How to Calculate Insulin Dose
A step-by-step walkthrough of the standard methods used to estimate insulin doses: weight-based starting doses, total daily dose (TDD), basal-bolus split, insulin-to-carb ratio (ICR), and correction doses. Educational guide — not a substitute for clinical guidance.
Step 1 — Estimate Total Daily Dose (TDD)
TDD is the total amount of insulin a person uses in a day — both basal and bolus combined. It is the foundation of all other dose calculations.
Weight-Based TDD Formula
The factor 0.5 u/kg/day is a common adult starting point. Type 2 patients new to insulin often start lower (0.1–0.2 u/kg/day). Type 1 patients may use 0.4–0.6 u/kg/day. Obese patients may require higher factors (0.6–1.0). These are starting estimates — individual responses vary widely.
Example: A 70 kg adult using 0.5 u/kg/day: TDD = 70 × 0.5 = 35 units/day
Step 2 — Split TDD into Basal and Bolus
In a basal-bolus regimen, TDD is divided between long-acting basal insulin (background coverage) and rapid-acting bolus insulin (meal coverage).
The 50/50 split is the standard starting point. Some providers start with 40% basal for Type 2 patients with predominantly postprandial hyperglycemia, or 60% basal for patients with significant fasting hyperglycemia. The split is adjusted based on fasting and post-meal glucose patterns over time.
Example (TDD = 35): Basal = 17.5 units/day · Bolus pool = 17.5 units/day
Step 3 — Calculate Insulin-to-Carb Ratio (ICR)
The ICR tells you how many grams of carbohydrate one unit of rapid-acting insulin covers.
500 Rule
The 500 Rule gives an estimated ICR. For a TDD of 35: ICR = 500 ÷ 35 ≈ 14 g/unit — meaning one unit covers approximately 14 grams of carbohydrate. The 450 Rule is sometimes used for patients on Regular insulin or with lower sensitivity.
Step 4 — Calculate Insulin Sensitivity Factor (ISF)
The ISF (also called correction factor) tells you how many mg/dL one unit of rapid-acting insulin will lower blood glucose.
1800 Rule
For a TDD of 35: ISF = 1800 ÷ 35 ≈ 51 mg/dL per unit. For Regular insulin, the 1500 Rule is used instead. The ISF is used to calculate correction doses when blood glucose is above target.
Step 5 — Calculate a Meal Bolus
Example: Eating 60g of carbs with an ICR of 14: 60 ÷ 14 = 4.3 units → round to 4 units (or 4.5 if your pen allows half-units).
Step 6 — Add a Correction Dose (if needed)
Example: BG = 220 mg/dL, Target = 100, ISF = 51: (220 − 100) ÷ 51 = 2.35 → round to 2.5 units. Add to meal bolus. Always subtract insulin on board (IOB) before correcting.
Quick Reference Summary
| Step | Formula | Example (70 kg, TDD = 35) |
|---|---|---|
| TDD | Weight × 0.5 | 35 units/day |
| Basal | TDD × 50% | 17–18 units/day |
| ICR | 500 ÷ TDD | ~14 g/unit |
| ISF | 1800 ÷ TDD | ~51 mg/dL/unit |
| Meal bolus | Carbs ÷ ICR | 60g ÷ 14 = 4 units |
| Correction | (BG − Target) ÷ ISF | (220 − 100) ÷ 51 = 2 units |
These are starting estimates only. Real insulin management involves ongoing titration based on blood glucose patterns, carbohydrate content, activity, illness, stress, and other factors. Work with your diabetes care team to adjust doses based on your actual glucose data.
Sources
- American Diabetes Association. Standards of Medical Care in Diabetes — 2024. Sections 7 and 9.
- Walsh J, Roberts R. Pumping Insulin. 5th ed. Torrey Pines Press; 2012.
Last reviewed: June 2025