Calculating Insulin Requirements
Initial Total Daily Dose (TDD)
For patients with type 1 diabetes, start with 0.5 units/kg/day as the initial total daily insulin dose, with approximately 50% allocated to basal insulin and 50% to bolus (prandial) insulin. 1
Weight-Based Calculation
- Standard starting dose: 0.5 units/kg/day for metabolically stable adults with type 1 diabetes 2, 1
- Range for newly diagnosed type 1 diabetes: 0.5-1.0 units/kg/day 2
- Lower doses (0.2-0.6 units/kg/day): Required for young children and those in the "honeymoon period" with continued endogenous insulin production 2
- Higher doses (up to 1.5 units/kg/day): Needed during puberty due to growth hormone and sex hormone effects 2, 1
- Increased requirements: During pregnancy, medical illness, ketoacidosis presentation, or steroid use 2, 1
Distribution Formula
Divide the TDD as follows:
- Basal insulin = 50% of TDD (though recent evidence suggests 30% may be more accurate) 1, 3
- Bolus insulin = 50% of TDD, divided among three meals 1
Calculating Insulin-to-Carbohydrate Ratio (ICR)
Use the formula ICR = 300/TDD for breakfast, and ICR = 400/TDD for lunch and dinner. 3
Standard Formulas
- Breakfast ICR = 300 ÷ TDD (accounts for higher insulin resistance in morning due to counter-regulatory hormones) 3
- Lunch and dinner ICR = 400 ÷ TDD 3
- Alternative formula from CGM data: ICR = 300 ÷ TDD for all meals 4
The ICR represents how many grams of carbohydrate are covered by 1 unit of insulin 5. For example, if TDD = 40 units, breakfast ICR = 300/40 = 7.5 (meaning 1 unit covers 7.5g carbs), while lunch/dinner ICR = 400/40 = 10 (1 unit covers 10g carbs) 3.
Calculating Correction Factor (Insulin Sensitivity Factor)
Use the formula: Correction Factor = 1500 ÷ TDD (in mg/dL) or 1960 ÷ TDD for optimal glucose control. 6, 4
Standard Formulas
- Correction factor = 1500 ÷ TDD (indicates how much 1 unit of insulin lowers blood glucose in mg/dL) 4
- Alternative formula = 1960 ÷ TDD (derived from optimal glucose control data) 6
For example, if TDD = 40 units, correction factor = 1500/40 = 37.5 mg/dL per unit (meaning 1 unit of insulin lowers glucose by approximately 38 mg/dL) 4.
Calculating Total Bolus Dose
Total bolus insulin = Meal bolus + Correction bolus 5
Component Calculations
- Meal bolus = Total carbohydrate grams ÷ ICR 5
- Correction bolus = (Current blood glucose - Target blood glucose) ÷ Correction factor 5
Example: For a patient eating 60g carbohydrates with blood glucose of 250 mg/dL, target of 100 mg/dL, ICR of 10, and correction factor of 50:
- Meal bolus = 60 ÷ 10 = 6 units
- Correction bolus = (250 - 100) ÷ 50 = 3 units
- Total bolus = 9 units 5
Relationship Between Dosing Parameters
The relationship between all three dosing factors can be represented as: 100 ÷ TBD = ICR = CF ÷ 4.5 4
This unified formula allows cross-checking of calculations for internal consistency 4.
Critical Pitfalls to Avoid
- Excessive insulin dosing: Many patients with poor control are actually overinsulinized; those receiving >0.6 units/kg/day often require dose reduction, not increase 7
- Using 500 rule for ICR: The traditional "500 rule" underestimates bolus insulin needs; use 300-400 rule instead 3
- Ignoring diurnal variation: Morning insulin resistance requires different ICR than afternoon/evening 3, 5
- Not accounting for insulin on board: Previous boluses still active can cause insulin stacking and hypoglycemia 5
- Fixed ICR throughout day: This ignores physiologic variations in insulin sensitivity 5
- Overestimating basal needs: Recent evidence suggests basal insulin should be approximately 30-40% of TDD, not 50% 3, 4