| Formula | 500 Γ· Total Daily Dose |
| Alternative formula | 450 Rule for short-acting insulin |
| Target post-meal BG change | < 2.8 mmol/L (50 mg/dL) |
| Check interval | 2 hours post-meal |
| Validation data points | Minimum 3 per meal slot before adjusting |
Key Takeaways
- The insulin-to-carb ratio (ICR) defines how many grams of carbohydrate one unit of rapid-acting insulin covers.
- The '500 Rule' is the standard starting formula for calculating your ICR.
- ICR varies by time of day, activity level, and illness β most people need multiple ratios.
- Always verify and adjust your ICR with guidance from your diabetes care team.
What Is an Insulin-to-Carb Ratio?
An insulin-to-carb ratio (ICR) is a personalised dosing guide that tells you how many grams of carbohydrate one unit of rapid-acting or short-acting insulin will cover. For example, an ICR of 1:15 means one unit of insulin manages 15 grams of carbohydrate. It is a cornerstone of flexible insulin therapy for people with Type 1 diabetes and insulin-dependent Type 2 diabetes.
The 500 Rule: Calculating Your Starting ICR
The most widely used starting method is the 500 Rule:
- Divide 500 by your total daily dose (TDD) of insulin.
- Example: TDD = 40 units β 500 Γ· 40 = 1 unit per 12.5 g carbs
Some clinicians use the 450 Rule for people using short-acting (human) insulin rather than rapid-acting analogues, as it acts more slowly.
Step-by-Step Calculation
- Step 1: Add up all insulin taken in a typical day (basal + all bolus doses).
- Step 2: Divide 500 by that number.
- Step 3: The result is the grams of carbs covered by 1 unit.
- Step 4: Test over 2β3 days: if blood glucose rises >2.8 mmol/L (50 mg/dL) after meals, your ratio may need to be stronger (lower number).
ICR Comparison Table
| TDD (units) | ICR (500 Rule) | Typical Profile |
|---|---|---|
| 30 | 1:17 | Child or very insulin-sensitive adult |
| 40 | 1:12 | Average adult with Type 1 |
| 60 | 1:8 | Insulin-resistant adult |
| 80 | 1:6 | High insulin requirement |
Why You May Need Different Ratios at Different Times
Dawn phenomenon and diurnal cortisol changes mean most people are more insulin-resistant in the morning. A common pattern is a stronger (lower number) breakfast ratio and a more generous (higher number) evening ratio. Accurately tracking meals alongside a reliable continuous glucose monitor or blood glucose meter from MDS Diabetes makes identifying these patterns straightforward.
Factors That Affect Your ICR
- Exercise: Aerobic activity increases insulin sensitivity β you may need less insulin per gram of carbs.
- Illness/stress: Counterregulatory hormones increase insulin resistance.
- Menstrual cycle: Resistance often rises in the luteal phase.
- Weight changes: Gaining or losing weight shifts your TDD and therefore your ICR.
- Carbohydrate type: High-fibre or low-GI foods raise glucose more slowly β timing matters as much as dose.
Testing and Fine-Tuning Your ICR
To validate your ICR: eat a measured meal of known carbohydrate content, dose accordingly, and check blood glucose 2 hours post-meal. A well-matched ratio should return you to within 2β3 mmol/L of your pre-meal reading. MDS Diabetes stocks a range of accurate glucose meters and test strips to support this process. Log at least three data points per meal time before adjusting.
ICR vs Correction Factor
The ICR deals with food; the correction factor (sensitivity factor) deals with high blood glucose. Both are needed for full mealtime dosing. The correction factor is calculated using the 100 Rule (mmol/L) or 1700 Rule (mg/dL) divided by TDD.