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A1C

General

Hemoglobin A1c (Glycated Hemoglobin)

A blood test that reflects average blood sugar levels over the past 2-3 months, used to diagnose and monitor diabetes and prediabetes.

Written by · Reviewed by the thecalcu.com team · Last updated July 4, 2026

What is A1C?

A1C (also called hemoglobin A1c, HbA1c, or glycated hemoglobin) is a blood test that measures the percentage of hemoglobin, the oxygen-carrying protein in red blood cells, that has glucose attached to it. Because red blood cells live for about 2-3 months, the A1C result reflects your average blood sugar level over that period, rather than a single snapshot.

A1C is the primary tool used to:

  • Diagnose prediabetes and type 2 diabetes
  • Monitor blood sugar control in people already diagnosed with diabetes
  • Assess whether a treatment plan (diet, medication, insulin) is working over time

Because higher average blood glucose leads to more glucose molecules binding to hemoglobin, a higher A1C percentage directly corresponds to poorer long-term blood sugar control.

Formula

A1C itself is measured directly from a blood sample in a lab, not calculated from other inputs. However, it converts to an estimated average glucose (eAG) using a standard linear formula derived from the ADAG study:

eAG (mg/dL) = 28.7 × A1C (%) − 46.7

eAG (mmol/L) = 1.59 × A1C (%) − 2.59

This lets patients relate their lab A1C result to the same units shown on a home glucose meter.

Worked Example

A patient receives an A1C result of 7.0%.

eAG = 28.7 × 7.0 − 46.7 = 200.9 − 46.7 = 154.2 mg/dL

Interpretation: An A1C of 7.0% corresponds to an estimated average blood glucose of about 154 mg/dL over the preceding 2-3 months, useful context for comparing against daily glucose meter readings. Use the A1C calculator or estimated average glucose calculator to run this conversion for any A1C value.

Key Things to Know

  • A1C reflects a weighted average: More recent blood sugar levels (the past 30 days) contribute more heavily to the A1C result than glucose levels from 2-3 months ago, because older red blood cells are gradually replaced by new ones.
  • A1C ranges guide diagnosis: Below 5.7% is normal, 5.7%-6.4% is prediabetes, and 6.5% or above (confirmed on repeat testing) indicates diabetes, the same thresholds used alongside fasting glucose and oral glucose tolerance tests.
  • A1C complements, not replaces, related metrics: Insulin resistance markers like HOMA-IR assess a different aspect of glucose metabolism (how well insulin is working), while A1C measures the downstream result (average blood sugar), doctors often use both together for a fuller picture.
  • Diet quality affects both A1C and glucose response: Foods with a high glycemic index cause faster, larger blood sugar spikes, which over time contribute to a higher A1C, making glycemic index a useful day-to-day lever for managing long-term A1C.
  • A1C targets are individualized: While under 7% is a common general target for people with diabetes, doctors may set higher targets (e.g., 7.5%-8%) for older adults or those with other health conditions where tighter control carries more risk than benefit.
  • Certain conditions distort A1C accuracy: Anemia, recent blood transfusion, and some hemoglobin variants can make the test unreliable, so results should always be interpreted alongside a patient's full clinical picture.

Related Calculators

A1c Calculator →Estimate your hemoglobin A1c percentage from average blood glucose using the inverse ADAG formula, with ADA diagnostic category context included instantly.
Estimated Average Glucose Calculator →Convert your A1c percentage into estimated average glucose (eAG) in mg/dL and mmol/L using the published ADAG study formula, with diabetes category context.

Related Terms

HOMA-IRHomeostatic Model Assessment of Insulin ResistanceGlycemic IndexGlycemic Index (GI)

Frequently Asked Questions

What is a normal A1C level?

A normal A1C is below 5.7%. A result between 5.7% and 6.4% indicates prediabetes, and 6.5% or higher on two separate tests indicates diabetes, per American Diabetes Association criteria. Most people with diagnosed diabetes aim for an A1C target below 7%, though targets are individualized based on age and other health conditions. Use the [A1C calculator](/a1c-calculator/) to check where your result falls.

How is A1C different from a regular blood glucose test?

A fingerstick or fasting glucose test measures your blood sugar at a single moment in time, which can vary widely based on your last meal or activity. A1C reflects the average percentage of hemoglobin proteins coated with glucose over the lifespan of red blood cells (about 2-3 months), giving a much more stable, long-term picture of blood sugar control. This is why A1C is used for diagnosis and ongoing monitoring rather than daily glucose checks alone.

What is eAG and how does it relate to A1C?

eAG (Estimated Average Glucose) translates the A1C percentage into the same mg/dL or mmol/L units used on a home glucose meter, making the result easier to relate to daily readings. It uses a standard linear conversion formula validated by a large international study (the A1c-Derived Average Glucose, or ADAG, study). The [estimated average glucose calculator](/estimated-average-glucose-calculator/) performs this conversion automatically from your A1C value.

How often should A1C be tested?

People without diabetes typically don't need routine A1C testing unless risk factors are present. Those with diabetes usually get tested every 3-6 months, with more frequent testing if treatment has recently changed or targets aren't being met. Because A1C reflects roughly a 3-month average, testing more often than every 3 months provides limited additional information.

Can anything other than blood sugar affect A1C results?

Yes. Conditions that affect red blood cell lifespan or hemoglobin, such as anemia, recent blood loss, pregnancy, kidney disease, and certain hemoglobin variants, can make A1C read falsely high or low regardless of actual average blood sugar. In these cases, doctors often rely more heavily on direct glucose monitoring or fructosamine testing instead of A1C alone.