Carb Factor Calculator
Compare your carb factor — the grams of carbohydrate one unit of insulin covers — across the 450, 500 and 550 rules, all from your total daily dose, so you can see which rule fits your insulin.
Your total insulin (basal + bolus) per day.
How to Use This Calculator
- Enter your total daily dose (TDD) — basal plus bolus units per day.
- Compare carb factors across the 450, 500 and 550 rules side by side.
- Pick a starting point — the 500 Rule for modern rapid-acting insulin — and confirm it with post-meal glucose checks and your care team.
A bigger number after the colon (e.g. 1:12 vs 1:8) means one unit covers more carbohydrate — you're more insulin-sensitive.
What Is the Carb Factor?
Your carb factor — also called the insulin-to-carb ratio (ICR) — is how many grams of carbohydrate one unit of insulin covers. The "magic number" rules estimate it by dividing a constant by your total daily dose. A higher constant gives a larger carb factor, meaning each unit covers more carbs:
Example: TDD 50 → 500 ÷ 50 = 1 unit per 10 g (450 → 9 g, 550 → 11 g).
Carb Factor by TDD
| TDD | 450 Rule | 500 Rule | 550 Rule |
|---|---|---|---|
| 30 | 1:15 g | 1:16.7 g | 1:18.3 g |
| 40 | 1:11.3 g | 1:12.5 g | 1:13.8 g |
| 50 | 1:9 g | 1:10 g | 1:11 g |
| 60 | 1:7.5 g | 1:8.3 g | 1:9.2 g |
A bigger number after the colon means one unit covers more carbohydrate (you're more insulin-sensitive).
Using Your Carb Factor to Dose Meals
Calculating a meal bolus
Once you know your carb factor, the meal dose is simple: insulin units = grams of carbohydrate ÷ carb factor. With a 1:10 ratio, a 60 g meal needs 60 ÷ 10 = 6 units. Add a correction with your sensitivity factor if your glucose is above target, or use the combined bolus calculator.
When one ratio isn't enough
Many people need different carb ratios at different meals. Breakfast often requires more insulin per gram of carb because of morning insulin resistance (the "dawn" effect), so a stronger ratio (a smaller number, e.g. 1:8) may be needed then, with a weaker one later in the day. Pumps and AID systems let you set time-of-day ratios — your care team helps tune them.
Frequently Asked Questions
What is a carb factor?
The carb factor — or insulin-to-carb ratio — is the grams of carbohydrate that one unit of rapid-acting insulin covers. A ratio of 1:10 means one unit handles 10 grams of carbs.
What's the difference between the 450, 500 and 550 rules?
They divide different constants by your TDD. The 500 Rule is standard for modern rapid-acting insulin. The 450 Rule is more conservative (often cited for Regular insulin), and 550 gives a larger carb factor for more insulin-sensitive people.
Which rule should I use?
Most people on rapid-acting insulin start with the 500 Rule. The best ratio is confirmed by checking post-meal glucose and refining with your care team — the rules just give a starting estimate.
How is this different from the 500-rule calculator?
This tool shows all three rules side by side so you can compare carb factors. For a single ratio and meal dosing, see the insulin-to-carb ratio calculator or the 450/500 rule calculator.
How do I use my carb factor to dose a meal?
Divide the carbohydrate in your meal by your carb factor. With a 1:10 ratio, a 45 g meal needs 45 ÷ 10 = 4.5 units. If your glucose is above target, add a correction dose using your sensitivity factor. The total is your mealtime bolus.
Can my carb ratio be different at breakfast?
Yes. Because of morning insulin resistance, many people need more insulin per gram of carbohydrate at breakfast — a stronger ratio such as 1:8 — and a weaker one later in the day. The magic-number rules give one average ratio; pumps and AID systems let you program different ratios by time of day, set with your care team.
Sources
- Walsh J, Roberts R. Pumping Insulin. The 500 Rule for carbohydrate factor.
- American Diabetes Association. Carbohydrate counting and insulin-to-carb ratios.
Last reviewed: June 2025