How to Calculate Insulin Dose: A Step-by-Step Guide
By Ryan Mitchell · Reviewed against ADA Standards of Care · Updated July 2026
Every insulin regimen — injections or pump — is built from the same four calculations. This guide walks through each one in order, with the formula, a worked example, and the calculator that automates it. By the end you'll understand exactly where numbers like "35 units a day" or "1 unit per 12 grams" come from.
Step 1 — Estimate Total Daily Dose (TDD) from body weight
The starting point for everything else is the total amount of insulin your body is likely to need in 24 hours, basal and bolus combined. For most adults, guidelines suggest a starting range of 0.4–0.6 units per kilogram of body weight per day, with 0.5 u/kg as the common midpoint.
Example: 80 kg × 0.5 = 40 units/day
Two important variations:
- Insulin-naïve Type 2: guidelines start far more conservatively — 0.1–0.2 u/kg/day of basal insulin, or a fixed 10 units at bedtime, titrated up from there.
- Pounds instead of kg: a common shortcut is TDD ≈ weight in pounds ÷ 4 (equivalent to ~0.55 u/kg).
Starting TDD chart by weight
| Weight | 0.4 u/kg (low) | 0.5 u/kg (mid) | 0.6 u/kg (high) | 0.2 u/kg (naïve T2) |
|---|---|---|---|---|
| 50 kg / 110 lb | 20 u | 25 u | 30 u | 10 u |
| 60 kg / 132 lb | 24 u | 30 u | 36 u | 12 u |
| 70 kg / 154 lb | 28 u | 35 u | 42 u | 14 u |
| 80 kg / 176 lb | 32 u | 40 u | 48 u | 16 u |
| 90 kg / 198 lb | 36 u | 45 u | 54 u | 18 u |
| 100 kg / 220 lb | 40 u | 50 u | 60 u | 20 u |
| 110 kg / 243 lb | 44 u | 55 u | 66 u | 22 u |
| 120 kg / 265 lb | 48 u | 60 u | 72 u | 24 u |
➜ Try it yourself: Insulin Dose by Weight Calculator · TDD Calculator
Step 2 — Split TDD into basal and bolus
Roughly 40–50% of TDD is basal — long-acting insulin (glargine, detemir, degludec) covering fasting periods and overnight. The remaining 50–60% is the bolus pool — rapid-acting insulin divided across meals.
Example: 40 u TDD → 20 u basal once daily + 20 u bolus pool → roughly 6–7 u per meal before carb adjustments.
➜ Basal-Bolus Calculator · Basal Percentage Calculator · Long-Acting Insulin Calculator
Step 3 — Derive your insulin-to-carb ratio (500 Rule)
The insulin-to-carb ratio (ICR) tells you how many grams of carbohydrate one unit of rapid-acting insulin covers, so meal doses scale with what you actually eat.
Example: 500 ÷ 40 ≈ 1 unit per 12 g of carbs. A 60 g-carb dinner → 60 ÷ 12 = 5 units.
Using Regular (short-acting) insulin instead of a rapid analog? Use the 450 Rule. Some clinicians prefer 550 for very rapid analogs — compare on the Carb Factor Calculator. Full guide: Insulin-to-Carb Ratio Explained.
Step 4 — Derive your correction factor (1800 Rule)
The insulin sensitivity factor (ISF, also called correction factor) tells you how far one unit of rapid-acting insulin drops your blood glucose, so you can correct high readings safely.
Example: 1800 ÷ 40 = 45 mg/dL per unit. Reading 240, target 120 → (240 − 120) ÷ 45 ≈ 2.5 units — before subtracting insulin on board.
Always subtract active insulin from a correction to avoid stacking — see the Insulin on Board Calculator. Full guide: ISF Explained.
Putting it all together — a full worked example
Maria, 75 kg, Type 1, using rapid-acting + long-acting insulin
TDD = 75 × 0.5 = 37.5 u/day
Basal = 37.5 × 50% ≈ 19 u glargine at bedtime
ICR = 500 ÷ 37.5 ≈ 1:13 — lunch with 65 g carbs → 5 u
ISF = 1800 ÷ 37.5 = 48 — reading 220, target 120 → +2 u correction
Lunch dose = 5 + 2 = 7 units (minus any insulin on board)
➜ Run this entire chain in one tool: Insulin Dose Calculator
Calculating insulin dose: Type 1 vs Type 2 diabetes
The four steps above apply to full replacement in Type 1. In Type 2, where the pancreas still makes some insulin, therapy usually starts smaller and simpler — often basal insulin alone — and only builds toward a full basal-bolus regimen if glucose stays above target. Here's how the calculation differs:
| Factor | Type 1 diabetes | Type 2 diabetes |
|---|---|---|
| Starting TDD | 0.4–0.5 u/kg/day, full basal-bolus from day one | 0.1–0.2 u/kg/day or a fixed 10 units basal |
| First regimen | Basal + mealtime bolus + correction | Usually basal insulin alone, at first |
| Carb counting | Essential — meals dosed by ICR | Often not needed early; added if bolus is started |
| Titration signal | Pre-meal and post-meal readings | Mainly fasting morning glucose |
| Correction dose | Routine, using ISF | Introduced later, if on bolus insulin |
Dedicated tools: Type 1 insulin calculator · Type 2 insulin calculator.
How to calculate an insulin dose based on your blood sugar
Dosing from a blood sugar reading means the correction (Step 4) component: how much extra insulin brings a high reading back to target. The formula is (current BG − target BG) ÷ ISF, then subtract any insulin still active from a previous dose.
Example: reading 260, target 120, ISF 50, 1 unit still on board → (260 − 120) ÷ 50 = 2.8 → minus 1 IOB ≈ 1.8 units. At a meal, add this to the carb dose from Step 3. Run it on the correction dose calculator.
The step everyone forgets: titration
Formulas produce starting numbers. The dose that actually manages your glucose emerges over 2–6 weeks of supervised adjustment: fasting readings tune the basal dose (a common outpatient protocol adds 2 units every 3 days until fasting glucose reaches target), 2-hour post-meal readings tune each meal's ICR, and correction outcomes tune the ISF. If your prescribed numbers differ from the formulas on this page, that's expected — titrated reality beats population math.
Sources & References
- American Diabetes Association. Standards of Medical Care in Diabetes — 2024. Section 9. Link
- Walsh J, Roberts R. Using Insulin. Torrey Pines Press. (500/1800 Rules.)
- NIDDK. Insulin Medicines & Treatments. Link
Last reviewed: July 2026 · See our methodology