Health⏱ 6 min read
How to Calculate Insulin-to-Carb Ratio for Type 1 Diabetes
The insulin-to-carb ratio tells you how many units of insulin to take per gram of carbohydrate. Here's how the 500 rule works, how to check it, and what affects it.
For people with Type 1 diabetes using mealtime (bolus) insulin, the insulin-to-carb ratio (ICR) is a fundamental daily calculation. Getting it right means stable blood glucose; getting it wrong means highs or dangerous lows.
What the Insulin-to-Carb Ratio Means
ICR = 1 unit of insulin : X grams of carbohydrate
An ICR of 1:10 means:
1 unit of insulin is needed for every 10g of carbohydrate.
For a meal with 60g of carbohydrate:
Insulin dose = 60 / 10 = 6 units
ICR varies by individual and often by time of day:
Many people need more insulin per gram at breakfast
(insulin resistance is typically higher in the morning)
The 500 Rule (Starting Point for ICR)
The 500 Rule estimates ICR for those on multiple daily injections (MDI):
ICR = 500 / Total Daily Dose (TDD) of insulin
Example: total daily insulin (basal + bolus) = 40 units/day
ICR = 500 / 40 = 12.5
→ approximately 1:12 or 1:13 (1 unit per 12-13g carbs)
For insulin pumps, use 450 instead of 500:
ICR = 450 / TDD
These are starting estimates. The ICR must be validated
and adjusted based on actual blood glucose outcomes.
Always work with a diabetes care team when adjusting insulin.
Counting Carbohydrates Accurately
Carbohydrates are on all UK food labels as "carbohydrates"
(the "of which sugars" sub-figure is included within total carbs —
do not add them together)
Calculating carbs in a meal:
Pasta 150g cooked (30g carbs per 100g): 150 x 0.30 = 45g carbs
Tomato sauce 100g (8g carbs per 100g): 8g carbs
Garlic bread 50g (40g carbs per 100g): 20g carbs
Total: 73g carbs
At ICR 1:10: 73 / 10 = 7.3 → round to 7 units
(Always round in the direction your readings suggest you need)
Key carb-containing foods:
High carb: bread, rice, pasta, potato, cereals, fruit, milk, beans
Low/zero carb: meat, fish, eggs, cheese, non-starchy vegetables, oils
Checking Your ICR Is Working
Test method: isolated meal check
1. Start with blood glucose in range (4-7 mmol/L)
2. Eat a meal with known carbs (weighed and calculated)
3. Dose using your ICR
4. Check glucose 2 hours after eating
If 2-hour glucose is:
In target (5-9 mmol/L): ICR is correct for this meal size ✓
Above 10 mmol/L: ICR too weak — need more insulin per gram
Below 4 mmol/L: ICR too strong — need less insulin per gram
Repeat with different meal sizes to confirm.
Different foods with same carb content can have different
blood glucose impact (glycemic index effect) — this variation
is a normal and expected challenge in carb counting.
Factors That Change ICR Day to Day
- Exercise: Physical activity increases insulin sensitivity — ICR often needs adjusting on active days
- Illness: Infection causes insulin resistance — may need significantly more insulin per gram
- Hormonal cycles: Insulin requirements vary significantly across the menstrual cycle
- Time of day: Dawn phenomenon means breakfast often requires more insulin per gram than dinner
- Stress: Psychological stress releases cortisol, which reduces insulin sensitivity