Methodology & Formulas

Every formula used on this site — with the source, the equation, a worked example, and known limitations.

All formulas produce educational estimates only. They are not prescriptions. Individual results depend on factors no formula can fully model. Consult your diabetes care team before acting on any estimate.

Total Daily Dose (TDD)

Formula

TDD = Weight (kg) × dose factor (units/kg/day)

Dose factor: 0.4–0.6 u/kg/day for most adults. This calculator uses 0.5 u/kg (midpoint) for Type 1 and 0.2 u/kg as a conservative start for insulin-naïve Type 2.

Worked example: 70 kg adult with Type 1 → TDD = 70 × 0.5 = 35 units/day. Typical split: ~50% basal (17–18 u), ~50% bolus pool (17–18 u).

Source: ADA Standards of Medical Care in Diabetes 2024, Section 9 (Pharmacologic Approaches); AACE Comprehensive Type 2 Diabetes Management Algorithm 2020.

Limitations: Does not account for current insulin regimen, existing TDD, activity level, dietary habits, concurrent medications, renal function, or degree of insulin resistance.

Insulin-to-Carb Ratio — The 500 Rule

Formula

ICR = 500 ÷ TDD

Result = grams of carbohydrate covered by 1 unit of rapid-acting insulin.

Worked example: TDD = 35 → ICR = 500 ÷ 35 ≈ 14.3g carbs per unit. If eating 45g carbs at a meal: bolus = 45 ÷ 14.3 ≈ 3 units.

Source: Walsh J, Roberts R, Bailey T. "Guidelines for Optimal Bolus Calculator Settings in Adults." J Diabetes Sci Technol. 2011;5(1):129–135. Also cited in ADA/AACE patient education materials. The 450 Rule is an alternative used by some clinicians.

Limitations: Assumes consistent carbohydrate absorption. Dietary fat and protein content, glycemic index, and meal timing all affect postprandial glucose beyond what a simple carb count captures.

Insulin Sensitivity Factor (ISF) — 1800 / 1500 Rule

Formula

ISF = 1800 ÷ TDD   (rapid-acting analogs)
ISF = 1500 ÷ TDD   (Regular / human insulin)

Result = mg/dL blood glucose drop per 1 unit of insulin.

Worked example (rapid): TDD = 35 → ISF = 1800 ÷ 35 ≈ 51 mg/dL per unit. To correct from 220 to 100 mg/dL: (220 − 100) ÷ 51 ≈ 2.4 units.

Source: Davidson PC, Hebblewhite HR, Steed RD, Bode BW. "Analysis of guidelines for basal-bolus insulin dosing: basal insulin, correction factor, and carbohydrate-to-insulin ratio." Endocr Pract. 2008;14(9):1095–1101. The 1700 Rule is a variant sometimes used for mixed insulin.

Limitations: ISF can vary by time of day (dawn phenomenon), exercise, and stress. The formula gives a single average estimate; real-world ISF often needs individual tuning.

Correction Dose

Formula

Correction Dose = (Current BG − Target BG) ÷ ISF

Result rounded to nearest 0.5 unit, per common clinical practice.

Worked example: Current BG = 220 mg/dL, Target = 100 mg/dL, ISF = 51 → (220 − 100) ÷ 51 = 2.35 → rounded to 2.5 units.

Source: ADA Standards of Care; Hirsch IB. "Insulin Analogues." N Engl J Med. 2005;352:174–183.

Limitations: Does not account for insulin on board (IOB) from recent doses, meal contributions, or exercise. Clinical bolus calculators in pumps/CGM systems apply IOB subtraction that this simple formula omits.

HOMA-IR (Insulin Resistance)

Formula

HOMA-IR = (Fasting Glucose × Fasting Insulin) ÷ 405   [mg/dL]
HOMA-IR = (Fasting Glucose × Fasting Insulin) ÷ 22.5   [mmol/L]

Glucose in mg/dL or mmol/L; insulin in μU/mL (mIU/L).

Interpretation (approximate): <1.0 = optimal sensitivity; 1.0–1.9 = early resistance; ≥2.0 = significant insulin resistance; ≥2.9 = strong resistance. Thresholds vary by population.

Source: Matthews DR, Hosker JP, Rudenski AS, et al. "Homeostasis model assessment: insulin resistance and β-cell function from fasting plasma glucose and insulin concentrations in man." Diabetologia. 1985;28(7):412–419.

Limitations: Validated for fasting, steady-state conditions only. Not applicable to insulin-treated patients, those with liver disease, or in non-fasting states. Does not directly measure insulin sensitivity — it models it.

QUICKI (Insulin Sensitivity Index)

Formula

QUICKI = 1 ÷ [log(Fasting Insulin μU/mL) + log(Fasting Glucose mg/dL)]

Uses log base 10. Higher values = greater insulin sensitivity.

Interpretation: Normal ≈ 0.45; mild resistance ≈ 0.30–0.45; significant resistance <0.30. Values are not directly comparable to HOMA-IR.

Source: Katz A, Nambi SS, Mather K, et al. "Quantitative Insulin Sensitivity Check Index: A Simple, Accurate Method for Assessing Insulin Sensitivity in Humans." J Clin Endocrinol Metab. 2000;85(7):2402–2410.

Insulin Unit Conversions

U-100 Insulin (standard concentration)

mL = Units ÷ 100   |   Units = mL × 100

U-100 means 100 units per 1 mL. This is the standard concentration for most insulin products in the US. U-200, U-300, and U-500 concentrations exist — always state the concentration explicitly.

Source: FDA Guidance on Insulin Product Labeling; ADA insulin storage and handling recommendations.

Important: Mixing up concentrations is a serious medication error risk. U-500 is 5× more concentrated than U-100. Always confirm concentration with your pharmacist.

Day Supply

Formula

Days Supply = Total Units in Package ÷ Daily Units Used

E.g., one 10 mL vial of U-100 = 1,000 units. At 35 units/day: 1,000 ÷ 35 = 28.6 → 28 days supply.

Source: Standard pharmacy dispensing calculation. See also: American Society of Health-System Pharmacists (ASHP) insulin dispensing guidelines.

Master Reference List

  1. American Diabetes Association. Standards of Medical Care in Diabetes — 2024. Diabetes Care. 2024;47(Suppl 1). Link
  2. AACE/ACE. Comprehensive Type 2 Diabetes Management Algorithm 2020. Link
  3. Walsh J, Roberts R, Bailey T. "Guidelines for Optimal Bolus Calculator Settings in Adults." J Diabetes Sci Technol. 2011;5(1):129–135.
  4. Davidson PC et al. "Analysis of guidelines for basal-bolus insulin dosing." Endocr Pract. 2008;14(9):1095–1101.
  5. Matthews DR et al. "Homeostasis model assessment." Diabetologia. 1985;28(7):412–419.
  6. Katz A et al. "Quantitative Insulin Sensitivity Check Index (QUICKI)." J Clin Endocrinol Metab. 2000;85(7):2402–2410.
  7. NIDDK. Insulin, Medicines, & Other Diabetes Treatments. Link

Last reviewed: June 2025 · All formulas verified against primary sources above