Calculator

Enter insulin type, TDD, meal carbohydrates, and your current/target BG. Then press Calculate. No “live” option is used; the calculator runs only when you click Calculate (or press Enter).

Rapid-acting uses 500 rule for ICR and 1800 rule for ISF . Regular uses constants (450/1500) as an option.
mg/dL mmol/L
Switching units converts the numbers you already entered.

Step 1 — Enter inputs

Units/day (all insulin).
Grams of carbohydrate in the meal/snack.
Use your individualized target.
Helper example (weight ÷ 4 or 0.55×kg).

Step 2 — Results

“1 unit covers ___ grams of carbohydrate”. (Calculated)
“1 unit lowers BG by ___ mg/dL”. (Calculated; always shown in mg/dL)
Carb grams ÷ ICR.
(Current − Target) ÷ ISF. May be negative if below target.
Total = carb coverage + correction, but not below 0.
How this calculator works

 

  • Carb coverage dose = (grams of carbohydrate) ÷ (grams covered by 1 unit).
  • Correction dose = (current BG − target BG) ÷ (ISF).
  • Total mealtime dose = carb coverage + correction (commonly not less than 0).

For rapid-acting insulin, the calculator estimates: ICR = 500 ÷ TDD and ISF = 1800 ÷ TDD.

Clinical practice varies; confirm with your diabetes plan and clinician guidance.

Disclaimer: All calculations must be confirmed before use. The results are not a substitute for clinical judgment. Perinatology.com and affiliated parties are not responsible for any errors or consequences arising from use of this calculator. If you are not willing to accept the terms in the Disclaimer, please do not use this website.
References
Davidson PC, et al. Analysis of guidelines for basal-bolus insulin dosing: basal insulin, correction factor, and carbohydrate-to-insulin ratio. Endocr Pract. 2008;14(9):1095–1101. PMID: 19158048.

UCSF Diabetes Teaching Center. “Calculating Insulin Dose” (Rule of 500; Rule of 1800). © 2025 The Regents of the University of California.  ACCESSED 12/21/2025