For health professionals — starting-point ratios derived from Total Daily Dose (TDD)
What TDD is
Total Daily Dose = all basal/long-acting insulin + all bolus/rapid-acting insulin the patient takes over 24 hours (from their pump report or injection log, averaged over a representative week).
Insulin-to-Carb Ratio (ICR) — grams of carb covered by 1 unit
Rule of 500
Rule of 450 (more conservative)
Insulin Sensitivity Factor (ISF) — mmol/L drop per 1 unit
Rule of 1800
Rule of 1500 (more conservative)
⚠️ Starting point only
These formulas give a first estimate to titrate, not a final prescription. Confirm ICR/ISF against the patient's actual post-meal and correction glucose response — ratios typically differ through the day (breakfast is often more insulin-resistant).
The method, explained
ICR — Rule of 500 / 450
ICR (g/unit) = constant ÷ TDD.
The constant of 500 assumes roughly a "500 g carbohydrate equivalent" is metabolised per unit of insulin over 24h;
450 is a more conservative alternative sometimes used as a safer starting point, e.g. for those more insulin-sensitive.
Both are widely used in diabetes education as an initial estimate — neither is inherently "correct" for a given person.
ISF — Rule of 1800 / 1500
ISF (mg/dL/unit) = constant ÷ TDD, historically 1800 for rapid-acting analogue insulin and 1500 for older
regular/short-acting insulin. Converting to mmol/L (divide by 18, the standard mg/dL↔mmol/L factor) gives the figures shown above.
Why TDD drives both
Someone using more insulin per day overall is, by definition, less insulin-sensitive — so each additional unit should move
their glucose or cover their carbs by proportionally less. Both rules scale inversely with TDD for that reason.
Rule of 500/450 and Rule of 1800/1500 are general, widely taught diabetes-education conventions (not specific to any single
jurisdiction's clinical guideline). Confirm ratios against the patient's actual glucose response before finalising.