Carb-to-Insulin Ratio Calculator
Estimate your insulin-to-carb ratio — grams of carbohydrate covered by one unit of insulin — from your total daily dose using the 500/450 rule.
How it works
Insulin-to-carb ratio (g per unit) = rule constant ÷ total daily dose · Meal bolus = carbs ÷ ratio
The insulin-to-carb ratio tells you how many grams of carbohydrate one unit of mealtime insulin covers, so you can dose for what you eat. The quick estimate divides a constant by your total daily insulin dose — 500 for rapid-acting insulin, 450 for regular insulin. Someone who uses more insulin overall covers more carbs per unit (a larger ratio). Enter a meal’s carbs to see the estimated bolus. As with all insulin figures, this is only a textbook starting point that a clinician must confirm and tune.
Worked example
For a total daily dose of 40 units with rapid-acting insulin, the rule of 500 gives 500 ÷ 40 = 12.5, so one unit covers about 12 g of carbohydrate. A meal with 60 g of carbs would need roughly 60 ÷ 12.5 = 4.8 units.
Frequently asked questions
What does a 1:12 ratio mean?
It means one unit of insulin is expected to cover about 12 grams of carbohydrate. To cover a meal, divide the meal’s carbohydrate grams by the ratio — 48 g of carbs at 1:12 needs about 4 units.
Why 500 for rapid and 450 for regular?
The constants come from clinical experience with each insulin’s potency. Rapid-acting analogs use 500 (sometimes higher); older regular insulin, which is slower and slightly less efficient for meals, uses 450. Match the constant to your insulin.
Can my ratio be different at different meals?
Yes. Many people are more insulin-resistant in the morning and need a smaller ratio (more insulin per carb) at breakfast than at dinner. The single number from this rule is an average starting point; your team may set different ratios by time of day.
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Educational estimate only — not medical advice, and not a substitute for a clinician, pharmacist or product label. Dosing, fluid and kidney decisions must be confirmed by a qualified professional using the actual drug, patient and situation. Do not use this tool to self-treat.