A1C to Blood Sugar Calculator

The A1C to average glucose formula from the ADAG study

An A1C result and an average blood sugar describe the same thing in two different languages. The A1C test reads the percentage of your hemoglobin that has glucose attached to it. The number you see on a glucose meter, in mg/dL or mmol/L, reads the sugar dissolved in the plasma at the moment you test. In 2008 the ADAG study, short for A1C Derived Average Glucose, published a linear regression that maps one onto the other, and the American Diabetes Association adopted it the same year. The formula in mg/dL units is:

eAG mg/dL = 28.7 x A1C - 46.7

Run the numbers for a 7.0% A1C: 28.7 x 7.0 - 46.7 = 154.2 mg/dL, which the ADA rounds to 154. That is the average glucose a person with a 7.0% A1C carried across the study window, and it is the same arithmetic the calculator above performs. The regression was fitted on 507 volunteers across 10 international centers, mixing people with type 1 diabetes, type 2 diabetes and no diabetes at all, so the line holds across a wide range of glycemia rather than for one narrow group.

The study indexed average glucose against continuous glucose monitor readings and frequent fingersticks collected over roughly three months, which is why the result is called an estimated average glucose rather than a prediction. It reports what the A1C mathematically implies, not what any single day looked like. The ADAG paper appeared in Diabetes Care in 2008 under Nathan and colleagues, and the ADA still publishes the same coefficients in its Standards of Care today.

mg/dL and mmol/L: the two unit systems

The United States reports glucose in milligrams per deciliter. Most of the rest of the world reports millimoles per liter. The ADAG team fitted a second regression directly in SI units so nobody has to convert twice:

eAG mmol/L = 1.59 x A1C - 2.59

For a 7.0% A1C that gives 1.59 x 7.0 - 2.59 = 8.5 mmol/L, the same reading as 154.2 mg/dL once you move between unit systems. The bridge between the two is the molar mass of glucose, about 180.16 grams per mole, which puts 1 mmol/L at 18.016 mg/dL. Divide 154.2 by 18.016 and you land on 8.56, which rounds to the 8.5 the direct regression produces, so the two routes agree.

Which unit you need depends on where you live and what your meter shows. A meter bought in the United States prints mg/dL, so the first formula is the one to keep at hand. A reader in the United Kingdom, Canada or Australia sees mmol/L, so the second applies. The calculator handles both, and converting one into the other by hand only takes one division by 18.016.

CDC diagnostic thresholds and the eAG at each boundary

The CDC uses fixed A1C cutoffs to sort results into three bands, and each band maps to an estimated average glucose. The table below pairs the CDC diagnostic wording with the eAG that the 28.7 and 1.59 regressions produce at the edges of each band.

Three boundary readings make the table concrete. At 5.7% the formula gives 28.7 x 5.7 - 46.7 = 116.9 mg/dL, which rounds to 117. At 6.4% it gives 28.7 x 6.4 - 46.7 = 137.0 mg/dL, the top of the prediabetes band. At the 6.5% diabetes threshold it gives 28.7 x 6.5 - 46.7 = 139.9 mg/dL, which rounds to the 140 the ADA quotes in its own conversion materials. These cutoffs come from the CDC diabetes testing page, last reviewed in 2024, and they match the ADA Standards of Care.

A lab report that shows an A1C of 6.0% therefore sits 0.3 points inside the prediabetes band, with an eAG of 28.7 x 6.0 - 46.7 = 125.5 mg/dL, and the same reading in SI units is 1.59 x 6.0 - 2.59 = 7.0 mmol/L. Borderline results deserve a repeat test, because the CDC recommends confirming a diabetes diagnosis with a second A1C or a fasting plasma glucose before treatment starts.

Running the formula in reverse: glucose to A1C

The regression runs both ways. Solving eAG = 28.7 x A1C - 46.7 for A1C gives the inverse formula the blood sugar side of the calculator uses:

A1C % = (eAG mg/dL + 46.7) / 28.7

Take a meter that reports a 30 day average of 130 mg/dL. Plugging it in: (130 + 46.7) / 28.7 = 6.2%, after rounding. A meter average of 154 mg/dL gives (154 + 46.7) / 28.7 = 7.0%, and a reading of 100 mg/dL gives (100 + 46.7) / 28.7 = 5.1%. Each of those maps cleanly back through the forward formula, which is the point: the two directions are one line, not two different models.

