A1C of 5.1% is an average glucose of 100 mg/dL

Estimated average glucose at A1C 5.1%

100 mg/dL

In mmol/L: 5.5 mmol/L
A1C entered: 5.1%
ADA range: below the range the ADA defines as prediabetes

The estimated average glucose behind an A1C of 5.1% is 100 mg/dL, or 5.5 mmol/L. That A1C would appear as approximately 32 mmol/mol on a report using IFCC units.

Below where the ADA sets its prediabetes range. Worth knowing that A1C also shifts with anaemia, pregnancy, kidney disease and some haemoglobin variants, so the context around the number often matters more than the digit itself.

Each additional tenth of a percentage point adds about 3 mg/dL to the estimated average, so 5.2% works out to roughly 103 mg/dL. That step is smaller than the 0.5-point tolerance NGSP allows on the assay, which is worth holding in mind before reading much into a one-tenth difference.

A1C looks back over roughly the previous two to three months, because it tracks glycation in red blood cells and those cells circulate for about 120 days before being replaced.

Laboratory methods differ from one another, so comparing a result from one lab against a result from another introduces a spread that has nothing to do with your glucose. Same-lab comparisons are the cleanest kind.

Bringing the printed report to an appointment beats bringing a screenshot of a calculator, because your clinician can read it against everything else in your chart.

Comparing an older report against a newer one sometimes means comparing units as well as values — a percentage against mmol/mol, or mg/dL against mmol/L. Convert before concluding anything.

Convert a different A1C value

A1C to estimated average glucose, 4.6% to 5.6%
A1C	eAG (mg/dL)	eAG (mmol/L)
4.6%	85 mg/dL	4.7 mmol/L
4.7%	88 mg/dL	4.9 mmol/L
4.8%	91 mg/dL	5.1 mmol/L
4.9%	94 mg/dL	5.2 mmol/L
5.0%	97 mg/dL	5.4 mmol/L
5.1%	100 mg/dL	5.5 mmol/L
5.2%	103 mg/dL	5.7 mmol/L
5.3%	105 mg/dL	5.8 mmol/L
5.4%	108 mg/dL	6 mmol/L
5.5%	111 mg/dL	6.2 mmol/L
5.6%	114 mg/dL	6.3 mmol/L

Computed with eAG (mg/dL) = 28.7 x A1C - 46.7 (Nathan 2008). mmol/L is derived from mg/dL using the glucose factor 18.0182.

Estimated average glucose (ADAG / Nathan) — eAG (mg/dL) = 28.7 x A1C - 46.7. Verified 2026-07-30 against Nathan et al., Diabetes Care 2008 (ADAG study) (A population regression: in the ADAG study, individuals at the same A1C had meaningfully different mean glucose. The A1C assay itself carries about +/- 0.5 percentage points under NGSP certification.)

Official sources

Nathan DM et al., Translating the A1C Assay Into Estimated Average Glucose Values, Diabetes Care 2008 — The ADAG study that produced the eAG equation used on this page.
American Diabetes Association — Standards of Care in Diabetes — Where the A1C ranges quoted here are defined.
NGSP — A1C assay standardisation and certification criteria — Why two lab results half a point apart can mean nothing changed.

Frequently asked questions

An A1C of 5.1% equals what blood sugar?

100 mg/dL is the estimate, which is 5.5 mmol/L. It follows directly from the ADAG regression applied to 5.1%, and represents a population average rather than your personal figure.

Which range does the ADA place 5.1% in?

The ADA describes 5.1% as below the range the ADA defines as prediabetes. That is a definition of a numeric range rather than a diagnosis: the ADA asks clinicians to confirm results with a repeat test and to read them alongside your history.

Is 5.1% the same in mmol/mol?

Some countries report A1C in IFCC units rather than NGSP percent. 5.1% is approximately 32 mmol/mol, so a report from the UK, Australia or much of Europe may show that number instead.

What time period does an A1C of 5.1% describe?

Roughly the previous two to three months of glucose exposure, weighted towards the most recent weeks, because A1C tracks glycation across the lifespan of red blood cells.

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