Carb-to-Insulin Ratio Calculator
Enter your total daily insulin (TDI). We'll calculate a starting insulin-to-carb ratio (ICR) and insulin sensitivity factor (ISF) using the standard 500 / 1800 rules. For T1D and T2D on multiple daily injections.
⚠ Educational only. Insulin-dose decisions belong with your endocrinologist or diabetes educator.
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Insulin-to-Carb Ratio (start)
Starting estimate
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ISF mg/dL per 1u
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ISF mmol/L per 1u
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Estimated basal (units)
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Estimated bolus (units)
How the rules work
- 500 rule (ICR): 1 unit covers (500 ÷ TDI) grams of carbs. T2D often uses 450 instead — slightly more conservative.
- 1800 rule (ISF, mg/dL): 1 unit lowers blood glucose by (1800 ÷ TDI) mg/dL. Some clinicians use 1500 for T1D on rapid analogs.
- 100 rule (ISF, mmol/L): 1 unit lowers blood glucose by (100 ÷ TDI) mmol/L.
- Basal/bolus split: typically ~50/50, but ranges 40–60% basal. Adjust based on overnight CGM data.
Common questions
What is an insulin-to-carb ratio?
The ICR tells you how many grams of carbs 1 unit of rapid-acting insulin covers. A ratio of 1:10 means 1 unit per 10g carbs. Your clinician sets it based on TDI and refines it with CGM data.
What is the 500 rule?
ICR = 500 / total daily insulin. For 50 TDI: ICR = 1:10. It's a starting point — your real ratio often differs by meal time.
What is the 1800 rule?
ISF (mg/dL) = 1800 / TDI. ISF (mmol/L) = 100 / TDI. For 50 TDI: 1u drops glucose by 36 mg/dL (2.0 mmol/L).
How should I use the result?
Educational only. Always discuss dose changes with your endocrinologist or diabetes educator. Real ratios are refined from CGM + meal logging over a few weeks.