Managing Blood Sugar: How to Convert A1C to Average Glucose (eAG)

A1C Explained Like You Are 10

Red blood cells carry oxygen around your body. Sugar in the blood can stick to a protein inside those cells called hemoglobin. The A1C test reports the percentage of hemoglobin with glucose attached. Because red blood cells live for months, A1C gives a long-view picture instead of a single snapshot.

Think of daily glucose readings as photographs and A1C as a short movie made from many photographs. A movie can show the general story, but it cannot tell you exactly what happened in every scene. Recent weeks also influence A1C more than the oldest weeks in the roughly three-month window.

How A1C Converts to Estimated Average Glucose

A commonly used relationship is eAG in mg/dL = 28.7 × A1C − 46.7. To convert mg/dL to mmol/L, divide by 18. The result is an estimate based on a study relationship, not the arithmetic average of readings from one person's glucose meter.

Example: for A1C 6.5%, calculate 28.7 × 6.5 − 46.7 = 139.85, or about 140 mg/dL. Dividing by 18 gives about 7.8 mmol/L. Try another value with the A1C to glucose calculator.

Screening Categories Are Not Personal Targets

For diagnosing type 2 diabetes in non-pregnant adults, NIDDK lists below 5.7% as normal, 5.7% to 6.4% as the prediabetes range, and 6.5% or above as the diabetes range. A diagnosis normally requires confirmation unless clear symptoms are present. These screening cutoffs are not the same thing as a treatment target. A safe target can differ with age, pregnancy, medications, hypoglycemia risk, other health conditions, and a clinician's judgment.

Why A1C and Meter Readings May Not Match

• A meter reading measures one moment; A1C reflects a longer period.
• Testing mostly before meals can miss after-meal peaks.
• Recent illness, medication changes, or a new routine may affect recent readings before A1C fully changes.
• Anemia, kidney or liver disease, recent blood loss or transfusion, pregnancy, and some hemoglobin variants can affect interpretation or test accuracy.
• Every laboratory test and home meter has measurement uncertainty.

Do not change medication from a calculator result. If laboratory A1C and glucose-monitoring data disagree, discuss both with a qualified healthcare professional.

Sources, Limits, and Conclusion

• NIDDK: The A1C Test and Diabetes
• NIDDK: Diabetes tests and diagnosis
• NIDDK: Hemoglobin variants and A1C

A1C and eAG express related information in different units. Conversion can make a laboratory percentage easier to understand, but it cannot recreate an individual's exact glucose pattern. Use the estimate to prepare questions, then interpret the actual test with your care team.

Practical takeaway: Keep the laboratory percentage, converted eAG, and home-monitoring pattern in separate columns when you track them. They describe different views of glucose. Record the date and units so that mg/dL is never confused with mmol/L.

Questions to Take to an Appointment

Useful questions include: Was this test performed with an NGSP-certified laboratory method? Could anemia, a hemoglobin variant, pregnancy, kidney disease, blood loss, or a recent transfusion affect my result? How does the value compare with my glucose meter or continuous monitor? What target is appropriate for me, and when should the test be repeated? Writing down medicines, symptoms, and recent glucose patterns can help the clinician interpret the number in context.