How to Calculate Insulin Dose: A Step-by-Step Guide
Medically reviewed by Dr. Chhaya Makhija, MD, DipABLM — October 2026
By Ryan Mitchell
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.
The insulin dose calculation method is the same four steps clinicians use: estimate total daily dose, split it into basal and bolus, derive the carb ratio and correction factor, then check the result against real readings. Each step is below.
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. The ADA Standards of Care 2026 give typical totals of 0.4–1 unit per kilogram of body weight per day and name 0.5 u/kg/day as the usual start for a metabolically stable adult with type 1 diabetes. Newly diagnosed type 1 adults often need only 0.2–0.6 u/kg/day.
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 |
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.
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 clinics use other constants, such as 450 or 550, and the Carb Factor Calculator compares them side by side. 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 (2.5 mmol/L) 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 | About 0.5 u/kg/day (0.2–0.6 when newly diagnosed), 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.
How the calculation changes in special situations
The four steps assume an adult starting full insulin replacement at home. Several common situations use a different starting number, and the table shows where each one comes from.
| Situation | Starting total daily dose | What changes |
|---|---|---|
| Hospital, not critically ill, eating | 0.3–0.6 u/kg/day | Half basal, half mealtime (ADA 2026); 80% of the home dose if already on insulin |
| Children, new type 1 | 0.5–1.0 u/kg/day | Low end for young, lean, prepubertal children; higher in puberty or after DKA |
| Pregnancy | Set by the obstetric diabetes team | Needs rise through the second and third trimesters and drop sharply after delivery |
| Type 2, starting basal only | 0.1–0.2 u/kg/day or 10 units | No carb ratio or correction at first; titrate against fasting glucose |
| Kidney disease or steroids | Adjust from the usual estimate | Kidney disease lowers needs; glucocorticoids raise them by about half |
How to calculate a premixed (70/30) insulin dose
Premixed insulin combines a long-acting and a short-acting part in one injection, so you calculate one total and split it by time of day. A common teaching split gives two-thirds of the daily total before breakfast and one-third before the evening meal. For a 36-unit total: 24 units before breakfast and 12 units before dinner. Meals then need to come at steady times, because the insulin is already in place.
How to turn units into mL for a syringe
Standard insulin is U-100, which means 100 units in each mL. Divide the units by 100 to get mL: 12 units = 0.12 mL, and 35 units = 0.35 mL. Insulin syringes are already marked in units, so you only need the mL figure for exam questions or a non-insulin syringe. For U-40 veterinary insulin, divide by 40 instead.
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. Each step in this guide has a matching tool in our insulin dose calculator collection.
Frequently Asked Questions: How to Calculate Insulin Dose
How do I calculate my total daily insulin dose?
Multiply body weight in kilograms by a dose factor. A metabolically stable adult with Type 1 diabetes typically starts near 0.5 units per kg per day, so an 80 kg person would start around 40 units. Insulin-naive Type 2 patients usually start far lower, at 0.1–0.2 units per kg or a fixed 10 units of basal. The result is a starting estimate that gets titrated over weeks, not a final dose.
What is the 500 Rule?
Divide 500 by your total daily dose to estimate how many grams of carbohydrate one unit of rapid-acting insulin covers. On a 40-unit TDD that gives 500 ÷ 40 = 12.5, so roughly 1 unit per 12–13 g of carbohydrate. Some clinicians use 450 instead of 500, which gives a slightly stronger ratio.
What is the 1800 Rule?
Divide 1800 by your total daily dose to estimate your correction factor — how far one unit of rapid-acting insulin lowers blood glucose in mg/dL. On a 40-unit TDD that is 1800 ÷ 40 = 45 mg/dL (2.5 mmol/L) per unit. Use 1500 instead of 1800 for regular (short-acting) human insulin, and divide by 18 if you work in mmol/L.
Is the calculation different for Type 2 diabetes?
Yes, and this is where generic calculators mislead people most. Insulin-naive Type 2 patients start conservatively on basal insulin alone, commonly 0.1–0.2 units per kg per day or a flat 10 units, then titrate against fasting glucose. Type 2 patients with marked insulin resistance may eventually need 0.5–2.0 units per kg — a far wider range than the Type 1 starting window.
Can I change my own insulin dose using these formulas?
No. These are the same formulas clinicians use to set a starting point, and they are useful for understanding where your numbers came from, but they take no account of your current glucose, kidney function, activity, illness or other medications. Dose changes belong with the prescriber who knows your history.
How do you calculate insulin dose in units to mL?
Divide the dose in units by the insulin strength. Standard U-100 insulin holds 100 units per mL, so 10 units = 0.1 mL and 25 units = 0.25 mL. For U-40 insulin used in pets, divide by 40, so 10 units = 0.25 mL. Always use a syringe marked for the same strength as the insulin.
How do you calculate a 70/30 insulin dose?
Work out a total daily dose first, then split it. A common teaching split gives two-thirds before breakfast and one-third before the evening meal, so a 30-unit total becomes 20 units in the morning and 10 units in the evening. Your prescriber sets the actual total and split.
Sources & References
- American Diabetes Association. Standards of Care in Diabetes — 2026. Section 9. Link
- Walsh J, Roberts R. Using Insulin. Torrey Pines Press. (500/1800 Rules.)
- American Diabetes Association. 16. Diabetes Care in the Hospital: Standards of Care in Diabetes — 2026. Link
- University of Iowa Stead Family Children's Hospital. Calculating Total Daily Dose. Link
- UCSF Diabetes Teaching Center. Calculating Insulin Dose. Link
- NIDDK. Insulin Medicines & Treatments. Link
Written by Ryan Mitchell · Last updated: October 2026