A1C Calculator
Convert HbA1c to estimated average blood glucose (eAG) or the other way, using the ADAG standard formula. Interpretation bands cover normal, prediabetes, and diabetes thresholds.
Last updated: September 2026
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About this calculator
HbA1c (glycated hemoglobin) measures the average blood glucose exposure of hemoglobin over the red blood cell lifespan (~120 days). The ADAG (A1C-Derived Average Glucose) study (Nathan et al., 2008) established the near-linear conversion: eAG (mg/dL) = 28.7 × A1C(%) − 46.7. This calculator uses that exact formula in both directions.
The ADA diagnostic thresholds are: <5.7% normal, 5.7–6.4% prediabetes, ≥6.5% diabetes on two confirming tests. HbA1c is not a real-time glucose reading — it is a rolling ~3-month average, weighted more toward the most recent month. Conditions that shorten red-cell lifespan (hemolytic anemia, recent transfusion, hemoglobin variants like SS, SC, or thalassemia) can produce falsely low A1C for the actual glucose average, and iron-deficiency anemia can raise it. In those cases, fructosamine or continuous glucose monitoring (CGM) is used instead.
A useful anchor point: A1C 6% ≈ 126 mg/dL average, A1C 7% ≈ 154, A1C 8% ≈ 183 mg/dL. Each 1% rise in A1C corresponds to about 29 mg/dL higher average glucose.
How to use
Example — A1C 5.7% to eAG. eAG = 28.7 × 5.7 − 46.7 = 163.6 − 46.7 = 116.9 mg/dL. This is the borderline normal-to-prediabetes reading; consider lifestyle intervention. Example — average glucose 154 mg/dL to A1C. A1C = (154 + 46.7) / 28.7 = 200.7 / 28.7 = 7.0%. That is a common target for adults with type 2 diabetes on treatment per ADA Standards of Care.
Frequently asked questions
How accurate is the A1C to eAG conversion?
The ADAG formula fits 507 individuals with a 95% CI of ±15 mg/dL. Individual glycation rates vary, so the number is a good average but any single reading has natural spread. CGM data over 14 days is the more precise picture of glucose exposure.
Why does A1C not reflect my recent glucose?
Red blood cells live ~120 days and A1C is their average glycation. About 50% of the A1C signal comes from the last 30 days, 25% from days 30–60, and 25% from days 60–120. A one-week binge or a one-week fast will not move A1C much; sustained changes over 6–8 weeks will.
What A1C target should I aim for?
General ADA target for adults with diabetes is <7%. Older adults, frail patients, or those with severe hypoglycemia history often target 7.5–8%. Younger, otherwise healthy patients may target <6.5%. Individualize with the care team; overly-tight targets increase hypoglycemia risk.
Can non-diabetes conditions affect A1C?
Yes. Hemoglobin variants (SS, SC, thalassemia), iron deficiency, kidney failure, recent transfusion, and pregnancy can all skew A1C. When suspected, fructosamine (2–3 week glucose average) or 14-day CGM is used instead.