What Is A1C? Your 3-Month Average, Explained
If a lab report or a doctor has mentioned your “A1C,” this page explains — in plain words — what the test measures and what the number means.
What the A1C test is
A1C — also called HbA1c or hemoglobin A1C — is a blood test.
It measures the percentage of your hemoglobin that has sugar attached to it. Hemoglobin is the protein inside red blood cells that carries oxygen around your body. Sugar in the blood — called glucose — sticks to hemoglobin over time.
So the A1C number is a percentage. For example, an A1C of 6% means that about 6 out of every 100 hemoglobin proteins have sugar attached.
Why it covers the past 2 to 3 months
Red blood cells live for about 3 months before the body replaces them. Because sugar sticks to hemoglobin for the life of the cell, the test picks up sugar that attached weeks ago as well as sugar from today.
That is why an A1C result reflects your average blood sugar over the past 2 to 3 months. It is an average built up over time — not a snapshot of one single moment, the way a finger-stick reading is.
The ranges the CDC publishes
The U.S. Centers for Disease Control and Prevention (CDC) publishes these ranges for the A1C test:
Only a clinician can diagnose anything. These ranges are general information about how the test is read — a single number on its own is not a diagnosis. A result in the diabetes range is normally confirmed with repeat testing before any diagnosis is made.
Turn an A1C into an estimated average glucose
Because A1C is a percentage, it can be hard to picture. This tool converts an A1C into an estimated average glucose (eAG) — the same kind of number a glucose meter shows. Type in an A1C below:
Formula (American Diabetes Association): eAG in mg/dL = 28.7 × A1C − 46.7. The mmol/L figure is mg/dL ÷ 18. This tool runs entirely on this page — nothing you type is saved or sent anywhere.
Why an average can hide the ups and downs
An average smooths everything out. Two people can have the same A1C with very different days: one person's blood sugar may stay fairly steady, while another's may swing between high peaks and low dips — and both patterns can land on the same average.
So an A1C tells you the overall picture of the last 2 to 3 months, but it does not show your daily highs and lows, or when they happened. That is one reason a single A1C number never tells the whole story on its own.
When to talk to a doctor
If your result is in the prediabetes or diabetes range — or if you simply don't understand your result — talk to your doctor or pharmacist.
They know your full picture: your other test results, your medicines, and your health history. This page cannot do that. A short conversation with them is the right next step for anything about your own result.
Sources
The facts on this page come from these sources:
- CDC — A1C and diabetes testing, including the published ranges.
- MedlinePlus (U.S. National Library of Medicine) — the A1C test: what it measures and what the result reflects.
- American Diabetes Association — estimated average glucose (eAG) and the formula used in the converter above.
Facts checked November 2026.