A1C to Average Blood Glucose (eAG): The ADA Formula
Your lab report says your A1C is 7.0%, but your glucose meter reads in mg/dL. The two numbers measure the same thing — your average blood sugar — on different scales, and one simple equation converts between them. This guide walks through the ADA-endorsed formula, the diagnostic bands, and the unit conversions, so you can read either number with confidence.
What A1C and eAG actually measure
A1C (also written HbA1c) is the percentage of your hemoglobin that has glucose attached to it. Because red blood cells live about three months, A1C reflects your average blood glucose over roughly the past 8–12 weeks — not a single moment. That is why a fasting finger-stick can look fine while A1C tells a longer story.
eAG — estimated average glucose — is that same long-run average expressed in the units your glucose meter uses (mg/dL in the US, mmol/L internationally). The A1C-Derived Average Glucose (ADAG) study of 2008 established the linear relationship that labs now use to print an eAG value right next to your A1C.
This is general educational information, not medical advice, diagnosis, or treatment. A1C and eAG are screening and monitoring tools; only a qualified clinician can interpret them in your context. Always consult your doctor before acting on any number here.
The ADA / NGSP formula
The conversion endorsed by the National Glycohemoglobin Standardization Program (NGSP) and the American Diabetes Association is a straight line:
eAG (mg/dL) = 28.7 × A1C − 46.7
To get the international unit, divide by 18:
eAG (mmol/L) = eAG (mg/dL) ÷ 18 (more precisely ÷ 18.0156)
And to go backwards — from a known average glucose to an A1C estimate — rearrange the same equation:
A1C (%) = (eAG + 46.7) ÷ 28.7
That is the entire model. The ADAG study reported a strong fit (r² = 0.84), which is why it became the reporting standard, but it is still an average relationship — individual biology can shift where you sit on the line (more on that below).
Worked example: A1C 7.0%
Suppose your lab reports A1C = 7.0%.
Step 1 — eAG in mg/dL:
28.7 × 7.0 − 46.7 = 200.9 − 46.7 = 154 mg/dL
Step 2 — eAG in mmol/L:
154 ÷ 18 ≈ 8.6 mmol/L
Step 3 — sanity-check the reverse:
(154 + 46.7) ÷ 28.7 = 200.7 ÷ 28.7 ≈ 7.0% ✓
So an A1C of 7.0% corresponds to an average blood glucose of about 154 mg/dL (8.6 mmol/L). That is meaningfully higher than the 100 mg/dL a lot of people assume "7" implies — a common misreading the formula clears up instantly.
The ADA diagnostic bands
The ADA Standards of Care use three A1C categories for screening adults. These thresholds are the reason A1C is so widely used — a single blood draw places you in a band:
| Category | A1C | Approx. eAG (mg/dL) | Approx. eAG (mmol/L) |
|---|---|---|---|
| Normal | below 5.7% | below ~117 | below ~6.5 |
| Prediabetes | 5.7% – 6.4% | ~117 – 137 | ~6.5 – 7.6 |
| Diabetes | 6.5% or higher | ~140 and up | ~7.8 and up |
The eAG columns are computed from the same 28.7 × A1C − 46.7 formula, rounded. A diagnosis is normally confirmed with a repeat or a second test type — a single value in the diabetes band is not, on its own, a diagnosis. Your clinician decides.
A quick conversion reference
If you prefer to read off a chart rather than run the arithmetic:
| A1C | eAG (mg/dL) | eAG (mmol/L) |
|---|---|---|
| 5.0% | 97 | 5.4 |
| 6.0% | 126 | 7.0 |
| 6.5% | 140 | 7.8 |
| 7.0% | 154 | 8.6 |
| 8.0% | 183 | 10.2 |
| 9.0% | 212 | 11.8 |
| 10.0% | 240 | 13.4 |
Every row is just 28.7 × A1C − 46.7, then ÷ 18 for the mmol/L column. Round eAG to a whole mg/dL and A1C to one decimal place, as labs do.
eAG is not the same as your CGM's GMI
If you wear a continuous glucose monitor (CGM), the app may show a Glucose Management Indicator (GMI) — and it will often differ from a lab A1C. That is expected, not an error. GMI comes from a different equation:
GMI (%) = 3.31 + 0.02392 × mean CGM glucose (mg/dL)
For a CGM mean of 150 mg/dL, GMI = 3.31 + 0.02392 × 150 ≈ 6.9% — close to, but not identical to, the 7.0% lab-A1C that maps to an eAG of 154. Studies find that roughly one in five people show a GMI and a lab A1C that differ by 0.5 percentage points or more.
The takeaway: eAG is derived from your lab A1C; GMI is derived from your CGM data. They estimate the same underlying thing by different routes, and labelling a CGM's GMI as "your A1C" is the single most common mistake in this area. Treat them as two related readings, not one.
When A1C can mislead
Because A1C depends on red blood cells and hemoglobin, several conditions can push it away from your true average glucose — high or low:
- Anemia and iron deficiency
- Hemoglobinopathies (sickle cell trait/disease, HbC, HbE)
- Recent blood transfusion or significant blood loss
- Pregnancy (second and third trimesters)
- Chronic kidney disease, especially on erythropoietin
- Any condition with high red-blood-cell turnover
In these situations A1C — and therefore the eAG it produces — may not reflect your real glucose control, and your clinician may rely more on direct glucose measurements. This is exactly why the number is a starting point for a conversation, not a verdict.
Putting it together
- eAG and A1C are the same average on two scales; convert with eAG = 28.7 × A1C − 46.7 (÷ 18 for mmol/L).
- Reverse it with A1C = (eAG + 46.7) ÷ 28.7.
- ADA bands: normal < 5.7%, prediabetes 5.7–6.4%, diabetes ≥ 6.5%.
- A CGM's GMI uses a different formula and commonly differs from lab A1C — don't conflate them.
- Certain conditions make A1C unreliable; a number is never a diagnosis on its own.
To convert your own value both ways — with the mmol/L and IFCC mmol/mol equivalents and the ADA category filled in automatically — use the A1C calculator.
Sources: NGSP — HbA1c and eAG conversion, eAG = 28.7 × A1C − 46.7 (ngsp.org/A1ceAG.asp); American Diabetes Association — eAG/A1C calculator and Standards of Care glycemic thresholds (professional.diabetes.org). This article is educational and not a substitute for professional medical advice.