A1C Calculator

What this A1C calculator does

This calculator converts between A1C (also called HbA1c, or glycated hemoglobin %) and the corresponding estimated average glucose (eAG) in both mg/dL (the unit used in the US) and mmol/L (the unit used in most of the rest of the world). It also flags where the A1C falls in the standard diagnostic categories — normal, prediabetes, diabetes — and whether the value sits within the ADA’s general treatment target of under 7 %.

The conversion uses the formula published in the A1C-Derived Average Glucose (ADAG) study by Nathan and colleagues in 2008, which the American Diabetes Association adopted as the standard translation between the lab number and an everyday glucose average:

eAG (mg/dL) = (28.7 × A1C) − 46.7 eAG (mmol/L) = (1.59 × A1C) − 2.59

Both formulas are linear and exact arithmetic — there is no guessing or curve-fitting on your specific data.

How to use the A1C calculator

1. Pick the direction: A1C → eAG (you have a lab result and want to know what it means in everyday glucose units), or eAG → A1C (you have an average from a meter or CGM and want to know what A1C it predicts).
2. Enter the value. A1C is a percentage, typically 4–14 %; eAG is in mg/dL, typically 70–350.
3. The headline stat shows the converted value in both mg/dL and mmol/L. The interpretation badge classifies the A1C against ADA thresholds.
4. The reference table at the bottom shows A1C 5 / 6 / 7 / 8 / 9 / 10 / 12 % with their corresponding eAGs, useful as a quick mental reference.
5. Tap Copy summary to paste the result into a logbook, message, or telehealth note.

What A1C actually measures

A1C is the percentage of hemoglobin molecules in your red blood cells that have a glucose molecule covalently attached — a process called glycation. Hemoglobin in a red cell circulates for roughly 120 days before the cell is recycled by the spleen. The longer a red cell is exposed to higher glucose concentrations, the more of its hemoglobin gets glycated. Because the red-cell population in your body is a continuous mix of young and old cells, the A1C value reflects a weighted average glucose over the last 8–12 weeks, with the most recent 4 weeks contributing about half the total.

That weighted-average nature is why A1C cannot be “spiked” by a single good or bad day, why it doesn’t move quickly after a change in medication or diet, and why testing more often than every three months rarely tells you anything new.

Why the ADA introduced eAG

A1C is reported as a percentage, which is hard to mentally connect with the fingerstick numbers people see daily. In 2008 the ADAG study sampled detailed glucose data from over 500 people using both continuous glucose monitoring and frequent self-monitoring, then fitted a linear relationship between mean glucose and A1C. The result — the formulas above — let labs and providers report eAG alongside A1C so patients see the lab number translated into the same units as their meter.

For example, an A1C of 7 % corresponds to an eAG of about 154 mg/dL — meaning that if you tested glucose every minute for the last three months and averaged all of those readings, you’d get roughly 154 mg/dL.

Why eAG differs from a single fingerstick

A single fingerstick captures a moment — fasting, post-meal, after exercise, at 3 AM — and any of those moments can be wildly above or below the average. The eAG is the long-term average of all those moments. Two people with the same 7 % A1C and the same 154 mg/dL eAG can have very different glycaemic variability: one bouncing between 80 and 280 mg/dL, the other steady at 130–180. Continuous glucose monitoring (CGM) reveals that variability; A1C alone does not.

Accuracy considerations

A1C is generally accurate, but several conditions distort it:

• Iron-deficiency anaemia, B12/folate deficiency: A1C reads artificially high.
• Hemolytic anaemias, recent transfusion, EPO therapy: A1C reads artificially low.
• Hemoglobinopathies (sickle-cell trait, hemoglobin C, beta- thalassemia): assay interference, direction depends on the variant and the method.
• Chronic kidney disease, pregnancy, recent significant blood loss: shifts toward lower readings.

If any of these apply to you, fructosamine, glycated albumin, or direct CGM averaging may be better tools — discuss with your provider.

Privacy

This calculator does its arithmetic in JavaScript on your device. There is no fetch call, no analytics on the values you enter, no server-side logging. Your A1C, eAG, and any interpretation stay on this device. The page works the same way offline once loaded.