A1C Calculator

What A1C is, and why it converts to a glucose number at all

Glucose in the bloodstream attaches itself to haemoglobin inside red blood cells, and once attached it stays there for the life of the cell. A1C — also written HbA1c — is simply the percentage of haemoglobin carrying that sugar. Red cells survive about 120 days, so the test is a slow-moving average of the last two to three months rather than a snapshot, which is exactly what makes it useful for judging control and useless for judging this morning.

The conversion shown here comes from the A1C-Derived Average Glucose study published by the American Diabetes Association in 2008. Researchers fitted continuous glucose monitoring data from 507 participants against their A1C results and found a strikingly linear relationship: estimated average glucose in mg/dL equals 28.7 multiplied by A1C, minus 46.7. Reverse it and A1C equals average glucose plus 46.7, divided by 28.7. For mmol/L, divide the mg/dL figure by 18.016.

Worked example: an A1C of 6.5%

Take 28.7 x 6.5 = 186.55, then subtract 46.7 to get 139.85 — call it 140 mg/dL, or 7.8 mmol/L. That is the average glucose level a continuous monitor would have recorded across roughly the last three months, day and night. It also sits exactly on the diagnostic threshold: 6.5% is the point at which the ADA labels a confirmed result as diabetes.

Worked example in reverse: a meter average of 154 mg/dL

Add 46.7 to get 200.7, divide by 28.7, and the answer is 7.0%. This is the single most quoted pairing in diabetes care, because 7% is the general A1C target for many adults and 154 mg/dL (8.6 mmol/L) is what that target feels like day to day. If your meter has been averaging 180 mg/dL, the same arithmetic gives 7.9% — a full point above target, and a concrete reason to review the plan rather than wait for the next blood draw.

How the categories are drawn

Below 5.7% is normal, 5.7% to 6.4% is prediabetes, and 6.5% or above meets the criterion for diabetes when confirmed by a second test. Those cut-offs are diagnostic thresholds, not treatment targets. Someone already living with diabetes is not aiming for 5.6%; they are usually aiming below 7%, and their clinician may set a higher or lower number depending on age, other conditions and how often blood sugar runs low.

Two labelling systems, one test

Clinics in the United States report A1C as a percentage. The UK, Ireland and much of Europe switched to IFCC units, reported in mmol/mol, where 6.5% appears as 48 mmol/mol and 7.0% as 53 mmol/mol. The bridge is mmol/mol = (percentage - 2.15) x 10.929. If a lab result looks impossibly large, check which scale it is on before panicking — a "48" is a 6.5%, not a catastrophe.

Where the estimate breaks down

The formula assumes red blood cells with a normal lifespan. Anything that shortens or lengthens that lifespan pushes A1C away from the true average: iron-deficiency anaemia can raise it, haemolytic anaemia and recent blood loss or transfusion lower it, and advanced kidney disease, pregnancy and haemoglobin variants such as HbS, HbC or HbE all interfere. In those situations clinicians lean on fructosamine, direct glucose monitoring or time-in-range from a sensor instead.

Even in a straightforward case, an eAG is an average and averages hide their spread. Two people can both convert to 154 mg/dL, one gliding smoothly between 120 and 190 and the other swinging from 50 to 300. The second person has the same A1C and a far worse day. Use this conversion to translate a lab number into something intuitive, then take the pattern — not the single figure — to whoever manages your care.