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How much insulin given on 500

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insulin 500 rule carbohydrate ratio calculation

The "Rule of 500" for Insulin-to-Carbohydrate Ratio

This isn't about giving 500 units of insulin - it's a formula used to calculate how many grams of carbohydrate 1 unit of rapid-acting (bolus) insulin will cover.
Formula:
Insulin-to-Carb Ratio (ICR) = 500 / Total Daily Dose (TDD) of insulin
Steps:
  1. Calculate TDD - add up all insulin taken in 24 hours (basal/long-acting + all bolus/rapid-acting doses).
  2. Divide 500 by the TDD.
  3. The result is the number of grams of carbohydrate covered by 1 unit of rapid-acting insulin.
Worked example (from Harrison's / diabetes management sources):
  • Basal (e.g., glargine): 26 units at night
  • Bolus (e.g., rapid-acting): 8 units at breakfast, lunch, dinner = 24 units
  • TDD = 26 + 24 = 50 units
  • ICR = 500 / 50 = 10
  • Result: 1 unit of rapid-acting insulin covers 10 g of carbohydrate (written as 1:10)
Using the ratio to dose a meal:
  • Bolus dose = grams of carbohydrate in the meal ÷ ICR
  • Example: a meal with 80 g carb, ICR of 1:10 → 80 / 10 = 8 units of bolus insulin
  • A correction dose (based on the "1800 rule" for insulin sensitivity factor) is added on top if blood glucose is above target.
Notes on accuracy:
  • This is a starting estimate, not a fixed rule. It's commonly used in insulin pump settings and multiple daily injection regimens for carbohydrate coverage - Harriet Lane Handbook, 23rd ed.
  • Individual insulin sensitivity varies by meal (e.g., often lower ICR/more insulin needed at breakfast due to dawn phenomenon), so ratios are frequently adjusted per meal time.
  • Studies in children (PMC5478012) found the 500 rule can be too conservative in young/prepubertal kids, who often need more insulin (lower ICR) than the formula predicts - it should be individualized with a diabetes educator or endocrinologist based on actual glucose response.
If you actually meant a blood glucose of 500 mg/dL (severe hyperglycemia) and how much correction insulin to give, that's a different question involving correction factor (the "1800 rule": 1800/TDD = mg/dL drop per unit) plus urgent evaluation for DKA/HHS - let me know if that's what you're asking and I can go into that instead.
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