What this number is
HbA1c measures the percentage of your red blood cells' hemoglobin that has glucose stuck to it — a process called glycation. Because red blood cells live around three months, A1c works like a slow-exposure photograph of your average blood sugar over roughly the last 8–12 weeks. The calculator converts that percentage into estimated average glucose (eAG) — the same units your meter or CGM uses, so the two finally speak the same language.
The reference table
| HbA1c | eAG (mg/dL) | Commonly used clinical category* |
|---|---|---|
| 5.0% | 97 | Below 5.7% — typical range |
| 5.6% | 114 | |
| 5.7% | 117 | 5.7–6.4% — often labeled "prediabetes" |
| 6.4% | 137 | |
| 6.5% | 140 | 6.5%+ — diabetes-range, per ADA criteria |
| 7.0% | 154 | |
| 8.0% | 183 |
*These are the categories clinicians commonly use (American Diabetes Association criteria). A single number is never a diagnosis — labs repeat tests, rule out interferences, and read results in context. That part belongs to your doctor.
Why your meter and your A1c can disagree
An A1c of 6.0% converts to an eAG of about 126 mg/dL — but that's an average. A day that swings between 85 and 180 and a day that sits steadily near 125 can produce the same average. That's why a single fasting reading, a high morning number, or one post-meal spike can look out of step with your lab result while both are telling the truth. The ADAG study that produced this formula also found honest scatter between individuals — two people with identical true averages can have A1c values a few tenths apart simply because of how their red blood cells behave.
Sources
- Nathan DM, et al. Translating the A1C assay into estimated average glucose values (the ADAG study). Diabetes Care. 2008. pubmed.ncbi.nlm.nih.gov/18540046
- American Diabetes Association. Standards of Care in Diabetes — diagnosis and classification criteria. diabetes.org/about-diabetes/a1c