One caution applies to the inverse direction. A meter average is only as representative as the readings that went into it. If you test mostly in the morning before breakfast, your average carries more fasting values than a true round the clock picture would, and it will flatter your A1C estimate. The ADAG study averaged continuous monitor data, so a hand picked sample of fingersticks reproduces that average only when it covers fasting, post meal and bedtime readings in realistic proportions.

When a meter average and a lab A1C disagree

Two averages that should agree often do not, and each gap has a known cause. The first is sampling. A meter average only counts the days you tested, while an A1C integrates every hour of the last two to three months. A week of disciplined readings bookended by a holiday of spikes shows up in the A1C and vanishes from the meter average.

The second is biology. Hemoglobin variants such as sickle cell trait, and conditions that change red blood cell turnover, including pregnancy, recent blood loss or kidney disease treated with erythropoietin, shift the A1C without shifting real glucose. The National Institute of Diabetes and Digestive and Kidney Diseases lists anemia and hemoglobin variants among the conditions that can make A1C results inaccurate. In those cases a clinician may rely on a fructosamine test or a CGM derived glucose management indicator instead.

The third is timing. Glucose attached to hemoglobin in the last 30 days contributes about 50% of the A1C value, the 31 to 90 day window about 30%, and days 91 to 120 about 20%, because red blood cells live roughly 120 days. A sharp change in control therefore moves the A1C faster than a naive three month average suggests, and a meter average from the last two weeks will disagree with an A1C drawn the same day for that reason alone.

A1C goals and what they mean as average glucose

For most nonpregnant adults with diabetes the ADA sets an A1C goal below 7.0%, which the regression puts at an eAG of 154 mg/dL or 8.5 mmol/L. The CDC states the same goal on its A1C testing page: for most people with diabetes the A1C goal is 7% or less. Goals move with the person. Older adults with limited life expectancy or recurrent hypoglycemia may carry a looser target such as below 8.0%, while a young adult early in the disease may aim tighter.

Reading a target through the eAG lens makes it concrete. A 6.5% goal means living at or below 139.9 mg/dL on average, which the formula shows as 28.7 x 6.5 - 46.7 = 139.9. A 7.0% goal means 154.2 mg/dL. Neither number says anything about individual days, and that is the point the ADA makes when it recommends pairing A1C with time in range data for anyone using a CGM.

One more anchor helps when you talk to a clinician. An A1C of 5.7% corresponds to 116.9 mg/dL, which is 6.5 mmol/L, and an A1C of 6.4% corresponds to 137.0 mg/dL, or 7.6 mmol/L. Anything between those two values is where the CDC places prediabetes, and preventing progression from that band is the single largest modifiable factor for avoiding type 2 diabetes.

Unit conversion is the other half of the job, and a short reference makes it mechanical. Divide mg/dL by 18.016 to reach mmol/L, or multiply by 18.016 to go back. The pairs below are the ones that come up most in lab reports and meter averages.

Every row runs the same division. The 100 mg/dL row is 100 / 18.016 = 5.55 mmol/L, and the 154 mg/dL row is 154 / 18.016 = 8.55, which matches the eAG that a 7.0% A1C carries. If your meter shows mmol/L and your lab shows A1C, reading across this table is faster than running the regression twice.

A1C to eAG reference table across the clinical range

A1C values land in bands that are easier to read with the numbers side by side. The table below covers the range most lab reports fall into, from a solid normal reading up to a result that signals diabetes has been present for a while. Each row applies the same 28.7 and 1.59 regressions.

Reading down the table shows how fast the glucose equivalent climbs. Each whole A1C point adds about 28.7 mg/dL to the average, so the distance from 6.0% to 8.0% is 2 x 28.7 = 57.4 mg/dL, which the table confirms: 182.9 minus 125.5 is 57.4. That slope is the coefficient itself, and it is the quickest mental shortcut for estimating a conversion without a calculator.

The same slope works in reverse. Every 28.7 mg/dL of sustained average glucose change moves the A1C by one point, so a plan that drops a meter average from 182 to 154 mg/dL is aiming at roughly a one point A1C improvement, from 8.0% down to 7.0%. Framing a goal as a glucose target rather than a percentage is often the easier conversation to have with a care team.