How this instrument works
Hemoglobin A1c measures how much sugar has chemically attached itself to the hemoglobin inside your red blood cells over their roughly three-month lifespan. Because that attachment happens gradually and doesn't reverse, the fraction of glycated hemoglobin works out to a durable record of average blood sugar exposure over the prior two to three months — a single draw standing in for thousands of individual readings you'd otherwise need a meter to capture.
The trouble is that a percentage doesn't mean much next to a meter's mg/dL display. The A1c-Derived Average Glucose study put continuous monitors and frequent finger-sticks on hundreds of participants for months alongside their lab draws, then fit a straight line connecting the two: multiply the A1c percentage by 28.7, subtract 46.7, and the result lands in the same mg/dL units a home meter uses. That line is what turns 'your A1c is 7%' into something a patient can compare directly against the number on their own screen.
Treat the output as a useful translation, not a promise. The equation describes a population-average relationship; plenty of individuals sit consistently off that line even when both their A1c and their true average glucose are measured correctly, a mismatch clinicians sometimes call the glycation gap. Kidney disease, certain anemias, and some hemoglobin variants can widen the gap further by changing how long red cells circulate or how they take up sugar, so this figure supplements a meter and a clinician's read rather than replacing either.
- Enter Hemoglobin A1c as a percentage, exactly as it appears on the lab report.
- Read Estimated average glucose in mg/dL — the equivalent your own meter would show on average.
- Compare that figure against your logged meter readings over the same stretch of weeks to see how closely they track.
- Re-run the conversion each time a new A1c comes back rather than reusing an older percentage.
Worked example — A1c of 7.0%
An A1c reading of 7.0%. Multiply by 28.7: 7.0 times 28.7 equals 200.9. Subtract the constant of 46.7 and the line settles at 154.2 mg/dL — the average blood sugar that lab percentage corresponds to, close to the neighborhood many meters would show across weeks of routine checks.
A tighter A1c of 6.0% works out to 6.0 times 28.7, or 172.2, minus 46.7, giving 125.5 mg/dL. Push the same math the other way to a less controlled 9.0%: 9.0 times 28.7 is 258.3, minus 46.7 lands at 211.6 mg/dL — each single point of A1c corresponds to roughly 28.7 mg/dL of average glucose, a useful rule of thumb on its own.
Questions
What does A1c actually measure?
The share of hemoglobin inside red blood cells that has sugar chemically bonded to it. Because red cells live for months and the bonding accumulates without reversing, the percentage reflects average blood sugar exposure over roughly the preceding two to three months rather than any single day's reading.
Why convert A1c to mg/dL at all?
A lab percentage is hard to relate to daily life; a mg/dL figure sits right next to the numbers a glucose meter already reports. Translating one into the other lets a patient line up a quarterly lab result against months of self-monitored readings on the same scale, rather than comparing two incompatible units by feel.
How reliable is the eAG conversion for any one person?
It's a solid population-average relationship, built from participants who wore continuous monitors for months alongside their lab draws, but individual agreement varies. Some people's A1c consistently runs higher or lower than their measured average glucose would predict — clinicians call this pattern the glycation gap — so treat eAG as a helpful estimate, not an exact substitute for logged meter data.
Can a condition make my A1c misleading?
Yes. Anything that shortens or lengthens how long red blood cells circulate — kidney disease, certain anemias, recent blood loss or transfusion, some inherited hemoglobin variants — can shift A1c away from what your true average glucose would suggest, independent of this equation's own limits. Flag any of these to whoever is interpreting your results.
Is a lower eAG always better?
Generally yes for long-term risk reduction, but not without limit — a target that's too aggressive raises the risk of dangerous low blood sugar, especially on insulin or certain oral medications. What counts as a good target depends on age, other health conditions, and treatment plan, so this is a conversation to have with the person managing your care.
References
- Nathan DM et al. (ADAG Study Group). 'Translating the A1C Assay Into Estimated Average Glucose Values.' Diabetes Care. 2008 (PubMed)
- American Diabetes Association — 'Diagnosis and Classification of Diabetes,' Standards of Care in Diabetes—2025, Diabetes Care (PMC)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.