A1C measures the percentage of your red blood cells' hemoglobin that has glucose stuck to it. Because red blood cells live around three months, the number reflects your average glucose over that window — weighted toward the most recent few weeks. An A1C of 7% corresponds to an estimated average glucose of about 154 mg/dL; each additional percentage point adds roughly 29 mg/dL to that average.
For most adults with diabetes the commonly cited target is below 7%, but the right number for you depends on your age, hypo awareness, and life circumstances — that conversation belongs with your care team.
What A1C does well: it is the long-run scoreboard, measured the same way everywhere, and it is the number most of the long-term outcome research is built on. What it hides: everything about variability. An A1C of 6.8% can mean steady lines or a rollercoaster of spikes and lows that happen to average out. That blind spot is exactly why Time in Range grew up alongside it.
A few practical notes:
- The last month counts more than the first — a rough week right before the draw nudges it up.
- Anemia, pregnancy, and some hemoglobin variants can skew the reading.
- It is a lagging indicator: changes you make today take weeks to show up in it. Your CGM shows them tomorrow.
