Insulin Calculator Formulas
A complete reference for every formula used across this site. Each formula includes the equation, variable definitions, a worked example, source, and a link to the corresponding calculator.
Dosing & Regimen Formulas
Total Daily Dose (TDD) — Weight-Based Starting Estimate
Variables: Body weight in kilograms. The factor 0.5 is a conservative starting point; some protocols use 0.3–0.4 for insulin-sensitive patients and 0.6–0.7 for insulin-resistant patients.
Example: 70 kg → TDD = 70 × 0.5 = 35 units/day
Source: ADA Standards of Medical Care 2024, Section 9.
Basal Dose
The basal-bolus split is approximately 50/50 as a starting point. Actual split is titrated based on fasting glucose patterns.
Example: TDD = 35 → Basal = 17.5 units (rounded to 18)
Total Bolus Pool & Per-Meal Dose
Splits the bolus pool equally across three meals as a starting point. Actual meal doses are refined using carb counting and the ICR.
Example: TDD = 35 → Bolus pool = 17.5 → Per meal = ~5.8 units
Ratio & Correction Formulas
Insulin-to-Carb Ratio (ICR) — 500 Rule
ICR tells you how many grams of carbohydrate one unit of rapid-acting insulin covers. Used for rapid-acting analogues (lispro, aspart, glulisine).
Example: TDD = 35 → ICR = 500 ÷ 35 = 1:14 (1 unit per 14 g carbs)
Source: Walsh J, Roberts R. Pumping Insulin. 5th ed. 2012.
Insulin-to-Carb Ratio — 450 Rule (Regular Insulin)
Used with Regular (short-acting) insulin due to its slower, longer action profile.
Example: TDD = 35 → ICR = 450 ÷ 35 = 1:12.9
Meal Bolus Dose
Example: 60 g carbs, ICR = 1:14 → Meal bolus = 60 ÷ 14 = 4.3 units (round to 4.5)
Insulin Sensitivity Factor (ISF) — 1800 Rule
ISF (also called correction factor) = how many mg/dL one unit lowers blood glucose.
Example: TDD = 35 → ISF = 1800 ÷ 35 = 51 mg/dL/unit
Source: Walsh J, Roberts R. Pumping Insulin. 5th ed. 2012.
ISF — 1500 Rule (Regular Insulin)
Example: TDD = 35 → ISF = 1500 ÷ 35 = 43 mg/dL/unit
ISF — 100 Rule (mmol/L)
Example: TDD = 35 → ISF = 100 ÷ 35 = 2.86 mmol/L/unit
Correction Dose
Always subtract Insulin on Board (IOB) from the calculated correction before injecting to avoid stacking.
Example: BG = 230 mg/dL, Target = 100, ISF = 51 → (230 − 100) ÷ 51 = 2.55 units (round to 2.5)
Glucose & A1c Conversion Formulas
A1c → Estimated Average Glucose (ADAG Formula)
Developed by Nathan et al. (2008) from the ADAG study correlating A1c to CGM-measured average glucose. The constant 28.7 converts the A1c % to mg/dL units; 46.7 is the intercept.
Example: A1c = 7.0% → eAG = (28.7 × 7.0) − 46.7 = 200.9 − 46.7 = 154 mg/dL
Source: Nathan DM et al. Diabetes Care. 2008;31:1473–1478.
Average Glucose → A1c (Reverse ADAG)
Example: eAG = 154 mg/dL → A1c = (154 + 46.7) ÷ 28.7 = 200.7 ÷ 28.7 = 7.0%
mg/dL ↔ mmol/L Glucose Conversion
Example: 180 mg/dL ÷ 18.016 = 9.99 mmol/L
Insulin Resistance Formulas
HOMA-IR
For mmol/L glucose: divide by 22.5 instead of 405. A score of ~1.0 represents normal insulin sensitivity in a healthy non-diabetic adult.
Example: Glucose = 95, Insulin = 12 → (95 × 12) ÷ 405 = 2.81 (borderline elevated)
Source: Matthews DR et al. Diabetologia. 1985;28:412–419.
