dL per unit | 8.3 g per unit |
| 80 units | 22.5 mg/dL per unit | 6.3 g per unit |
| 100 units | 18 mg/dL per unit | 5 g per unit |
Limits
- Starting estimates only: illness, hormones, exercise, and site absorption move day-to-day needs.
- Uses mg/dL throughout; multiply mmol/L by 18 to convert.
- The 1800 constant targets rapid-acting insulin; regular insulin often runs on 1500.
- No dose renders below target: the correction clamps at zero.
- Not medical advice; confirm any regimen or ratio change with a clinician.
Common mistakes
- Using the 1800 rule for regular insulin instead of rapid-acting.
- Correcting with glucose below target instead of treating the low with carbs.
- Stacking correction doses faster than insulin action time, which drives lows hours later.
- Treating ISF as fixed when sensitivity drops with inactivity, illness, or steroids.
- Rounding each half of the bolus instead of the total.
Try it yourself
Quick checks. Answers sit tucked under each question.
TDD 60 units?
ISF 30 mg/dL per unit, carb ratio 8.3 g per unit.
TDD 60, glucose 350, target 140, no meal?
Correction dose 7.0 units.
TDD 100, glucose 200, target 100?
ISF 18, correction 5.6 units, total 5.5 after rounding.
Questions
What is insulin sensitivity factor?
The expected drop in glucose from one unit of insulin. An ISF of 45 means one unit should lower blood glucose about 45 mg/dL.
What is the 1800 rule?
ISF = 1800 / TDD, a common starting estimate for rapid-acting insulin, where TDD counts every basal and bolus unit in a day.
Why 500 for carbohydrates?
The 500 rule, ICR = 500 / TDD, estimates grams of carbs covered per unit. Very insulin-sensitive people often see 400 or 450 used instead.
Should I round the dose?
Half-unit pens step in 0.5 units, whole-unit pens in 1, and pumps in hundredths. The tool rounds the total to 0.5 to match common pens.
Does this work for type 2 diabetes?
The arithmetic applies whenever mealtime insulin is used, including type 2. Needs vary more widely, so calibration with glucose data matters even more.
Is this medical advice?
No. It is a teaching estimate of standard rules. Change doses, ratios, or targets only with your diabetes care team.
Sources
- Indian Health Service: Carbohydrate Counting handout (PDF)
- NPAIHB: Calculating the Correction Dose (PDF)