A1C Calculator.

A1C (glycated hemoglobin) is the percentage of your hemoglobin that has glucose stuck to it. Because red blood cells live about three months, A1C reflects your average blood sugar over roughly the previous 8–12 weeks — a single number that smooths out the daily spikes and dips. It is the marker doctors use to diagnose and monitor diabetes.

Estimated average glucose (eAG) is that same A1C expressed in the mg/dL or mmol/L units you see on a meter or CGM. The ADA introduced eAG precisely so patients could connect their lab A1C to the day-to-day numbers they already recognize. An A1C of 7% is not "7 anything" you measure at home — it corresponds to an average glucose of about 154 mg/dL (8.6 mmol/L).

The conversion comes from the 2008 A1C-Derived Average Glucose (ADAG) study, which continuously monitored glucose in hundreds of participants and fit a linear regression: eAG (mg/dL) = 28.7 × A1C − 46.7. This calculator applies that equation forward (A1C → glucose) and inverted (glucose → A1C). To use it in the glucose→A1C direction, enter your CGM or meter average, not a single spot reading.

For fructosamine, we use the common approximation A1C ≈ 0.017 × fructosamine (µmol/L) + 1.61. Fructosamine reflects glycation of serum proteins over the past 2–3 weeks, so it is useful when A1C is unreliable or when you need a faster read on a recent change.

A1C assumes an average red-cell lifespan. Anything that shortens or lengthens that lifespan skews the result: iron-deficiency anemia and recent blood loss can raise A1C relative to true glucose; hemolytic anemia, pregnancy, chronic kidney disease, and recent transfusion can lower it. Hemoglobin variants (common in people of African, Mediterranean, or Southeast Asian ancestry) can throw off some A1C assays entirely.

If your CGM average and your lab A1C tell noticeably different stories, that gap is itself information — bring it to your clinician rather than trusting either number alone. This tool estimates the statistical relationship; it does not diagnose diabetes.