Hemoglobin A1c Calculator — HbA1c to Average Blood Glucose
Convert your HbA1c lab result into an estimated average blood glucose (eAG) in both mg/dL and mmol/L, and see where it falls on the ADA diagnostic scale.
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3-month average blood glucose estimated from HbA1c using the ADA formula
- 1
Slope × HbA1c
28.7 × 6.5 = 186.5 - 2
Estimated average glucose (eAG)
186.5 − 46.7 = 140ADA 2008 formula (Nathan et al., Diabetes Care): eAG (mg/dL) = 28.7 × HbA1c% − 46.7.
How does this calculator work?
HbA1c reflects average blood glucose over ~3 months. The ADA formula eAG (mg/dL) = 28.7 × A1c% − 46.7 converts it to the same units as a glucose meter. Below 5.7% is normal; 5.7–6.4% is prediabetes; 6.5%+ is in the diabetes range. Always confirm diagnosis with a clinician.
Formula
How this is calculated
Hemoglobin A1c (HbA1c or glycated haemoglobin) measures the percentage of haemoglobin molecules in your blood that have glucose attached to them. Because red blood cells live for roughly 3 months, the HbA1c value reflects your average blood glucose level over that period — unlike a fasting or random glucose test, which is only a snapshot.
The American Diabetes Association (ADA) published an empirically derived conversion in 2008, based on a study that simultaneously measured HbA1c and continuous glucose monitoring in hundreds of participants: eAG (mg/dL) = 28.7 × A1c% − 46.7. For mmol/L, divide by 18.018. The resulting eAG figure is expressed in the same units as a home blood glucose meter reading, which many patients find more intuitive than a percentage.
The ADA diagnostic thresholds are: below 5.7% normal, 5.7–6.4% prediabetes (also called impaired glucose regulation), and 6.5% or above in the diabetes range. However, HbA1c is a screening and monitoring tool, not a standalone diagnostic — conditions that affect red blood cell turnover (anaemia, haemoglobinopathies, recent blood loss) can cause falsely high or low readings. Always interpret results in consultation with a healthcare professional.
Frequently asked questions
For adults without diabetes the ADA considers an HbA1c below 5.7% normal. Values from 5.7% to 6.4% suggest prediabetes, and 6.5% or above on two separate tests meets the diagnostic threshold for diabetes.
People with diabetes who are meeting treatment goals typically test every 6 months; those whose therapy has changed or who are not at target may need testing every 3 months. Your doctor will advise based on your situation.
Conditions that alter red blood cell lifespan — iron-deficiency anaemia, haemolytic anaemia, sickle-cell trait or certain haemoglobin variants — can skew the result. Recent blood transfusions also interfere. If your A1c seems inconsistent with your self-monitored glucose readings, discuss alternative testing methods with your doctor.
Also known as
TG we-Calculate Editorial Team. (2026). Hemoglobin A1c Calculator — HbA1c to Average Blood Glucose [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/hemoglobin-a1c-calculator
TG we-Calculate Editorial Team. "Hemoglobin A1c Calculator — HbA1c to Average Blood Glucose." TG we-Calculate. 2026. https://we-calculate.com/calculator/hemoglobin-a1c-calculator.
TG we-Calculate Editorial Team, "Hemoglobin A1c Calculator — HbA1c to Average Blood Glucose," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/hemoglobin-a1c-calculator
@misc{wecalculate_hemoglobin_a1c_calculator, title = {Hemoglobin A1c Calculator — HbA1c to Average Blood Glucose}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/hemoglobin-a1c-calculator}}, year = {2026}, note = {TG we-Calculate} }
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