eAG (Average Glucose) to A1c Calculator

Convert estimated average glucose back to HbA1c: A1c (%) = (eAG + 46.7) / 28.7. ADAG / Nathan 2008 inverse.

Inputs

Result

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How to use this calculator

  • Enter your average glucose (CGM 14-day, fingerstick log, or known mean).
  • Pick units.
  • Read the equivalent A1c — for trending, not lab replacement.

About this calculator

Reverse of the A1c → eAG mapping. Useful when interpreting a CGM 2-week or 90-day average and you want a comparable A1c estimate. Same source: Nathan DM et al., Diabetes Care 2008;31(8):1473. Formula: A1c = (eAG + 46.7) / 28.7. The CGM-derived "GMI" (Glucose Management Indicator) uses a slightly different formula (GMI = 3.31 + 0.02392 × mean glucose mg/dL; Bergenstal 2018) but agrees within ~0.3% A1c. **Not medical advice.**

Frequently asked

GMI vs. eAG-derived A1c?+
GMI (Bergenstal 2018, Diabetes Care 41:2275) uses CGM mean glucose: GMI = 3.31 + 0.02392 × mean (mg/dL). eAG-A1c (Nathan) is the older direct linear inverse. They agree within ~0.2-0.4%.
Why does my CGM say a different A1c?+
Modern CGMs use GMI. Same input glucose → 0.1-0.4% different A1c. Both are valid estimates; lab A1c is the gold standard.
Mean glucose units?+
mg/dL is standard in US, India, France. mmol/L (Canada, UK, EU). 1 mmol/L ≈ 18.018 mg/dL.
Why 28.7 / 46.7?+
These are the regression coefficients from Nathan 2008 ADAG study. They are not from physiology — they're empirical fit.
Children / pregnancy?+
A1c targets and conversions differ in pediatrics and pregnancy (where fructosamine or 2-hr OGTT is preferred). Use clinician guidance.
Is this medical advice?+
No. Educational tool. A1c interpretation requires clinical context.

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