How to calculate A1C from average glucose

The arithmetic

Start from the published relationship between A1C and estimated average glucose: eAG in mg/dL equals 28.7 times A1C, minus 46.7. Rearranging for A1C gives A1C equals your average glucose plus 46.7, all divided by 28.7. So an average of 154 mg/dL gives (154 + 46.7) / 28.7, which is 6.99, or 7.0% to the precision anyone reports. If your readings are in mmol/L, multiply by 18.0182 first to get mg/dL, then run the same arithmetic — that keeps you on one equation instead of two.

What you cannot do at home

You can calculate an estimate from your own averages, which is what this page describes. What you cannot do is calculate the measurement a laboratory makes: a lab A1C measures the proportion of your haemoglobin that has become glycated, which is a physical property of your blood, not an arithmetic function of the readings you happened to take. Those two things track each other well enough to be useful and diverge often enough to matter. If a number is going to inform a decision, the lab value is the one that counts.

Getting the average right first

The output is only as good as the average you feed it, and this is where most home calculations go wrong. A handful of fasting readings is not an average of your day, because they systematically sample your lowest values; testing mostly after meals biases the other way. A continuous glucose monitor over a full fortnight gives a genuinely representative mean, and for CGM data there is a better-suited equation anyway — GMI, from Bergenstal 2018, which was designed for exactly that source of data.

Official sources

• Nathan et al., Translating the A1C Assay Into Estimated Average Glucose Values, Diabetes Care 2008
• American Diabetes Association — Standards of Care in Diabetes

Frequently asked questions