A1C Calculator guide
Type an A1C percentage or an average glucose and get the other one instantly, in mg/dL, mmol/L, and IFCC mmol/mol, using the same ADAG formula the American Diabetes Association uses.
What your A1C number means
A1C (also written HbA1c) is the percentage of your hemoglobin that has glucose stuck to it. Red blood cells live about three months, so A1C reflects your average blood sugar over roughly the past 2-3 months, weighted toward the most recent weeks. It doesn't need fasting, which is why doctors like it for screening.
The catch: nobody thinks in "percent glycated hemoglobin." You think in the mg/dL your meter or CGM shows. Estimated average glucose (eAG) translates one into the other.
The ADAG formula
The conversion comes from the A1c-Derived Average Glucose (ADAG) study, led by David Nathan and published in Diabetes Care in 2008. Researchers at 10 centers tracked 507 adults (type 1, type 2, and no diabetes) for three months with continuous glucose monitors plus fingersticks, then compared the average to their A1C. The best fit was a straight line: eAG (mg/dL) = 28.7 x A1C - 46.7, with R² = 0.84.
To go the other way, rearrange it: A1C = (eAG + 46.7) / 28.7. For mmol/L, divide mg/dL by 18.016 (the molar mass of glucose over 10), which works out to about 1.59 x A1C - 2.59.
Many labs outside the US report A1C in mmol/mol (the IFCC unit). The official NGSP-IFCC master equation is IFCC = (A1C % - 2.15) x 10.929. The calculator shows all three.
Worked example
Your lab report says A1C 7.0%. eAG = 28.7 x 7.0 - 46.7 = 154.2, so about 154 mg/dL, or 154.2 / 18.016 = 8.6 mmol/L. In IFCC units: (7.0 - 2.15) x 10.929 = 53 mmol/mol.
Now reverse it. Your CGM says your 90-day average is 154 mg/dL. (154 + 46.7) / 28.7 = 6.99, so about 7.0%. The two directions agree because they are the same line.
Reference points worth memorizing: 5.7% is about 117 mg/dL, 6.5% is about 140 mg/dL, and 8.0% is about 183 mg/dL.
The ranges, and what they do and don't mean
The American Diabetes Association uses these cut points: below 5.7% is normal, 5.7-6.4% is prediabetes, and 6.5% or higher is in the diabetes range. For many non-pregnant adults who already have diabetes, the ADA's general treatment goal is below 7%, but your own target may be higher or lower depending on age, other conditions, and risk of low blood sugar. That's a conversation with your care team, not a number to copy from a website.
A single high result is not a diagnosis. The ADA calls for a confirming test (a repeat A1C or a different glucose test) unless there are clear symptoms with very high blood sugar.
When A1C and your meter disagree
eAG is a population average. In the ADAG data, people with an A1C of 7% had measured averages ranging from about 123 to 185 mg/dL (the 95% range around 154). If your CGM average and your lab A1C don't line up, neither is necessarily wrong.
Several conditions distort A1C itself: iron-deficiency anemia can push it up; blood loss, transfusion, hemolysis, late pregnancy, and advanced kidney disease can push it down; and some hemoglobin variants, such as sickle cell trait, interfere with certain lab methods. If you have any of these, your clinician may lean on CGM data or fructosamine instead.
Common mistakes
Changing medication because of this conversion. Never adjust insulin or other diabetes drugs based on a calculator. Dosing depends on patterns, lows, and timing that an average hides.
Comparing eAG to a fasting number. Fasting glucose is one moment; eAG averages every hour, including after meals. An eAG of 154 with fasting readings around 120 is common.
Ignoring lows. A "good" A1C can hide frequent low blood sugars balanced by highs. If you have lows, symptoms, or big swings, bring your meter or CGM data to your appointment.
Waiting it out. If you land in the prediabetes range, that's the most fixable point. Ask about a CDC-recognized lifestyle change program; the Diabetes Prevention Program trial cut progression to type 2 diabetes by 58% with modest weight loss and activity.
How we calculate: sources
Frequently asked questions
How do you convert A1C to average blood sugar?
Use the ADAG formula: estimated average glucose (mg/dL) = 28.7 x A1C - 46.7. An A1C of 7% equals about 154 mg/dL (8.6 mmol/L).
What A1C is normal?
The American Diabetes Association defines below 5.7% as normal, 5.7-6.4% as prediabetes, and 6.5% or higher as the diabetes range. A diagnosis needs a confirming test.
What is an A1C of 6.5 in blood sugar?
About 140 mg/dL (7.8 mmol/L): 28.7 x 6.5 - 46.7 = 139.9. That is the ADA threshold for the diabetes range.
How do I convert average glucose back to A1C?
A1C = (average glucose in mg/dL + 46.7) / 28.7. A 90-day CGM average of 154 mg/dL works out to about 7.0%.
Why doesn't my A1C match my meter or CGM average?
eAG is a population average. In the ADAG study, people with an A1C of 7% had real averages from about 123 to 185 mg/dL. Anemia, kidney disease, pregnancy, and hemoglobin variants can also skew A1C.
What A1C goal should I aim for?
For many non-pregnant adults with diabetes, the ADA's general goal is below 7%, but targets are individualized. Agree on yours with your clinician, and never adjust medication based on a calculator.
When should I see a doctor about my A1C?
If your A1C is 5.7% or higher and you haven't discussed it, if you're pregnant, or if it changed a lot. Seek same-day care for glucose over 300 mg/dL or symptoms like extreme thirst, vomiting, or confusion.
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