What is my A1C if my average blood sugar is 140?

The forward equation, from the ADAG study published by Nathan and colleagues in 2008, is estimated average glucose in mg/dL = 28.7 x A1C - 46.7. Rearranged for A1C it becomes A1C = (average glucose + 46.7) / 28.7. At 140 mg/dL that gives 6.505, which is 6.5 percent at the precision anyone reports. If your readings are in mmol/L, multiply by 18.0182 first so you stay on one equation rather than two.

This estimates an A1C; it does not reproduce one. A laboratory A1C measures the proportion of your haemoglobin that has become glycated, which is a physical property of your blood. A figure calculated from readings is an inference from the readings you happened to take, and the two track each other well enough to be useful and diverge often enough to matter.

Which makes the quality of the average the main thing to check. A handful of fasting readings systematically samples your lowest values; testing mostly after meals biases the other way. A continuous monitor over a full fortnight gives a genuinely representative mean - and for that data there is a better-suited equation, GMI, published by Bergenstal in 2018 precisely so the CGM figure would stop being called an estimated A1C.

On the result itself: 6.5 percent is a boundary the ADA uses as one of its diagnostic criteria, normally with a confirmatory test or unambiguous symptoms. It is also exactly where a single number tells you least, because NGSP certification allows a certified assay to differ from the reference method by roughly half a percentage point - wider than the gap people worry about at a boundary.