The A1C Calculator is a free online tool that converts an A1C (HbA1c) percentage into the estimated Average Glucose value you would see on a blood glucose meter, in either mg/dL (the standard unit in the United States) or mmol/L (the standard unit across the United Kingdom, the European Union, Australia, Canada, and most of the rest of the world). It works in both directions, so you can also enter an average glucose number and see the equivalent A1C. The calculator uses the formula published by the American Diabetes Association from the 2008 ADAG (A1c-Derived Average Glucose) study, the same formula used by the ADA's own glucose conversion calculator and by clinical lab reports across the United States.
A1C, sometimes written HbA1c, glycated hemoglobin, or glycohemoglobin, measures the percentage of hemoglobin molecules inside your red blood cells that have glucose chemically bonded to them. Glucose attaches to hemoglobin slowly and irreversibly while the red blood cell circulates, and red blood cells live for roughly 120 days, so the A1C result is a weighted time-average of every glucose value you carried over the previous two to three months. The most recent month counts roughly twice as much as the month before, so a recent change in glucose control is reflected in your A1C, but not as quickly as you might expect. This is why clinicians wait at least three months between A1C tests when adjusting treatment.
The relationship between A1C and average glucose was put on a firm clinical footing by the 2008 A1c-Derived Average Glucose (ADAG) study, led by David M. Nathan and published in Diabetes Care. The study followed 507 participants with type 1 diabetes, type 2 diabetes, or no diabetes for three months, collecting roughly 2,700 glucose values per person from a combination of continuous glucose monitoring and seven-point fingerstick profiles. A simple linear regression of mean glucose against A1C produced the formula eAG (mg/dL) = 28.7 × A1C (%) − 46.7, with an R-squared of about 0.84. That means roughly 84% of the variability in average glucose at a given A1C is explained by A1C alone, and the remaining 16% is individual biology. The same formula is used by this calculator and by every standard lab report that prints "estimated average glucose" alongside the A1C result.
Reading the diagnostic ranges. The American Diabetes Association Standards of Care define three A1C ranges for diagnosis. Below 5.7% is normal. 5.7% to 6.4% is prediabetes — a state where insulin resistance is climbing and beta-cell function is starting to fail, and where lifestyle changes can often prevent or significantly delay progression to type 2 diabetes. 6.5% or higher, confirmed on two separate tests, is diabetes. For people who already have diabetes, the typical treatment target is below 7%, although ADA explicitly endorses individualized targets: tighter (below 6.5%) for younger, healthier adults at low risk of hypoglycemia, and more relaxed (below 8% or below 8.5%) for older adults with limited life expectancy, severe hypoglycemia history, advanced complications, or extensive comorbidities. The diagnostic chip in this calculator's result card uses the ADA general thresholds; your personal target should always be set with your clinician.
Why eAG matters to patients. An A1C result of 7.5% does not mean very much intuitively — percentages of glycated hemoglobin are not a concept anyone meets in daily life. But that same A1C expressed as eAG — 169 mg/dL or 9.4 mmol/L — is exactly the kind of number you see on the meter every day. Reporting the two side by side makes it instantly clear whether your long-term average is in the same ballpark as your daily readings, whether your readings on the meter cluster above or below the long-term mean, and how much room you have to your personal target. The ADA pushed clinical labs and electronic health record systems to start reporting eAG alongside A1C precisely for this reason: it closes the loop between the daily numbers patients see and the quarterly summary their endocrinologist talks about.
Converting between mg/dL and mmol/L. The conversion factor is fixed by chemistry: one mole of glucose weighs 180.182 grams, so one millimole per liter equals 180.182 mg per liter, which equals 18.0182 mg per deciliter. To convert mg/dL to mmol/L, divide by 18.0182. To convert mmol/L to mg/dL, multiply by 18.0182. For example, an eAG of 154 mg/dL is 154 / 18.0182 = 8.546 mmol/L, which rounds to 8.6 mmol/L; conversely an eAG of 7.0 mmol/L is 7.0 × 18.0182 = 126.1 mg/dL, which rounds to 126 mg/dL. This calculator displays both units side by side so you do not need to do the arithmetic yourself.
