Carb to Insulin Ratio Calculator

Formula used

How to use this calculator

1. Enter total meal carbohydrates in grams.
2. Select your ratio method: use your prescribed ratio, or estimate from TDD.
3. Choose a rounding step that matches your pen or pump.
4. Optional: enable correction and enter current and target glucose.
5. Provide your ISF if you know it, or enter TDD to estimate ISF.
6. Press Calculate to see results above this form.
7. Export your results with the CSV or PDF buttons.

Professional insight

Why a carb ratio matters

A carb ratio links food to rapid insulin. If your ratio is 12 g per unit, a 60 g meal needs 5.0 units before any correction. Ratios can vary by time of day, activity, hormones, and illness, so the same person may use 10 g/unit at breakfast and 15 g/unit at dinner.

Estimating a starting ratio from daily dose

When a prescribed ratio is not available, educators often use the 500 rule: grams per unit ≈ 500 ÷ TDD. With a total daily dose of 40 units, the estimate is 12.5 g/unit. With 60 units, it is 8.3 g/unit. Treat this as a baseline to discuss with a clinician, not a self adjustment plan.

Meal bolus calculation and rounding

The meal bolus equals carbs ÷ ratio. For 75 g and a 10 g/unit ratio, the meal bolus is 7.5 units. Many pens dose in 0.5 or 1.0 unit steps, while pumps may allow 0.05 to 0.1. Rounding to 0.5 units would keep 7.5 unchanged, but rounding 1.67 units becomes 1.5 or 2.0 depending on the chosen step.

Adding a glucose correction dose

If current glucose is above target, a correction may be added using an insulin sensitivity factor, ISF. The correction equals (current − target) ÷ ISF. Example: 180 mg/dL current, 110 target, ISF 45 gives (70 ÷ 45) = 1.56 units. If current is 95 and target is 110, the correction is negative, about −0.30 units with ISF 50.

Estimating sensitivity with the 1800 rule

For rapid insulin, an educational ISF estimate is 1800 ÷ TDD. With TDD 40, ISF ≈ 45 mg/dL per unit; with TDD 60, ISF ≈ 30. Lower ISF values mean each unit has a stronger glucose lowering effect, increasing the need for careful verification and documented trends.

Interpreting totals and tracking outcomes

Total bolus equals meal plus correction. In the lunch example, 5.00 + 1.56 = 6.56 units. Record carbs, dose, timing, and 2 to 3 hour post meal daily readings to see patterns. Frequent lows, missed highs, or large corrections suggest the need for professional review, safer targets, and revised ratios based on real data.

FAQs

1) Is this calculator safe to use for dosing decisions?

No. It is educational and cannot account for insulin type, active insulin, illness, exercise, or clinician targets. Use only clinician-approved settings and confirm with your care team.

2) What does “grams per unit” mean?

It is how many grams of carbohydrate are covered by one unit of rapid insulin. A lower number means more insulin per gram, and a higher number means less.

3) When should I use the 500 and 1800 rules?

They are common education estimates based on total daily dose. They may help start a discussion with a clinician but should not replace prescribed ratios or sensitivity factors.

4) Why can the correction dose be negative?

If current glucose is below target, the formula yields a negative value. This typically suggests reducing insulin or adding carbs per your plan, not blindly “subtracting” insulin.

5) What rounding step should I choose?

Match your device. Many pens deliver 0.5 or 1.0 unit steps, while pumps may allow finer increments. Consistent rounding improves logs and reduces calculation mistakes.

6) What data should I track to improve accuracy?

Log carbs, dose, timing, pre-meal glucose, and 2–3 hour post-meal readings. Note activity and illness. Patterns over several days are more reliable than single values.