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HbA1c and BMI calculator (average blood sugar and body mass index)
Convert an HbA1c percentage into the average blood sugar it represents, and get your body mass index in the same pass — the two numbers most metabolic risk models start from.
HbA1c converts to estimated average glucose with eAG (mg/dL) = 28.7 × A1c − 46.7. An HbA1c of 5.7–6.4% is prediabetes and 6.5% or above meets the diagnostic threshold for diabetes. BMI is weight in kilograms divided by height in metres squared: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight and 30 or above obesity.
HbA1c & BMI calculator
From a lab report or point-of-care test, as a percentage.
HbA1c to average blood sugar conversion
The ADAG study formula converts A1c to estimated average glucose: eAG mg/dL = 28.7 × A1c − 46.7, or eAG mmol/L = 1.59 × A1c − 2.59. A 6.0% A1c is roughly 126 mg/dL (7.0 mmol/L) average glucose over the previous two to three months.
What HbA1c level is prediabetes or diabetes?
Below 5.7% is normal, 5.7% to 6.4% is prediabetes and 6.5% or higher on two occasions meets the diagnostic threshold for diabetes. For most adults already treated for diabetes, a target under 7% is typical, individualised by age and comorbidity.
BMI categories and their limits
Under 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, 30 and above obesity. BMI does not distinguish muscle from fat and understates risk in South Asian populations, where thresholds of 23 and 27.5 are often used. Waist circumference adds information BMI misses.
Why the two numbers belong together
HbA1c and BMI are both inputs into insulin-resistance and cardiovascular models. Tracking them together shows whether a weight change is actually moving glucose control — which one reading of either can never tell you.
Frequently asked questions
- What is the formula to convert A1c to average glucose?
- Estimated average glucose in mg/dL = 28.7 × A1c − 46.7. In mmol/L it is 1.59 × A1c − 2.59.
- Is an HbA1c of 5.9% bad?
- 5.9% sits in the prediabetes range (5.7–6.4%). It is a signal to act on diet, activity and weight, and to retest — not a diabetes diagnosis.
- How is BMI calculated?
- BMI = weight in kilograms ÷ (height in metres)². For 70 kg at 1.70 m that is 70 ÷ 2.89 = 24.2.
Source
Nathan DM et al. Translating the A1C assay into estimated average glucose values. Diabetes Care 2008;31:1473–1478. WHO. Obesity: preventing and managing the global epidemic, 2000.
This page is educational. It is not a diagnosis and does not replace a conversation with a qualified clinician.
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