What the A1c calculator does
HbA1c (usually just “A1c”) is a blood test that reflects average blood glucose over the previous two to three months, reported as a percentage. Day-to-day glucose meters and continuous monitors, on the other hand, report glucose in mg/dL or mmol/L. This calculator converts between the two: enter an A1c to see the estimated average glucose it corresponds to, or enter an average glucose to see the A1c it would produce. It uses the ADAG study formula adopted by the American Diabetes Association, and runs in your browser.
The formula
Estimated average glucose (mg/dL) = 28.7 × A1c − 46.7
Estimated average glucose (mmol/L) = 1.59 × A1c − 2.59
A1c 7.0% → 28.7 × 7 − 46.7 = 154 mg/dL (8.6 mmol/L)
Average 126 mg/dL → (126 + 46.7) ÷ 28.7 = 6.0%The relationship comes from the 2008 A1c-Derived Average Glucose (ADAG) study, which compared frequent glucose measurements with A1c in about 500 people. It is a population average; an individual's A1c can sit somewhat above or below the line for the same glucose.
A1c to average glucose reference
| A1c % | mg/dL | mmol/L | Interpretation (ADA) |
|---|---|---|---|
| 5.0 | 97 | 5.4 | Normal |
| 5.7 | 117 | 6.5 | Prediabetes range begins (5.7–6.4) |
| 6.0 | 126 | 7.0 | Prediabetes |
| 6.5 | 140 | 7.8 | Diabetes diagnosis threshold |
| 7.0 | 154 | 8.6 | Common treatment target for many adults |
| 8.0 | 183 | 10.1 | Above target |
| 9.0 | 212 | 11.8 | |
| 10.0 | 240 | 13.4 | |
| 12.0 | 298 | 16.5 |
What A1c measures
Glucose in the blood attaches to haemoglobin in red blood cells, and the more glucose there is, the more haemoglobin becomes glycated. Red cells live about 120 days, so the proportion of glycated haemoglobin reflects the average glucose over that span, weighted toward the most recent month. It needs no fasting and is not affected by a single high or low reading, which makes it the standard for diagnosing diabetes and monitoring control.
When A1c and meter readings disagree
- Anaemia and blood loss change red-cell turnover; iron deficiency raises A1c, recent bleeding or transfusion lowers it.
- Haemoglobin variants (sickle trait, thalassaemia) can interfere with some lab methods.
- Kidney disease and pregnancy alter red-cell lifespan.
- Glucose variability: someone swinging between 60 and 250 can have the same A1c as someone steady at 155. Time-in-range from a continuous monitor captures what A1c hides.
- Meter sampling bias: testing mostly before meals under-samples post-meal peaks, so the meter average looks lower than the A1c implies.
Using the conversion sensibly
- Use it to translate a lab result into the units you see daily, so a target like “under 7%” becomes “average under about 154 mg/dL”.
- Use a CGM's 90-day average (the “glucose management indicator”, GMI, uses a similar formula) as a between-visits estimate — but expect the lab A1c to differ by up to half a point.
- Do not use it to diagnose. Diagnosis requires a lab A1c or a fasting/oral glucose test, interpreted by a clinician.