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Insulin / Carb Ratio

Insulin-to-Carb Ratio (ICR): how many grams of carbs 1 unit covers. Insulin Sensitivity Factor (ISF / correction factor): how many mg/dL 1 unit drops your sugar. Both derived from your Total Daily Dose.

40 U/day

Sum of all insulin (basal + bolus) per day.

Insulin-to-Carb Ratio
1 unit : 12.5 g carbs
Insulin Sensitivity Factor
1 unit drops 45 mg/dL

Always confirm with your endocrinologist before adjusting doses. Individual ratios vary by time-of-day, exercise, illness.

How the 500 and 1800 rules work

For rapid-acting analogues (lispro, aspart, glulisine): ICR = 500 ÷ TDD grams of carbs covered by 1 unit; ISF = 1800 ÷ TDD mg/dL drop from 1 unit. For regular insulin use 450 and 1500 respectively. TDD = total daily dose (basal + bolus, units/day). These are starting estimates — refine with CGM data over 2–4 weeks.

Worked example

A type-1 patient on TDD 40 units of insulin lispro. ICR = 500/40 = 1 unit per 12.5 g carbs. ISF = 1800/40 = 1 unit drops glucose by 45 mg/dL. Eating 2 chapatis + dal + sabzi (~50 g carbs) needs 4 units. If pre-meal glucose is 250 mg/dL and target 110, correction = (250−110)/45 ≈ 3 additional units. Total bolus = 7 units. Indian carb-heavy meals (rice, idli, dosa) push ICR work harder than Western low-carb meals.

When to use this

  • Newly diagnosed type-1 starting MDI (multiple daily injections)
  • Switching from sliding-scale to carb-counting regimen
  • Insulin pump initiation — pump needs ICR/ISF programmed
  • Gestational diabetes on intensive insulin

Always confirm starting doses with your endocrinologist. Pair with our glycemic load calculator to choose lower-GL Indian meals and reduce bolus variability.

FAQ

Why does ICR change throughout the day?

Insulin sensitivity drops in the morning (dawn phenomenon — cortisol surge). Most type-1 diabetics need 1:8 in morning, 1:12 lunch, 1:15 evening. Adjust by meal.

Should I use 500 rule or 450 / 550?

500 is the most common starting point. Actual ratio comes from CGM data + carb logs over 1-2 weeks. Endocrinologists tweak from there.

Type 2 — does this apply to me?

Only if you're on intensive insulin therapy (basal + bolus). Most type-2 diabetics on metformin / SGLT2i don't need ICR — diet management is primary.