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.

mg/dL is standard in US, India, France. mmol/L (Canada, UK, EU). 1 mmol/L ≈ 18.018 mg/dL.

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.

eAG (Average Glucose) to A1c Calculator

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

Result

Estimated A1c

6.99%

eAG 154 mg/dL = 154.0 mg/dL → A1c 6.99%. Diabetes range (≥6.5%).

Input eAG154 mg/dL
eAG in mg/dL154.0
Estimated A1c (%)6.99
ADA categoryDiabetes range (≥6.5%)

Step-by-step

eAG already in mg/dL = 154.
A1c = (eAG + 46.7) / 28.7 = (154.0 + 46.7) / 28.7 = 6.99%.

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 (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%.