PonyatnoHow it's calculated
Average glucose comes from the ADAG study formula (Nathan, 2008): eAG (mg/dL) = 28.7 × HbA1c (%) − 46.7, or in mmol/L — 1.59 × HbA1c − 2.59.
Conversion to IFCC units: mmol/mol = (HbA1c % − 2.15) × 10.929. The same equation is used in reverse when you enter mmol/mol.
Ranges follow the American Diabetes Association (ADA) Standards of Care: below 5.7% is normal, 5.7–6.5% is prediabetes, 6.5% and above is the diabetes range.
What to know
What HbA1c shows
Glucose slowly attaches to the hemoglobin inside red blood cells, and a red cell lives for about three months. That is why HbA1c reflects your average sugar over that period, not on the morning of the test: one week of sweets or stress before the blood draw barely moves it.
When the result can mislead
HbA1c reads falsely low when red cells live shorter than usual: after blood loss, with hemolysis, in pregnancy, after a transfusion. It can read high with iron-deficiency anemia and in kidney disease. Some hemoglobin variants interfere with certain lab methods. In these cases doctors rely on fasting glucose or a glucose tolerance test instead.
Average glucose is an estimate: two people with the same HbA1c can have real average sugars that differ by 20–35 mg/dL.
This calculator is a reference tool, not a diagnosis. Laboratory ranges differ, and the result depends on your health, medicines and other tests. Discuss it with your doctor.
FAQ
What is a normal A1C level?
According to the ADA, below 5.7%. Values from 5.7% to 6.5% are classed as prediabetes, and 6.5% or higher as the diabetes range. Your lab may print its own reference range — go by that first.
Do I need to fast for an A1C test?
No. It reflects your average sugar over months, so what you ate beforehand has almost no effect. If fasting glucose is drawn at the same time, though, that part does require fasting.
How often should A1C be checked?
With diabetes — usually every 3–6 months; with prediabetes — roughly once a year, unless your doctor advises otherwise. Testing more often than every three months makes little sense: the value doesn't have time to change.
Can diabetes be diagnosed from one A1C result?
No. A doctor makes the diagnosis, and it is usually confirmed with a repeat test or a different test. This calculator only shows which range your result falls in.
Sources
Formulas and cut-offs follow clinical guidelines — see sources below.
- Nathan DM et al. Translating the A1C assay into estimated average glucose values. Diabetes Care. 2008;31(8):1473–1478.
- American Diabetes Association. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes—2025. Diabetes Care. 2025;48(Suppl 1):S27–S49.
- IFCC/NGSP master equation: IFCC (mmol/mol) = (NGSP % − 2.15) × 10.929. Hoelzel W et al. Clin Chem. 2004;50(1):166–174.