A1c Calculator

Convert HbA1c to estimated average blood glucose and vice versa, with ADA classification, reference ranges, and a three-month average note.

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HbA1c (%)

Estimated average glucose

8.6 mmol/L

154.2 mg/dL

Diabetes — above individual target for most

HbA1c reflects your average blood glucose over the preceding 2–3 months. It is influenced most heavily by the most recent 4–6 weeks. An eAG of 8.6 mmol/L (154.2 mg/dL) represents an estimated daily average, not a single point-in-time reading.

A1c reference guide

A1c	mmol/L	mg/dL	Category
< 5.7%	< 6.5	< 117	Normal
5.7–6.4%	6.6–9.0	119–162	Pre-diabetes
< 7.0%	< 8.6	< 154	Diabetes target (many)
7.0–7.9%	8.6–10.1	154–183	Above target
≥ 8.0%	≥ 10.1	≥ 183	Significantly above target

A1c targets vary by individual clinical context. A result in the diabetes range should be confirmed by a healthcare provider and does not constitute a diagnosis on its own.

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Health — Medical

A1c Calculator

HbA1c (glycated haemoglobin) reflects average blood glucose over the preceding 2–3 months. This calculator converts A1c to estimated average glucose (eAG) in both mmol/L and mg/dL, and reverses the conversion, using the ADA-validated ADAG formula.

About this calculator

Reviewed March 2026

Methodology

ADAG formula: eAG (mg/dL) = 28.7 × A1c% − 46.7; eAG (mmol/L) = 1.5944 × A1c% − 2.5944. A1c classification thresholds: normal < 5.7%, pre-diabetes 5.7–6.4%, diabetes ≥ 6.5%, well-controlled < 7.0%, above-target ≥ 7.0%, significantly above ≥ 8.0%.

Limitations

eAG is an estimate; individual variation of ±0.5–1.5 mmol/L (10–30 mg/dL) is documented in validation studies.
A1c diagnostic thresholds may differ by clinical guideline (WHO uses ≥ 6.5%; some national guidelines vary).

Disclaimer

A1c results require clinical interpretation. Targets vary by individual clinical context and should be set in partnership with a healthcare provider.

Sources

See our calculation methodology and editorial standards for how this estimate is produced.

What HbA1c measures

Haemoglobin A1c forms when glucose in the bloodstream attaches to haemoglobin in red blood cells. Since red blood cells live approximately 90–120 days, the proportion of glycated haemoglobin reflects average blood glucose over that period. Higher average glucose means more glycation and a higher A1c percentage.

A1c is weighted toward the most recent 4–6 weeks because newly formed red blood cells represent the most recently glycated fraction. A major illness, blood transfusion, or haemolytic anaemia can invalidate A1c as a measure of average glucose.

The ADAG formula

The A1c-Derived Average Glucose (ADAG) study validated the relationship between A1c and average glucose measured by continuous glucose monitoring in a diverse population. The resulting formula — eAG (mg/dL) = 28.7 × A1c% − 46.7 — superseded earlier linear estimates and is the basis of the ADA eAG reporting standard. The conversion to mmol/L is eAG (mmol/L) = 1.5944 × A1c% − 2.5944.

Frequently asked questions

How often should A1c be tested?

For people with well-controlled diabetes meeting treatment targets, the ADA recommends testing at least twice per year. For people with poorly controlled diabetes or those whose therapy has changed, quarterly testing is recommended. For people without diabetes but with risk factors, testing frequency depends on clinical context.

Can A1c be falsely high or low?

Yes. Conditions that increase red blood cell lifespan (iron deficiency anaemia, hypothyroidism) can falsely elevate A1c. Conditions that decrease red blood cell lifespan (haemolytic anaemia, sickle cell disease, recent blood transfusion) can falsely lower it. In these cases, other measures such as fructosamine or continuous glucose monitoring are preferred.

A1c Calculator

A1c Calculator

What HbA1c measures

The ADAG formula

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