How to lower your A1C. The evidence base for lowering A1C is strong and broadly aligned across medical societies. Eating fewer ultra-processed refined carbohydrates and added sugars, and replacing them with vegetables, legumes, whole grains, and lean protein, lowers A1C reliably across diet patterns (Mediterranean, low-carb, plant-based, and DASH all work). 150 minutes of moderate aerobic activity per week, plus two to three sessions of resistance training, lowers A1C by an average of about 0.5–0.7%. Sleeping seven to nine hours per night and managing chronic stress have measurable effects too. On the medication side, metformin remains the cornerstone first-line treatment for type 2 diabetes, and the GLP-1 receptor agonists (semaglutide, dulaglutide, tirzepatide) and SGLT-2 inhibitors (empagliflozin, dapagliflozin) have driven significant A1C reductions plus cardiovascular and renal benefits in major trials. Continuous glucose monitoring is increasingly available at low cost and lets you see the moment-to-moment effect of every meal, walk, and night of sleep on your glucose, which often does more than any single intervention to motivate sustainable change.
Important accuracy caveats. The ADAG formula is an excellent average, but several conditions can shift your eAG away from the prediction in either direction. Anything that shortens red blood cell lifespan — hemolytic anemia, sickle cell disease, hereditary spherocytosis, recent significant blood loss, or recent blood transfusion — makes A1C read falsely low for a given true average glucose, because new red blood cells have had less time to glycate. Conditions that lengthen red blood cell lifespan — splenectomy, iron-deficiency anemia, vitamin B12 deficiency — can make A1C read falsely high. Hemoglobin variants (HbS, HbC, HbE, HbF in adults) interfere with some A1C assays. Late pregnancy lowers A1C because of increased red cell turnover. Advanced kidney disease and erythropoietin therapy both affect A1C reliability. If you have any of these conditions, an alternate measure such as fructosamine, glycated albumin, or CGM-derived Glucose Management Indicator (GMI) may be more reliable than A1C for monitoring control.
A1C versus CGM-derived GMI. Continuous glucose monitors compute a separate metric called the Glucose Management Indicator, defined as GMI (%) = 3.31 + 0.02392 × mean CGM glucose (mg/dL), from the 2018 Bergenstal study. GMI is similar to A1C but not identical; for any individual, the difference between A1C and GMI can be up to about 1 percentage point in either direction, mostly because of the same red-blood-cell-lifespan issues that shift A1C in the first place. If your CGM GMI and your lab A1C disagree by more than half a percentage point, that gap is information — not error — and it is worth a conversation with your clinician about whether to weight one measure more than the other when setting targets and adjusting treatment.
Glucose units around the world. mg/dL (milligrams per deciliter) is the standard reporting unit in the United States, parts of Latin America, and a handful of Middle Eastern and Asian countries. mmol/L (millimoles per liter) is the standard across the United Kingdom, Ireland, Australia, New Zealand, Canada, and most of continental Europe — including Germany, France, Spain, Italy, the Netherlands, Sweden, Norway, Denmark, Finland, Poland, Hungary, and Czechia. Many countries (China, Japan, India, South Africa, Egypt) report both, depending on the lab. International scientific publications increasingly report both side by side. This calculator switches between the two units with a single toggle and persists your preference on your device, so once you pick the unit you actually use, the calculator opens to it next time.
Medical disclaimer. This calculator is provided for informational and educational purposes only. It is not a medical device, it does not diagnose any condition, and its results are not a substitute for professional medical advice, diagnosis, or treatment. The diagnostic ranges shown reflect the general thresholds published by the American Diabetes Association in its current Standards of Care; your personal A1C target may differ. Always discuss your A1C results, your eAG, and any change to your treatment plan with a qualified healthcare professional. If you think you are having a medical emergency, call your local emergency number immediately. The conversion math used by this calculator comes from Nathan DM et al., "Translating the A1C Assay into Estimated Average Glucose Values," Diabetes Care 31(8):1473–1478 (2008).