How this calculator works
A1C (also called HbA1c or glycated hemoglobin) is a blood test that reflects average blood glucose over the previous two to three months, used to diagnose and monitor diabetes. Because the test result is a percentage rather than a glucose number, many people find it easier to think in terms of estimated average glucose (eAG) — the same information expressed in the mg/dL or mmol/L units a glucose meter uses.
This calculator converts between A1C and eAG in both directions using the formula from the ADAG (A1c-Derived Average Glucose) study, the research that established this specific relationship. This is a mathematical conversion, not a diagnostic tool: it does not account for conditions that affect A1C accuracy (such as anemia or certain hemoglobin variants), and it cannot replace an actual A1C blood test or a conversation with a healthcare provider about what your results mean.
The formula
eAG (mg/dL) = 28.7 × A1C(%) − 46.7eAG (mmol/L) = eAG(mg/dL) ÷ 18.0182Reverse: A1C(%) = (eAG(mg/dL) + 46.7) ÷ 28.7The 28.7 and 46.7 coefficients come from the ADAG study (Nathan et al., 2008), which measured average glucose using continuous monitoring and fingerstick testing alongside lab A1C values in over 500 people. 18.0182 is the standard conversion factor between mg/dL and mmol/L for glucose.
Worked example: A1C of 7%
- Multiply A1C by 28.7: 7 × 28.7 = 200.9.
- Subtract 46.7: 200.9 − 46.7 = 154.2 mg/dL.
- Convert to mmol/L: 154.2 ÷ 18.0182 ≈ 8.6 mmol/L.
- So an A1C of 7% corresponds to an average blood glucose of about 154 mg/dL (8.6 mmol/L) over the prior few months.
Frequently asked questions
What A1C level indicates diabetes?
Diagnostic thresholds are set by medical guidelines, not by this calculator, and depend on how and when the test is taken. Only a doctor, using laboratory-confirmed results and your full medical picture, can diagnose diabetes or prediabetes. This tool only converts between A1C and average glucose units.
Why doesn't my meter's average glucose match my A1C exactly?
eAG is a population-average relationship, and your own red blood cells, monitoring gaps, and glucose variability throughout the day can make your personal average differ somewhat from the eAG estimate. A1C also reflects roughly the prior three months, while a meter average may cover a shorter or differently-sampled period.
Can anything make an A1C result inaccurate?
Yes. Certain types of anemia, some hemoglobin variants, recent blood loss or transfusion, and some chronic conditions can all distort A1C independent of actual average glucose. If your A1C and your day-to-day glucose readings seem inconsistent, ask your doctor whether one of these factors might apply to you.
How often should A1C be tested?
Testing frequency is a medical decision based on your individual situation, typically every 3-6 months for people managing diabetes, but this varies by treatment plan and clinical guidance. Follow your healthcare provider's recommended schedule rather than any general rule.
Can I use this calculator instead of getting an A1C blood test?
No. This tool only performs the ADAG conversion between an A1C percentage and estimated average glucose — it requires an actual A1C value from a laboratory blood test as its input, and it does not measure glucose or diagnose anything itself. Enter an A1C percentage you already have to see its eAG equivalent, or run the reverse conversion from a meter's average; if you don't have a recent A1C result, there is nothing here to convert. Talk to a doctor about getting tested.