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Insulin dose calculation

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"insulin"[MeSH Terms] AND "dose"

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Below is the standard way to calculate insulin doses for a basal-bolus regimen. The method comes from the textbooks in the library. I didn't know your setting, so this is the general outpatient approach. Say if you need one of the variants listed at the end.

1. Total daily dose (TDD)

SituationStarting TDD
Newly diagnosed T1DM (conservative start)0.4 units/kg/day
Average non-obese T1DM0.5 to 0.8 units/kg/day
Obese, insulin-resistant, pubertal, late pregnancyHigher, up to 1 to 2 units/kg/day
Simple office estimateWeight (kg) × 0.7
The first two rows and the "higher" row are from the Washington Manual (p. 895) and Goodman & Gilman's (Insulin Dosing and Regimens). Goodman & Gilman's gives 0.4 to 0.7 units/kg/day for mixed T1DM and 1 to 2 units/kg/day for obesity, type 2 diabetes and puberty. The weight × 0.7 estimate is from the Textbook of Family Medicine 9e (Step 1 of basal-bolus initiation). Adjust the starting dose for hypoglycemia risk, renal function and age.

2. Split into basal and bolus

  • Basal is 40% to 50% of TDD. Use glargine, detemir, degludec or NPH.
  • Bolus is the remaining 50% to 60%, split across meals. Use a rapid-acting analog (lispro, aspart or glulisine).
  • A common prandial starting dose is 0.1 units/kg per meal. The total of the premeal doses should roughly equal the basal dose (Textbook of Family Medicine 9e; Washington Manual).

3. Correction (sensitivity) factor, "1800 rule"

  • Insulin sensitivity factor (ISF) = 1800 ÷ TDD. This is the mg/dL drop expected from 1 unit of rapid-acting insulin (Textbook of Family Medicine 9e).
  • Correction dose = (current glucose − target glucose) ÷ ISF.
  • Some sources use 1500 for regular insulin. That is from my own knowledge, not the retrieved text.

4. Carbohydrate ratio, "500 rule"

  • Grams of carbohydrate covered by 1 unit = 500 ÷ TDD (Harriet Lane Handbook).
  • A typical T1DM ratio is 1 unit per 10 to 15 g of carbohydrate, but it must be individualized (Harrison's, 22nd ed.).
  • Meal dose = (carbohydrate grams ÷ ratio) + correction dose.

Worked example: 70 kg adult with T1DM

  1. TDD = 70 × 0.5 = 35 units/day.
  2. Basal at 50% of TDD = about 17 units once daily. Bolus at 50% = about 18 units/day, or about 6 units per meal.
  3. ISF = 1800 ÷ 35 = about 50 mg/dL per unit.
  4. Carbohydrate ratio = 500 ÷ 35 = about 1 unit per 14 g.
  5. A meal with 60 g carbohydrate and a glucose of 250 mg/dL (target 120):
    • Meal dose = 60 ÷ 14 = 4.3 units.
    • Correction dose = (250 − 120) ÷ 50 = 2.6 units.
    • Total = about 7 units of rapid-acting insulin.

Titration and timing

  • Basal: adjust by 5% to 10% daily until fasting glucose is consistently below 130 mg/dL (Washington Manual). The Family Medicine text uses 1 unit/day until fasting glucose is below 100 mg/dL.
  • Basal in the hospital: continue basal insulin even when the patient is NPO.
  • Prandial timing: inject about 15 minutes before the meal. If pre-meal glucose is below 80 mg/dL, inject at the start of the meal. If the patient is not eating reliably, give the dose after the meal based on what was eaten (Goodman & Gilman's).
  • Basal-only start: for type 2 diabetes starting basal insulin, the usual start is 10 units/day or 0.1 to 0.2 units/kg/day. This is from my own knowledge, not the retrieved text.

Caveats

  • These formulas give a starting point. Individual variation is large, so titrate to glucose logs or CGM.
  • Lower the starting dose for renal impairment, frailty, poor intake or erratic meals. Hypoglycemia is the main risk.
  • Pregnancy needs a different approach. Bolus insulin rises from about 50% of TDD before conception to about 75% at 36 weeks (Creasy & Resnik's).

Recent evidence

A PubMed check of recent systematic reviews found no contradiction of the above. Two are worth knowing about: a 2026 network meta-analysis of insulin regimens in type 2 diabetes (PMID 41436667) and a 2025 meta-analysis of once-weekly versus once-daily insulin in type 1 diabetes (PMID 40186384). Weekly basal insulin changes dosing and conversion rules, so check current labeling if you are using it.
If you want a specific variant, tell me which: DKA or inpatient IV insulin infusion, hyperkalemia, a type 2 basal start, pediatric dosing, a sliding scale, or insulin pump settings.
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