How to Convert HbA1c to Average Blood Sugar

Average blood sugar from an HbA1c uses the ADAG formula: eAG (mg/dL) = 28.7 × HbA1c (%) - 46.7. An HbA1c of 7.0% is an estimated average glucose of 154 mg/dL. The number is called estimated average glucose, or eAG, and it is not the same as GMI, a separate estimate calculated from continuous glucose monitor data.

How do I convert my HbA1c to average blood sugar?

You put the lab HbA1c percentage through the ADAG equation from Nathan 2008. In mg/dL:

eAG = 28.7 × HbA1c - 46.7

In mmol/L:

eAG = 1.59 × HbA1c - 2.59

A 7.0% result works out as 28.7 × 7 - 46.7 = 154 mg/dL, or 8.5 mmol/L.

The common values worth remembering:

• 5.0% = 97 mg/dL (5.4 mmol/L)
• 6.0% = 126 mg/dL (7.0 mmol/L)
• 6.5% = 140 mg/dL (7.7 mmol/L)
• 7.0% = 154 mg/dL (8.5 mmol/L)
• 8.0% = 183 mg/dL (10.1 mmol/L)
• 9.0% = 212 mg/dL (11.7 mmol/L)
• 10.0% = 240 mg/dL (13.3 mmol/L)

The conversion also runs backwards. An average glucose of 140 mg/dL is about an HbA1c of 6.5%, because (140 + 46.7) ÷ 28.7 = 6.5.

HbA1c is not a blood sugar reading. It is a two to three month glycation record, and the ADAG formula is the translation labs use.

Why HbA1c tracks two to three months of glucose, not today

HbA1c is not a blood sugar reading from one moment. Glucose in the bloodstream enters red blood cells and slowly attaches to haemoglobin. The higher the average glucose, the more haemoglobin carries glucose. A red cell lives for about 120 days, so the percentage of glycated haemoglobin reflects the average glucose over the previous two to three months, not the past hour.

The record is weighted toward the most recent four to six weeks. Older red cells are continuously cleared and replaced, so their contribution fades. That is why a person whose last month improved will often have an HbA1c lower than what the previous two months alone would predict, but not yet at the new steady state.

What this looks like on a lab report: the HbA1c is printed as a percentage and many labs also print an estimated average glucose next to it. A 6.4% result will show something near 137 mg/dL.

eAG is not the same as GMI

eAG is not GMI. The two numbers can match by coincidence, but they are built from different data.

GMI was introduced for continuous glucose monitor users who kept trying to back-calculate an A1c from their sensor average. The GMI equation is 3.31 + 0.02392 × mean glucose in mg/dL (Bergenstal 2018). A CGM mean of 154 mg/dL gives a GMI of 7.0%, which happens to match the ADAG eAG at that point. In an individual, lab HbA1c and CGM-derived GMI often differ by a few tenths of a percent. That gap is not necessarily an error: it can reflect red cell lifespan, sensor accuracy or how often the sensor was worn.

The practical point: do not take a CGM mean, convert it to eAG, then call it lab HbA1c. If you want the lab-based average, start with the HbA1c. If you want a sensor-based estimate, use GMI.

When the simple conversion does not hold

The ADAG equation is a population fit. It works well in most adults with routine red cell biology, but it is not a personal kinetic measurement. If red cell survival changes, the relationship between HbA1c and average glucose breaks.

Conditions where a converted eAG should be read with a doctor rather than as a true glucose average:

• recent blood loss or transfusion
• haemolytic anaemia or shortened red cell survival
• haemoglobin variants, including sickle cell trait and thalassaemia
• pregnancy, especially the second and third trimester
• severe kidney disease or conditions affecting erythropoietin

In these situations the lab HbA1c can be falsely low or high relative to the person's actual average glucose. The calculator still runs, but the number can mislead.

One common mistake is to treat eAG as a target to hit at every fingerprick. It is an average. A person with an eAG of 154 mg/dL might sit near 110 mg/dL fasting and above 180 mg/dL after meals. The average hides the shape of the day.

Try it in your own lab report

Use the free HbA1c to average glucose calculator at HealthOS: https://healthosx.com/blood/calculators/hba1c-average-glucose. It runs in your browser with no account and gives the result in mg/dL. For mmol/L, use the separate unit converter on HealthOS.