What is A1C?
A1C (HbA1c) is the share of haemoglobin in your red blood cells that has glucose attached to it. Red blood cells live about 3 to 4 months, so the test reflects your average blood sugar over the last 2 to 3 months — not a single moment like a fasting or random glucose test.
How is A1C converted to average glucose?
With the equation from the ADAG study (2008), which measured glucose in 507 people over 3 months — the same equation the American Diabetes Association (ADA) uses:
Average glucose (mg/dL) = 28.7 × A1C − 46.7
Example: an A1C of 7% → 28.7 × 7 − 46.7 = 154 mg/dL, and dividing by 18 gives about 8.6 mmol/L. If your result is in mmol/mol, 53 mmol/mol = 7% and 48 mmol/mol = 6.5%.
What is a normal A1C?
| A1C | Meaning |
|---|---|
| Below 5.7% (below 39 mmol/mol) | Normal |
| 5.7 to 6.4% (39–47 mmol/mol) | Prediabetes |
| 6.5% or higher (48 mmol/mol or higher) | Diabetes range, confirmed with a repeat test |
For people with diabetes, the goal for most non-pregnant adults is below 7%, without frequent lows. A doctor may set a higher goal for older people or those with many lows, or a lower one if it can be reached safely.
The other way: from your average readings to A1C
If you have the average from your meter or sensor, the calculator reverses the same equation: A1C ≈ (average + 46.7) ÷ 28.7. For example, an average of 150 mg/dL → (150 + 46.7) ÷ 28.7 = about 6.9%. This means the average of all your readings across the day (fasting, before and after meals, bedtime) over the last 2–3 months, not your fasting reading alone: if most readings are fasting or before meals the average comes out lower than the true one, and if most are after meals, higher. Real A1C is only measured by a lab.
When is A1C less accurate?
Some conditions change how long red blood cells live or the type of haemoglobin, and can make the result read higher or lower than the truth:
- Severe anaemia, or recent blood loss or transfusion.
- Inherited blood disorders such as thalassaemia and sickle cell disease, which are common in Egypt and the Gulf.
- Kidney failure and liver disease.
- Pregnancy, and some medicines.
If any of these apply to you, tell your doctor — other tests may be used instead.
Frequently Asked Questions
Do I need to fast for an A1C test?
No. A1C reflects a 3-month average, so a meal before the test does not change it; you can take it at any time of day.
Should I enter my fasting or after-meal average?
Neither on its own: enter the average of all your readings across the day over the last 2–3 months. The ADAG equation was built from continuous glucose monitoring plus 7-point meter profiles (before and after each meal and at bedtime). The easiest choice is the 30- or 90-day average your meter, app or sensor shows, as long as you test at different times of day. If most readings are fasting, the result will usually come out lower than your real A1C; if most are after meals, higher.
How often should A1C be tested?
The ADA advises people with diabetes to test at least twice a year if their glucose is on target, and every 3 months if treatment has changed or it is not on target. Your doctor decides what suits you.
What is the difference between A1C and fasting glucose?
Fasting glucose is a single reading after at least 8 hours without food: normal is below 100 mg/dL, 100 to 125 is prediabetes, and 126 or higher is the diabetes range. A1C is a months-long average, so the two complement each other.
Can this calculator diagnose diabetes?
No. It explains what your number means. Diabetes is diagnosed by a doctor, usually with two tests. You can also keep an eye on your weight with the BMI calculator and your food with the calorie calculator, since weight and diet affect blood sugar the most.
Sources
- American Diabetes Association: eAG/A1C Conversion Calculator.
- Nathan et al., Diabetes Care 2008: Translating the A1C assay into estimated average glucose values (the ADAG study).
- CDC: A1C Test for Diabetes and Prediabetes.
- NDSS (Australia): Understanding HbA1c measurements (% ↔ mmol/mol).