What is A1C?
A1C (also written HbA1c) is a blood test that measures the percentage of your hemoglobin with glucose attached to it. Hemoglobin is the protein in red blood cells that carries oxygen, and red blood cells live for roughly 90 days. Because of that lifespan, the A1C test works like a three-month average of your blood sugar, which is exactly why it became the gold-standard test for diagnosing diabetes and tracking how well treatment is working.
Unlike a finger-stick reading, which tells you your glucose at a single moment, A1C cannot be skewed by what you ate the night before or whether you remembered to test that morning. It smooths out the daily ups and downs into one dependable percentage.
What does A1C measure?
When glucose circulates in your blood, some of it permanently sticks to hemoglobin, a process called glycation. The more glucose in your bloodstream over time, the more glycated hemoglobin builds up. The A1C percentage tells your care team how much sugar your red blood cells have absorbed over their lifetime. Higher average glucose means a higher A1C; better control means a lower one.
How does A1C convert to average glucose?
Because most people think in meter numbers rather than percentages, researchers developed a way to translate A1C into estimated average glucose, or eAG. The ADAG study produced the formula clinicians use: eAG (mg/dL) = 28.7 × A1C − 46.7. Divide by 18.0182 for mmol/L. A 7.0% A1C, for example, works out to about 154 mg/dL.
You can convert any value with our free A1C calculator, or look it up on the full A1C to eAG conversion chart. CGM users can also compare sensor data against lab results with the GMI to A1C calculator.
Normal A1C levels and diabetes diagnosis
Below 5.7% is normal. An A1C between 5.7% and 6.4% indicates prediabetes, a warning zone where lifestyle changes genuinely can reverse course. An A1C of 6.5% or higher, confirmed on a repeat test, indicates diabetes. In eAG terms, 5.7% corresponds to about 117 mg/dL and 6.5% to about 140 mg/dL of average glucose.
| Category | A1C | eAG (mg/dL) | What it means |
|---|---|---|---|
| Normal | Below 5.7% | Below 117 | No diabetes; keep up healthy habits |
| Prediabetes | 5.7% to 6.4% | 117 to 140 | At risk; lifestyle changes can reverse it |
| Diabetes | 6.5% or higher | 140 or higher | Diabetes range; follow your care plan |
One caveat worth knowing: certain conditions, anemia, recent blood loss, kidney disease, some hemoglobin variants, and pregnancy, can make A1C read higher or lower than your true average glucose. If your A1C ever seems out of step with your meter, mention it to your clinician.
How to lower your A1C
Lowering A1C takes steady habits rather than a perfect week: trim refined carbohydrates and sugary drinks, take a short walk after meals, build muscle with resistance exercise, protect your sleep, and follow the medication plan your clinician prescribes. Because A1C reflects three months of red blood cell turnover, expect meaningful movement to take about three months of consistent change. Roughly a 1% drop is a realistic, worthwhile milestone. Always talk with your doctor before changing anything about your treatment.
What is A1C: frequently asked questions
What is A1C in a blood test?
A1C is a blood test that measures the percentage of glycated hemoglobin. It reflects your average blood glucose over the previous 2–3 months and is the standard test for diagnosing and monitoring diabetes.
What does A1C measure?
A1C measures how much glucose is attached to hemoglobin in your red blood cells. Since red blood cells live about 90 days, A1C gives a 2–3 month average of blood sugar levels.
What is a normal A1C by age?
For most healthy adults, a normal A1C is below 5.7% regardless of age. A1C of 5.7–6.4% indicates prediabetes, and 6.5% or higher indicates diabetes. Older adults may have slightly higher individualized targets under medical guidance.
What A1C level is considered diabetes?
An A1C of 6.5% or higher on two separate tests indicates diabetes. An A1C between 5.7% and 6.4% indicates prediabetes, and below 5.7% is considered normal.
How to lower your A1C?
Lowering A1C typically involves reducing refined carbohydrates and added sugar, increasing physical activity, managing stress, improving sleep, staying hydrated, and following your clinician's medication plan. Ask your doctor before making changes.
What is the difference between A1C and eAG?
A1C is the percentage of glycated hemoglobin reported by the lab, while eAG (estimated average glucose) is the same result expressed as an everyday blood sugar number in mg/dL or mmol/L. The ADAG formula converts between them: eAG = 28.7 × A1C − 46.7.
How often should you check your A1C?
Most guidelines suggest every 3 months for people with diabetes who are changing treatment, and every 6 months for those stable at their goal. People with prediabetes are typically retested about once a year.
Key takeaways
- A1C measures the percentage of glycated hemoglobin and reflects your average blood glucose over the past 2 to 3 months.
- A normal A1C is below 5.7%. Prediabetes runs from 5.7% to 6.4%, and diabetes is 6.5% or higher on a confirmed test.
- A1C converts to estimated average glucose with the ADAG formula: eAG (mg/dL) = 28.7 × A1C − 46.7. A 7.0% A1C equals about 154 mg/dL.
- A1C cannot be skewed by a single meal, but conditions like anemia, recent blood loss, kidney disease, or pregnancy can distort the result.
- Lowering A1C takes consistent habits in diet, exercise, sleep, and medication, and meaningful change usually shows up over about three months.
Conclusion
The A1C test is the most reliable long-term snapshot of your blood sugar because it averages roughly three months of glucose exposure into a single percentage. Knowing what the test measures, where your number falls, and how it translates into everyday glucose numbers makes lab results far easier to act on. If your A1C is above the normal range, talk with your clinician about a plan; if it is in range, keep doing what works. Use the free A1C calculator to convert every result and track your progress between visits.