Insulin Calculator Formulas Reference
Medically reviewed by Dr. Chhaya Makhija, MD, DipABLM — October 2026
By Ryan Mitchell
Every equation used across this site, in one place, with its meaning and the tool that runs it. Bookmark this as your cheat sheet — full sourcing is on the methodology page.
Dosing formulas
Total Daily Dose (TDD)
Starting estimate of all insulin per day. Insulin-naïve Type 2: 0.1–0.2 u/kg or 10 u fixed. ➜ Dose by Weight · TDD
Basal / Bolus Split
➜ Basal-Bolus · Basal %
Insulin-to-Carb Ratio (ICR)
Grams of carb covered by 1 unit. ➜ ICR (500) · 450 Rule · Compare 450/500/550
Insulin Sensitivity Factor (ISF)
mg/dL that 1 unit lowers glucose. ➜ ISF Calculator
Correction & Meal Dose
Total bolus = meal + correction − insulin on board. ➜ Correction · Bolus · IOB
Insulin resistance & metabolic
HOMA-IR
➜ HOMA-IR · guide: What Is HOMA-IR?
QUICKI
➜ QUICKI
Monitoring & conversion
eAG ↔ HbA1c
➜ HbA1c Calculator
Glucose units
➜ Glucose Converter
Insulin units ↔ mL
➜ Units ↔ mL · Units ↔ mg Every formula on this page runs live in one of our online insulin calculators.
Every formula on one sheet
Each row below is implemented by a calculator on this site. Divisors differ between rules because they assume different insulins, so check which product your prescription names before picking one.
| What it gives you | Formula | Notes |
|---|---|---|
| Total daily dose (TDD) | Weight (kg) × 0.4–1.0 | Type 1 start 0.5; Type 2 start 0.1–0.2 |
| Basal / bolus split | Basal ≈ 40–50% of TDD | Pump users measure nearer 28% |
| Carb ratio — 500 Rule | 500 ÷ TDD | Rapid analogues (lispro, aspart) |
| Carb ratio — 450 Rule | 450 ÷ TDD | Regular human insulin |
| Sensitivity — 1800 Rule | 1800 ÷ TDD | mg/dL per unit, rapid analogues |
| Sensitivity — 1700 Rule | 1700 ÷ TDD | Davidson variant, slightly stronger |
| Sensitivity in mmol/L | 100 ÷ TDD | Same rule, SI units |
| Correction dose | (Current BG − Target) ÷ ISF | Subtract insulin on board |
| Carbohydrate dose | Carbs (g) ÷ carb ratio | Add the correction for the total bolus |
| HOMA-IR | (Glucose × Insulin) ÷ 405 | mg/dL; use ÷ 22.5 for mmol/L |
| QUICKI | 1 ÷ (log insulin + log glucose) | Fasting values, mg/dL |
| GMI from CGM | 3.31 + 0.02392 × mean glucose | mg/dL, 14 days of data |
| Estimated average glucose | 28.7 × A1c − 46.7 | mg/dL from A1c % |
| Glucose infusion rate | (Rate mL/hr × dextrose %) ÷ (6 × kg) | mg/kg/min |
| Units to volume | Units ÷ concentration | U-100: 20 u = 0.2 mL |
| Days supply | Total units ÷ units per day | Round down to a whole day |
TDD drives most of the chain: the carb ratio, the sensitivity factor and the basal split all derive from it, so an error in the total daily dose propagates into every later figure.
Frequently Asked Questions: Insulin Calculator Formulas Reference
What is the difference between the 450 Rule and the 500 Rule?
Both estimate the insulin-to-carb ratio by dividing a constant by total daily dose; they simply differ in how aggressive the result is. 500 is the more common starting constant, while 450 produces a stronger ratio — more insulin per gram of carbohydrate. Clinicians sometimes prefer 450 for people who are more insulin resistant.
Should I use 1800 or 1500 for my correction factor?
Use 1800 with rapid-acting analogues such as lispro, aspart or glulisine, and 1500 with regular (short-acting) human insulin, which is less potent per unit over the correction window. If you work in mmol/L rather than mg/dL, divide the result by 18.
Do these formulas work for Type 2 diabetes?
They apply once someone is on a full basal-bolus regimen, because they are all derived from total daily dose. They are far less useful for a Type 2 patient on basal insulin alone, where the dose is titrated against fasting glucose rather than calculated from a ratio.
Where do these formulas come from?
They are empirical rules of thumb that entered clinical practice through diabetes education literature and are reproduced in ADA-aligned teaching materials, not equations derived from first principles. That is precisely why every result they produce is a starting estimate to be titrated, and why two clinicians may reasonably use different constants.
Sources
- Matthews DR, Hosker JP, Rudenski AS, et al. "Homeostasis model assessment: insulin resistance and β-cell function from fasting plasma glucose and insulin concentrations in man." Diabetologia. 1985;28(7):412–419. — HOMA-IR.
- Katz A, Nambi SS, Mather K, et al. "Quantitative insulin sensitivity check index: a simple, accurate method for assessing insulin sensitivity in humans." J Clin Endocrinol Metab. 2000;85(7):2402–2410. — QUICKI.
- Bergenstal RM, Beck RW, Close KL, et al. "Glucose Management Indicator (GMI): A New Term for Estimating A1C From Continuous Glucose Monitoring." Diabetes Care. 2018;41(11):2275–2280. — GMI.
- Nathan DM, Kuenen J, Borg R, et al. "Translating the A1C assay into estimated average glucose values." Diabetes Care. 2008;31(8):1473–1478. — the eAG relationship. Link
- American Diabetes Association. Standards of Care in Diabetes — 2026. Diabetes Care. 2026;49(Suppl 1). Link — weight-based initiation, glycemic targets and titration principles.
- Walsh J, Roberts R. Pumping Insulin. Torrey Pines Press. — the 500 and 1800 Rules as taught in diabetes education practice.
Every formula above is also cross-referenced on our methodology page, which records the source behind each individual equation.
Written by Ryan Mitchell · Last updated: October 2026