Insulin-to-Carb Ratio Calculator (500 Rule)
Estimate your insulin-to-carb ratio (ICR) using the 500 Rule. Enter your total daily dose and see how many grams of carbohydrate 1 unit of insulin covers. Educational use only.
🍽️ Carb Ratio Calculator
Sum of all insulin (basal + all boluses) in a typical day. Calculate TDD →
ICR Estimate
(1 unit : X g)
(for entered carbs)
units/day
How to Use This Calculator
Know Your TDD
Add up all insulin you take in a typical day — basal units plus all meal and correction boluses. Use the TDD calculator if you need help.
Choose the Rule
The 500 Rule is standard for rapid-acting analogs (lispro, aspart, glulisine). Use 450 for Regular insulin or if you're highly insulin-sensitive.
Read Your ICR
The result shows grams of carbohydrate per unit. Example: ICR of 15 means 1 unit covers 15g carbs.
Optional Meal Bolus
Enter your meal carbs to see the estimated bolus dose. This does NOT include a correction component or account for insulin on board.
Verify the Ratio
Test by eating a known-carb meal, dosing the ICR estimate, and checking BG 2 hours later. Adjust up or down with your care team based on results.
Different Meals, Different Ratios
Many people use a lower ICR at breakfast (dawn phenomenon). Keep notes across meals to identify patterns — share with your diabetes team.
The 500 Rule Explained
ICR Formula
The number 500 represents the approximate grams of carbohydrate that TDD-worth of insulin can process per day (based on 1800 kCal/day carbohydrate with physiological glucose metabolism assumptions). If your TDD = 40 units: ICR = 500 ÷ 40 = 12.5 g/unit.
Meal Bolus Calculation
Example: 60g carb meal with ICR = 12.5 → Meal Bolus = 60 ÷ 12.5 = 4.8 units (round to nearest 0.5 unit). Note: most bolus calculators add the correction component to this. These are separate calculations — ICR covers the meal; ISF and correction dose handle high blood glucose.
Insulin on board (IOB) is not calculated here. Always check how much insulin is still active from previous doses before giving a meal bolus + correction. Stacking insulin without accounting for IOB is a leading cause of hypoglycemia.
Sources & References
- Walsh J, Roberts R, Bailey T. "Guidelines for Optimal Bolus Calculator Settings." J Diabetes Sci Technol. 2011;5(1):129–135.
- American Diabetes Association. Standards of Medical Care in Diabetes — 2024. Section 7. Link
- Bergenstal RM et al. "Effectiveness of Sensor-Augmented Insulin-Pump Therapy in Type 1 Diabetes." NEJM. 2010;363:311–320.
Last reviewed: June 2025
Frequently Asked Questions
What is the insulin-to-carb ratio?
ICR tells you how many grams of carbohydrate one unit of rapid-acting insulin covers at mealtime. An ICR of 1:15 means 1 unit covers 15g carbs. To calculate a meal bolus: divide the total carb grams by your ICR. Example: 60g ÷ 15 = 4 units bolus (carb component only — correction is separate).
What is the 500 Rule for carb ratio?
The 500 Rule estimates ICR by dividing 500 by your TDD: ICR = 500 ÷ TDD. A TDD of 40 gives ICR = 12.5 g/unit. This formula is derived from physiological insulin-glucose metabolism assumptions and is validated in clinical practice as a starting point for meal dosing. Some protocols use 450 for Regular insulin or 550 for very insulin-sensitive individuals. Your actual ICR requires real-world verification through post-meal monitoring.
How do I verify my carb ratio is correct?
To verify ICR: eat a meal with a known carb count (no high-fat or high-protein additions that slow digestion), dose using only the ICR calculation (pre-meal BG should be in target range), and check BG 2 hours post-meal. If BG is at or near pre-meal level (within ~30 mg/dL), ICR is accurate. If BG is above target, ICR likely needs to be lowered (more insulin per gram). If below target, ICR needs to be raised. Do this test with your care team's guidance.
Why do I need different carb ratios at different meals?
The dawn phenomenon causes higher insulin resistance in the morning — driven by overnight cortisol and growth hormone rises. This means more insulin is needed per gram of carb at breakfast than at dinner for many people. Pump users typically set separate ICRs for breakfast, lunch, and dinner. MDI users can approximate this by using a lower ICR (more insulin per gram) at breakfast. Verify each meal's ICR separately using post-meal glucose data.
Does carb ratio change over time?
Yes. ICR changes with significant weight change, fitness level, illness, stress, hormonal cycles (especially puberty, pregnancy, and menopause), and changes in medications (including GLP-1 receptor agonists). Any significant change in overall TDD should prompt a review of your ICR. Your diabetes care team should reassess ICR at every visit or when glucose patterns change consistently.