A1c

What is a normal A1c?

By GyroscopeUpdated November 14, 20266 sources

Under 5.7% (39 mmol/mol). An A1c of 5.7 to 6.4% (39 to 47) is the prediabetes range, and 6.5% (48) or more on two tests is diabetes. Anemia, recent blood loss and some hemoglobin variants can throw it off, so pair it with a fasting glucose. The lines are the same at every adult age.

Why the number confuses people

It’s a percent, but it isn’t a percent of anything you’ve heard of. Some labs report it in mmol/mol instead, sensor apps show a “GMI” that looks like it, and some reports print an “average glucose” beside it. All four can describe the same blood.

What the percent means

A1c is the share of the hemoglobin in your red cells that has sugar stuck to it. In someone with an A1c of 5.5%, 5.5 out of every 100 hemoglobin molecules carry sugar. Red cells pick it up as they age, so the number shows your average blood sugar over the last few months, with the most recent weeks counting most.

The study that linked A1c to average glucose followed 507 adults, 80 of them without diabetes, for 3 months each. Its formula is average glucose (mg/dL) = 28.7 × A1c − 46.7, so each 0.1 on your A1c is about 3 mg/dL of average blood sugar.

The whole normal range is a narrow window. The difference between an A1c of 5.0 and 5.6 is an average of about 17 mg/dL, about as much as a sensor ran above fingertip blood in one small trial.

When A1c gets it wrong

A1c assumes your red cells live a normal life span, and they don’t always. Anemia, iron levels, blood loss, a transfusion, some hemoglobin variants such as sickle cell trait, G6PD deficiency and HIV all change the link between A1c and blood sugar (American Diabetes Association, 2026). Kidney failure, liver disease, opioids and pregnancy can too (MedlinePlus). Even in healthy people, the average red cell was anywhere from 38 to 60 days old, and in a small study that spread alone was enough to shift A1c for the same blood sugar.

Two tests, and age

A diagnosis takes two abnormal results unless blood sugar is clearly high. Two A1c results of 6.5% or more count even when the fasting glucose is normal. A1c also drifts up about 0.1 points a decade in people without diabetes, but the lines don’t move with age.

You don’t need to fast for an A1c, and the ADA lists that, along with fewer swings from stress or illness, as its advantages over a fasting test. Convert any value on the A1c converter, see every value from 4.5 to 10 as its own page, or start with what an A1c of 5.7 means.