What A1C Actually Measures
Glucose sticks to hemoglobin slowly and irreversibly, so the percentage of 'glycated' hemoglobin reflects the average glucose concentration over the lifespan of a red blood cell, about 90–120 days.
That's A1C's superpower: unlike a fasting glucose, it can't be gamed by a good day or a skipped meal, and it catches elevations between tests.
The Conversion Math
eAG (mg/dL) = 28.7 × A1C − 46.7, from the landmark ADAG study, or divide mg/dL by 18 for mmol/L. An A1C of 7.0% equals roughly 154 mg/dL (8.6 mmol/L) of average glucose.
Averages hide swings: two people with identical A1C can have very different daily patterns. That's why continuous glucose monitoring complements A1C rather than replacing it.
When A1C Misleads
Anything that changes red-cell lifespan distorts A1C, anemia, blood loss, pregnancy, and some hemoglobin variants among them. In those cases, clinicians lean on fructosamine or direct glucose monitoring.
Diagnosis requires confirmation: 6.5%+ on two separate tests, or A1C plus a fasting or oral glucose tolerance test.
Sources
FAQ
What is a normal A1C?
Below 5.7%. 5.7–6.4% signals prediabetes; 6.5% or above on two tests is diagnostic for diabetes.
What glucose level equals A1C 7?
About 154 mg/dL (8.6 mmol/L) as a three-month average.
How often should A1C be tested?
Twice yearly when stable and in range; quarterly when treatment changes or targets are missed.
Please read this first: these results are educational estimates, not medical advice. For decisions about your health, talk to a qualified professional.