HOMA-%B (Beta-Cell Function)
Estimates remaining beta-cell secretory capacity. 100% = normal secretory function. Requires glucose in mmol/L (divide mg/dL by 18.016).
Example: Insulin = 12, Glucose = 5.3 mmol/L → (20 × 12) ÷ (5.3 − 3.5) = 240 ÷ 1.8 = 133%
QUICKI (Quantitative Insulin Sensitivity Check Index)
Higher QUICKI = more sensitive. Normal range ~0.339–0.450. Values < 0.30 suggest significant resistance.
Example: Insulin = 12, Glucose = 95 → 1 ÷ [log(12) + log(95)] = 1 ÷ [1.079 + 1.978] = 1 ÷ 3.057 = 0.327 (borderline)
Source: Katz A et al. J Clin Endocrinol Metab. 2000;85:2402–2410.
Glucose-to-Insulin Ratio (G:I)
Lower G:I = more insulin for the same glucose = more resistance. Reference: >12 good sensitivity, <4.5 suggests hyperinsulinemia.
Example: Glucose = 95, Insulin = 12 → 95 ÷ 12 = 7.9 (normal)
Triglyceride-to-HDL Ratio (TG:HDL)
A surrogate marker for insulin resistance and cardiovascular risk based on lipid panel (no insulin required). Thresholds apply to mg/dL values. Reference: <2.0 normal, >3.0 elevated resistance risk.
Unit conversion: TG mg/dL ÷ 88.57 = mmol/L | HDL mg/dL ÷ 38.67 = mmol/L
IV Insulin & Transition Formulas
IV Insulin Infusion Rate
Common concentrations: 0.5 / 1.0 / 2.0 units/mL. Common starting factors: 0.02–0.1 u/kg/hr (protocol-dependent).
Example: 80 kg × 0.05 = 4 units/hr ÷ 1.0 units/mL = 4 mL/hr
IV → Subcutaneous Transition (TDD from IV Rate)
Reduction accounts for the lower bioavailability and different pharmacokinetics of subcutaneous insulin. The first basal dose should be given 1–2 hours before stopping the IV infusion.
Insulin on Board (IOB)
Linear IOB Decay
Where t = elapsed time since injection (hours), DIA = duration of insulin action (hours, typically 3–6 hr for rapid-acting). Gives a straight-line decay to zero.
Parabolic / Exponential IOB Approximation
More physiologically realistic — rapid initial decay, slower tail. Used by many insulin pumps. Both models converge at t = 0 (100% active) and t = DIA (0% active).
Example: t = 2 hr, DIA = 4 hr → IOB% = (1 − 0.5)² = 0.25 = 25% active
Supply & Volume Formulas
Volume (mL) from Units
For standard U-100: mL = Units ÷ 100. For U-40: mL = Units ÷ 40.
Example: 50 units U-100 → 50 ÷ 100 = 0.5 mL
Vial Day Supply
Standard U-100 vial = 1000 units (10 mL × 100 units/mL).
Example: TDD = 35 → 1000 ÷ 35 = 28.6 days per vial
Pens Per Month
Standard pen cartridge = 300 units (3 mL).
Example: TDD = 35 → (35 × 30) ÷ 300 = 1050 ÷ 300 = 3.5 pens (round up to 4)
Pump Basal Rate
Starting estimate for a flat 24-hour basal rate profile. Real pump programming uses variable rates across the day.
Example: TDD = 35 → (17.5) ÷ 24 = 0.73 u/hr
Primary Sources
- Matthews DR et al. "Homeostasis model assessment." Diabetologia. 1985;28:412–419.
- Katz A et al. "Quantitative insulin sensitivity check index." J Clin Endocrinol Metab. 2000;85:2402–2410.
- Nathan DM et al. "Translating the A1C assay." Diabetes Care. 2008;31:1473–1478.
- Walsh J, Roberts R. Pumping Insulin. 5th ed. 2012. (500 Rule, 1800 Rule)
- American Diabetes Association. Standards of Medical Care — 2024.
Last reviewed: June 2025