The A1c test (also called HbA1c or glycated hemoglobin) is the most common blood marker doctors use to judge long-term blood sugar control. It does not tell you what your glucose is at this moment — it reflects your average blood glucose over roughly the past two to three months. This calculator converts that percentage into an estimated average glucose (eAG) figure you can read directly, and it also runs the math in reverse so you can turn a fingerstick or CGM average back into the A1c equivalent.
The equation behind the tool is the ADAG (A1c-Derived Average Glucose) relationship adopted by the American Diabetes Association: eAG (mg/dL) = 28.7 × A1c − 46.7, and the reverse, A1c = (eAG + 46.7) / 28.7. Enter your number, pick the direction, and both mg/dL and mmol/L results appear instantly.
How to Use
- Enter your A1c (HbA1c) as a percentage, for example
5.7. This is the number printed on your lab report. - Keep the Conversion set to A1c to average glucose to turn that percentage into an everyday glucose estimate.
- Or switch the Conversion to average glucose to A1c if you already know your typical reading — a fingerstick average or the average your continuous glucose monitor (CGM) shows you — and want to see what A1c it corresponds to.
- If you chose the reverse direction, type that average reading into Average Glucose (eAG) in mg/dL.
- Read the result card: it shows the estimated average glucose in mg/dL, the same value in mmol/L, the matching A1c percentage, and a plain-language range (Normal, Prediabetes, or Diabetes).
The normal cutoffs used on this page follow standard diabetes guidelines: Normal is below 5.7%, Prediabetes is 5.7% to 6.4%, and Diabetes is 6.5% or higher.
The A1c to eAG Formula
Your red blood cells carry hemoglobin, and glucose sticks to that hemoglobin over the cell's lifespan. The higher your average glucose, the more "glycated" the hemoglobin becomes — which is exactly what A1c measures. The ADAG study linked the percentage directly to a real average glucose number:
eAG (mg/dL) = 28.7 × A1c − 46.7
Worked example: an A1c of 7.0% gives 28.7 × 7.0 − 46.7 = 154.2 mg/dL (about 8.6 mmol/L). An A1c of 5.7% gives 28.7 × 5.7 − 46.7 = 116.9 mg/dL (about 6.5 mmol/L). Notice how a small change in the percentage — just 1.3 points — moves the estimated glucose by roughly 37 mg/dL, which is why the prediabetes and diabetes cutoffs matter so much.
To convert the other way, rearrange the formula: A1c = (eAG + 46.7) / 28.7. A typical glucose of 154 mg/dL works back out to (154 + 46.7) / 28.7 = 7.0%, confirming the math is symmetric.
For mmol/L, divide the mg/dL figure by 18.0182 (rounded to 18 in everyday use). So 154.2 mg/dL becomes 154.2 / 18.0182 ≈ 8.6 mmol/L.
How A1c Relates to Your Daily Readings
A1c smooths out the spikes and dips that a single fingerstick shows. A person can have a normal fasting reading yet a high A1c if their after-meal numbers run high all day, or a slightly high fasting number yet a normal A1c if everything else stays tight. Because the test reflects roughly three months, it is also the right tool for judging whether a new medication, diet change, or exercise habit is actually working over time — not after a single good day.
It is normal for the eAG here to differ a little from what your CGM reports as "GMI." CGM-based GMI uses a slightly different coefficient, and individual red-blood-cell lifespan, anemia, kidney disease, and certain medications can make A1c read a touch higher or lower than the true average. Treat the eAG as a close, evidence-based estimate, not a lab result.
Examples
Example 1 — Borderline prediabetes. A patient gets an A1c of 5.7%. Converting: 28.7 × 5.7 − 46.7 = 116.9 mg/dL (6.5 mmol/L). The range label reads Prediabetes (at the threshold), which is the point where lifestyle changes most reliably prevent progression.
Example 2 — Managed diabetes. A person with diabetes on medication has an A1c of 7.0%. The calculator shows 154.2 mg/dL (8.6 mmol/L) and the Diabetes range. For many adults this sits near the individualized target, though targets should be set with a clinician because hypoglycemia risk, age, and complications change the goal.
Example 3 — From CGM average back to A1c. Someone's CGM shows an average of 154 mg/dL. Switching the conversion to average glucose to A1c and entering 154 gives (154 + 46.7) / 28.7 = 7.0% — a useful sanity check when you want to predict what your next lab draw will show.
Example 4 — Very tight control. A reading of 100 mg/dL converts back to (100 + 46.7) / 28.7 = 5.1%, labeled Normal. Extremely tight control can raise low-blood-sugar risk, so it is worth discussing with a doctor rather than chasing a lower number on your own.
FAQ
What is a normal A1c?
A normal A1c is below 5.7%. Between 5.7% and 6.4% is classified as prediabetes, and 6.5% or higher on two separate tests generally meets the diagnostic threshold for diabetes. These are population cutoffs; your personal target depends on age, pregnancy, medications, and other health factors.
How do I convert A1c to average glucose (eAG)?
Use the ADAG formula: eAG in mg/dL equals 28.7 times your A1c minus 46.7. For example, A1c 7.0% becomes 28.7 × 7.0 − 46.7 = 154.2 mg/dL. To get mmol/L, divide the mg/dL result by about 18.
Can I convert my average glucose back to A1c?
Yes. Rearrange the formula to A1c = (eAG + 46.7) / 28.7. Enter your average reading in mg/dL using the reverse direction on this page and it returns the matching A1c percentage. This is handy for estimating your next lab result from a CGM average.
What is the difference between A1c and GMI?
A1c comes from a blood lab and reflects roughly three months of average glucose. GMI (Glucose Management Indicator) is a CGM-reported estimate that uses the sensor's own average and a slightly different equation. They usually agree within a few tenths of a percent but are not identical, and CGM is not a substitute for the laboratory A1c test.
Does A1c show my current blood sugar right now?
No. A1c measures average glucose over about two to three months, smoothed across the lifespan of your red blood cells. A single fingerstick or CGM value tells you what your glucose is this minute; A1c tells you the trend over the last few months.
When should I get my A1c tested?
Adults without diabetes are often screened every three years, and people with prediabetes or diabetes are tested more frequently — typically every three to six months — to check whether treatment is working. Your clinician decides the schedule based on your situation.
Why might my A1c not match my CGM average?
Conditions that change red blood cell turnover, such as anemia, kidney disease, recent blood loss, and some medications, can make A1c read higher or lower than the true average glucose. If the two disagree, your doctor may order additional tests to interpret the result correctly.
Disclaimer
This A1c calculator is for informational purposes only and is not medical advice, diagnosis, or treatment. The estimated average glucose and range labels are generated from the standard ADAG equation and should not replace a laboratory A1c test or professional medical judgment. Always discuss your A1c results, targets, medications, and any symptoms with a qualified healthcare provider before changing your diet, insulin, or other treatment.