How this instrument works
Hemoglobin A1c measures the share of hemoglobin inside red blood cells that has glucose permanently attached, and because red blood cells live for roughly 120 days, that share builds up as a running record of average blood glucose over the preceding two to three months. That makes A1c useful for judging longer-term control, but its native unit — a percentage — does not map intuitively onto the mg/dL figures a home glucose meter or continuous monitor reports every day.
The A1c-Derived Average Glucose (ADAG) study, led by Nathan and colleagues and published in Diabetes Care in 2008, closed that gap by having hundreds of participants wear frequent glucose monitoring alongside standard A1c testing, then fitting a straight line between the two: eAG in mg/dL equals 28.7 times the A1c percentage, minus 46.7. MedlinePlus's A1C test reference confirms labs increasingly report this estimated average glucose figure alongside the raw percentage specifically because patients find the mg/dL scale easier to relate to their own meter readings.
The formula is a population-level regression, not a personal guarantee — it describes the average relationship between A1c and measured glucose across the ADAG study's participants, and any one person's true average can sit somewhat above or below what the equation predicts. Conditions that speed up or slow down red blood cell turnover, certain anemias, and some hemoglobin variants can all shift A1c without a matching shift in actual average glucose, which is one reason day-to-day meter or continuous-monitor readings still matter alongside this estimate rather than being replaced by it.
- Enter Hemoglobin A1c (%) exactly as reported on your lab result.
- Read Estimated average glucose (mg/dL): the calculator applies 28.7 × A1c − 46.7 automatically.
- Compare the eAG figure to the mg/dL numbers your glucose meter or continuous monitor already reports, since both now sit on the same scale.
- Remember the result describes an average over roughly the past three months, not a single moment's blood sugar.
Worked example — A1c of 7%, 5.7%, and 10%
An A1c of 7%. Multiply first: 28.7 × 7 = 200.9. Subtract the constant: 200.9 − 46.7 = 154.2 mg/dL. That is the eAG figure reported alongside a 7% result — roughly what a meter would average if it recorded a reading every few minutes across three months.
An A1c of 5.7%, the threshold many guidelines use to flag prediabetes. Multiply: 28.7 × 5.7 = 163.59. Subtract 46.7: 163.59 − 46.7 = 116.89, reported as about 116.9 mg/dL. That sits noticeably higher than a single fasting reading might suggest, since eAG reflects an average across meals rather than a fasting snapshot.
An A1c of 10%, consistent with poorly controlled diabetes. Multiply: 28.7 × 10 = 287. Subtract 46.7: 287 − 46.7 = 240.3 mg/dL. At that level the estimated average sits well above typical target ranges for most of the day, every day, across the three-month window the A1c reflects.
Questions
What does eAG actually measure?
It is not a separate measurement — it is a conversion of the hemoglobin A1c percentage into the same mg/dL scale a glucose meter uses, based on the linear formula the 2008 ADAG study fitted between the two. It represents an estimated average blood glucose over roughly the preceding two to three months, the span reflected by A1c.
Why does A1c reflect about three months of glucose control?
Because it measures the share of hemoglobin, inside red blood cells, that glucose has permanently attached to, and red blood cells circulate for about 120 days before being replaced. A1c drawn today reflects a weighted average of blood glucose across that whole lifespan, leaning more heavily toward the most recent four to six weeks than toward three months back.
Can my actual average glucose differ from the eAG this formula predicts?
Yes. The ADAG formula is fitted from population averages, so an individual figure can sit above or below what the equation predicts. Anything that changes red blood cell turnover — certain anemias, some hemoglobin variants, recent blood loss or transfusion — can shift A1c without shifting true average glucose by the same amount, widening the gap between predicted eAG and a person's real average.
How is eAG different from a fasting glucose test?
Fasting glucose is a single snapshot taken after not eating for several hours; eAG is a three-month rolling average derived from A1c and includes the swings from meals, activity, and everything else across that period. A normal fasting glucose and an elevated eAG, or the reverse, can both happen, since the two are measuring different things over very different timescales.
What A1c level corresponds to a diabetes diagnosis?
Commonly cited guidelines flag an A1c of 5.7% to 6.4% as prediabetes and 6.5% or above, confirmed on repeat testing, as diabetes — corresponding to an eAG of roughly 117 to 137 mg/dL, and above about 140 mg/dL, in this formula. An actual diagnosis is made by a clinician using these cutoffs alongside other tests, not by this calculator.
Should I use eAG instead of my glucose meter readings?
No — the two answer different questions and work best together. eAG summarizes roughly three months into one number, while meter or continuous-monitor readings show what is happening right now and across each day. Neither replaces the other, and a clinician typically wants both when adjusting a treatment plan.
References
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.