A1c is calculated by measuring the percentage of hemoglobin in your blood that has glucose attached to it. The result reflects your average blood sugar over roughly two to three months, matching the lifespan of a red blood cell. Labs use specialized equipment to separate glycated hemoglobin from the rest, then express the ratio as a percentage. An A1c of 6%, for example, means 6% of your hemoglobin is coated with sugar.
What the Lab Actually Measures
Most labs measure A1c using a technique called ion-exchange chromatography. In simple terms, a machine pushes a blood sample through a column that separates hemoglobin into its different forms based on electrical charge. Hemoglobin molecules with glucose attached carry a slightly different charge than those without, so the machine can isolate them and measure what fraction of the total they represent. The result is your A1c percentage.
This percentage isn’t something you calculate at home from finger-stick readings. It’s a direct measurement of a physical property of your blood. Your daily glucose readings bounce around constantly, but A1c captures the cumulative exposure your red blood cells have had to glucose over their entire 90- to 120-day life cycle. That’s why it’s useful as a long-term marker rather than a snapshot.
Converting A1c to Average Blood Sugar
While A1c itself is measured in a lab, you can convert it into an estimated average glucose (eAG) that’s easier to compare with your meter readings. The formula is straightforward:
- In mg/dL: eAG = (A1c × 28.7) − 46.7
- In mmol/L: eAG = (A1c × 1.59) − 2.59
So an A1c of 7% translates to an estimated average glucose of about 154 mg/dL (or 8.6 mmol/L). An A1c of 6% works out to roughly 126 mg/dL. These aren’t precise daily targets but rather a way to bridge the gap between the percentage your doctor mentions and the numbers you see on your glucose meter.
Percentage vs. mmol/mol Units
If you’ve seen A1c reported in unfamiliar units, you’re likely looking at the international system (IFCC), which expresses A1c in mmol/mol rather than a percentage. The U.S. and many other countries use the percentage system (NGSP), while parts of Europe and other regions use mmol/mol. The conversion between them:
- Percentage to mmol/mol: IFCC = (10.93 × NGSP) − 23.50
- mmol/mol to percentage: NGSP = (0.09148 × IFCC) + 2.152
A 6.5% A1c equals 48 mmol/mol. A 5.7% equals 39 mmol/mol. If your lab report uses one system and your doctor references the other, these formulas let you translate between them.
What the Numbers Mean Diagnostically
The American Diabetes Association’s 2025 guidelines define three ranges:
- Normal: below 5.7% (below 39 mmol/mol)
- Prediabetes: 5.7% to 6.4% (39 to 47 mmol/mol)
- Diabetes: 6.5% or higher (48 mmol/mol or higher)
A diabetes diagnosis based on A1c requires a certified lab test, not a point-of-care device. A second confirmatory test is typically needed unless symptoms are already present alongside a high result.
How Accurate Are Home A1c Kits?
Home and point-of-care A1c devices exist and are reasonably close to lab results, though not identical. Testing by the New York State Department of Health found that two common point-of-care systems had coefficients of variation of 2% and 3%, compared to 1% for a central laboratory. In practical terms, if your true A1c is 7.0%, a point-of-care device might read anywhere from about 6.8% to 7.2%. That’s fine for tracking trends between lab visits but not reliable enough for an initial diagnosis.
Conditions That Skew the Result
Because A1c depends on hemoglobin inside red blood cells, anything that changes how long those cells survive will throw off the number, even if your actual blood sugar is well controlled. This is an important caveat that pure math can’t fix.
Conditions that shorten red blood cell lifespan, like hemolytic anemia, significant kidney disease, or recovery from major blood loss, will make A1c falsely low. The red blood cells don’t live long enough to accumulate a representative amount of glucose. Dialysis patients are particularly affected; glycated albumin is considered a more reliable marker in that group.
Iron deficiency anemia pushes A1c in the opposite direction, making it falsely high. This is especially relevant in pregnancy, where iron deficiency is common. The mechanism involves a compound called malondialdehyde, which increases during iron deficiency and accelerates the attachment of glucose to hemoglobin. A person with iron deficiency could test at 6.0% while their actual average glucose corresponds to something closer to 5.5%.
Hemoglobin variants also matter. The four most common, hemoglobin S (sickle cell trait), C, D, and E, can interfere with certain lab methods and produce falsely high or low readings depending on the assay. Not every lab method is affected by every variant. The NGSP maintains a list of which testing methods are reliable in the presence of specific variants. If you carry a hemoglobin trait, your doctor can select a method known to be accurate for your situation, or use an alternative marker like fructosamine.
Why You Can’t Just Average Your Meter Readings
A common question is whether you can reverse-engineer your A1c from daily glucose checks. You can get a rough estimate using the eAG formula in reverse: A1c = (average glucose in mg/dL + 46.7) ÷ 28.7. But this estimate is often off because meter readings capture only the moments you test, not the full 24-hour picture. Post-meal spikes, overnight lows, and the hours between checks all influence A1c but may never show up in your log.
Continuous glucose monitors get closer because they sample every few minutes, but even their averages don’t perfectly align with A1c. Individual biology plays a role: two people with identical average glucose levels can have different A1c values because their red blood cells glycate at slightly different rates. This natural variation means the formulas are population averages, not personal guarantees.