Insulin-to-Carb Ratio Calculator (500 Rule)
Medically reviewed by Dr. Chhaya Makhija, MD, DipABLM — October 2026
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
Enter meal carbs above to see your exact dose; otherwise a 60 g meal is shown as an example.
How to Use the Insulin-to-Carb Ratio Calculator (500 Rule)
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
Quick answer: the 500 Rule — sometimes written as the rule of 500 — estimates your insulin-to-carb ratio from your total daily dose: 500 ÷ TDD = grams of carbohydrate covered by one unit of rapid-acting insulin. It is a starting estimate, refined against real meal logs.
ICR Formula
The number 500 represents the approximate grams of carbohydrate that TDD-worth of insulin can process per day (an empirical convention from pump therapy, not a metabolic calculation). If your TDD = 40 units: ICR = 500 ÷ 40 = 12.5 g/unit.
Reading it the other way, this insulin-to-carb ratio chart maps common total daily doses to their 500-rule starting ratio (grams of carbohydrate per unit):
| TDD (units/day) | 20 | 25 | 30 | 40 | 50 | 60 | 75 | 100 |
|---|---|---|---|---|---|---|---|---|
| ICR (g per unit) | 25.0 | 20.0 | 16.7 | 12.5 | 10.0 | 8.3 | 6.7 | 5.0 |
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.
Carb Ratio by TDD — the 500 Rule at a Glance
Each card is a total daily dose and the carb ratio it produces (500 ÷ TDD). A higher TDD gives a "tighter" ratio — fewer grams covered per unit, because more insulin overall means more insulin resistance.
Meal Bolus Grid: Carbs ÷ Ratio = Units
Read down to your meal's carbs, across to your ratio — the cell is your carb bolus in units (nearest ½). Correction factor and basal tools built on the same TDD are in our correction and basal insulin calculators.
| Carbs ↓ / Ratio → | 1:6 | 1:10 | 1:12 | 1:15 | 1:20 |
|---|---|---|---|---|---|
| 20 g | 3.5 | 2 | 1.5 | 1.5 | 1 |
| 40 g | 6.5 | 4 | 3.5 | 2.5 | 2 |
| 60 g | 10 | 6 | 5 | 4 | 3 |
| 80 g | 13.5 | 8 | 6.5 | 5.5 | 4 |
| 100 g | 16.5 | 10 | 8.5 | 6.5 | 5 |
Carb bolus only — add correction via the Correction Dose Calculator, and subtract insulin on board. Regular insulin uses the 450 Rule instead.
Why two carb ratio charts disagree: 500 Rule vs 450 Rule
Printed carb ratio charts often give different answers for the same daily dose. The usual reason is that two formulas are in circulation, and they are meant for different insulins: the 500 Rule for rapid-acting insulin and the 450 Rule for Regular (short-acting) insulin. The 500 Rule divides 500 by the total daily dose; the 450 Rule divides 450. The 450 Rule always produces a stronger ratio, meaning fewer grams of carbohydrate covered by each unit.
| TDD (units/day) | 500 Rule | 450 Rule | Difference |
|---|---|---|---|
| 20 | 1 unit : 25 g | 1 unit : 22.5 g | 2.5 g |
| 30 | 1 unit : 16.7 g | 1 unit : 15 g | 1.7 g |
| 40 | 1 unit : 12.5 g | 1 unit : 11.3 g | 1.3 g |
| 50 | 1 unit : 10 g | 1 unit : 9 g | 1 g |
| 60 | 1 unit : 8.3 g | 1 unit : 7.5 g | 0.8 g |
| 80 | 1 unit : 6.3 g | 1 unit : 5.6 g | 0.6 g |
| 100 | 1 unit : 5 g | 1 unit : 4.5 g | 0.5 g |
The gap is largest at low daily doses: 2.5 g per unit at 20 units a day, against half a gram at 100 units a day. Using the rule written for the other insulin therefore matters most for people on smaller total doses.
Neither rule measures anything about you. Both produce a starting ratio that gets adjusted against what glucose actually does after meals. The choice between them follows the insulin in use: a chart built on the 500 Rule does not apply to someone taking Regular insulin.
Insulin-to-Carb Ratio Calculator (500 Rule): Limitations & Safety Notes
- The ratio covers carbohydrate only. High-fat, high-protein meals produce a delayed glucose rise that a carbohydrate ratio cannot anticipate.
- It is derived from total daily dose and inherits any error in that figure.
- Many people need a stronger ratio at breakfast than later in the day because of the morning rise in cortisol and growth hormone.
- A wrong ratio and a mis-counted meal look identical in the result. Confirm carb counting before changing the ratio.
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 Care in Diabetes — 2026. Section 9: Pharmacologic Approaches to Glycemic Treatment. Link
Written by Ryan Mitchell · Last updated: October 2026
Frequently Asked Questions: Insulin-to-Carb Ratio Calculator (500 Rule)
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; 1.7 mmol/L), 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.