How is A1C and eAG calculated?

Written by: Dr. Albana Greca Sejdini, MD, MMedSc

Medically reviewed by: Dr. Ruden Cakoni, MD, Endocrinologist

Last medically reviewed: July 2026

HbA1c and estimated average glucose describe the same general period of glucose exposure in different units. HbA1c is measured from glycated hemoglobin in a blood sample. eAG is not measured directly; it is calculated from HbA1c using a regression formula.

Quick Answer: The standard conversion is eAG (mg/dL) = 28.7 × A1C − 46.7. An A1C of 7.0% corresponds to an estimated average glucose of approximately 154 mg/dL, or 8.6 mmol/L. eAG is an estimate—not your current glucose, not a treatment dose, and not necessarily identical to the average shown by your meter or CGM.

A1C and eAG Calculator

Convert A1C to estimated average glucose, or convert a true average glucose value back to an estimated A1C. Do not enter one fasting or after-meal reading as though it were an average.

A1C → eAG

Average Glucose → Estimated A1C

Educational limitation: The formulas describe the average relationship found in the ADAG study. Individual A1C may be higher or lower than the calculated value because of red-blood-cell biology, hemoglobin variants, kidney disease, pregnancy, glucose variability, measurement differences, and other factors.

Need only a glucose-unit conversion? Use the mmol/L and mg/dL blood sugar converter.

What Does A1C Measure?

HbA1c—also called A1C, glycated hemoglobin, or hemoglobin A1C—measures the percentage of hemoglobin with glucose attached to it.

Hemoglobin is the oxygen-carrying protein inside red blood cells. Glucose attaches to hemoglobin through a natural nonenzymatic process called glycation. The higher glucose has been over time, the greater the proportion of glycated hemoglobin.

A laboratory reports A1C as:

  • NGSP percentage, such as 7.0%, which is commonly used in the United States; or
  • IFCC mmol/mol, such as 53 mmol/mol, which is commonly used in many other countries.

A1C does not require fasting. For diagnosis, it should be performed using an appropriate standardized laboratory method. Learn how it compares with other tests in the blood tests for diabetes guide.

Why Does A1C Reflect Approximately Two to Three Months?

Red blood cells circulate for roughly 120 days, but A1C is not a simple average of every day across four months. It is a weighted average because circulating blood contains red cells of different ages.

Glucose exposure during the most recent month contributes more strongly than exposure three or four months earlier. This is why a major treatment or glucose change can begin affecting A1C before 120 days have passed.

A1C does not directly show the current glucose level, hypoglycemia frequency, post-meal peaks, overnight patterns, or glucose variability. Two people can therefore have the same A1C but very different glucose patterns and safety risks.

a1c eAG daily glucose

How Is eAG Calculated From A1C?

The A1C-Derived Average Glucose, or ADAG, study compared laboratory A1C with frequent glucose measurements collected through continuous glucose monitoring and structured finger-stick testing. The resulting regression equation was:

eAG (mg/dL) = 28.7 × A1C (%) − 46.7

eAG (mmol/L) ≈ 1.59 × A1C (%) − 2.59

Example: A1C of 7%

eAG = (28.7 × 7) − 46.7 = approximately 154 mg/dL

154 ÷ 18.0182 = approximately 8.6 mmol/L

The reverse equation is:

Estimated A1C (%) = [eAG (mg/dL) + 46.7] ÷ 28.7

The relationship is strong at the population level but does not produce a perfect match for every person.

A1C to Estimated Average Glucose Chart

A1C IFCC eAG mg/dL eAG mmol/L
5%31 mmol/mol975.4
6%42 mmol/mol1267.0
7%53 mmol/mol1548.6
8%64 mmol/mol18310.2
9%75 mmol/mol21211.8
10%86 mmol/mol24013.4
11%97 mmol/mol26914.9
12%108 mmol/mol29816.5

The chart gives estimated averages. It does not mean glucose remained near that value throughout the day.

What A1C Levels Mean for Diagnosis

Diagnostic category A1C IFCC equivalent
Normal diagnostic rangeBelow 5.7%Below 39 mmol/mol
Prediabetes5.7%–6.4%39–47 mmol/mol
Diabetes range6.5% or higher48 mmol/mol or higher

Without unequivocal hyperglycemia, an A1C in the diabetes range normally requires confirmation with a repeat A1C or another abnormal diagnostic test.

These diagnostic categories should not be applied as treatment labels to a person already known to have diabetes. An A1C of 6.2% while using diabetes medicine does not automatically mean the condition has become “prediabetes.”

What Is a Good A1C Target?

For many nonpregnant adults with diabetes, a common treatment goal is below 7%. This is not the correct target for everyone.

A different goal may be appropriate according to age, pregnancy, diabetes duration, severe hypoglycemia, impaired awareness, heart or kidney disease, complications, medicine burden, and personal circumstances. A lower A1C is not automatically better when it is achieved through frequent low glucose.

Learn how goals are individualized in HbA1c Explained: Good Levels and Average Blood Sugar.

Why Might A1C and Home Glucose Readings Not Match?

  • the meter may be checked mainly while fasting and miss after-meal highs;
  • testing may be more frequent on “good” or “bad” days;
  • CGM data may be incomplete or cover a shorter period;
  • recent treatment changes may affect current readings before fully changing A1C;
  • glucose variability can produce the same average through different patterns;
  • A1C may be altered by red-cell or hemoglobin factors;
  • meter technique, strip storage, or laboratory variation may contribute.

Do not assume that the A1C is always wrong or that the meter is always wrong. Compare dates, data completeness, laboratory method, red-cell health, and glucose patterns. See A1C vs. Blood Sugar.

When Can A1C Be Falsely High or Falsely Low?

Situation Possible effect
Recent blood loss, hemolysis, or shortened red-cell survivalMay make A1C falsely low.
Iron-deficiency anemiaMay make A1C falsely high in some circumstances.
Blood transfusionCan make A1C difficult to interpret in either direction.
Erythropoietin treatment or dialysisMay alter red-cell turnover and A1C accuracy.
Hemoglobin variantsMay interfere with some assay methods or change interpretation.
PregnancyChanges red-cell turnover; pregnancy uses specific glucose tests and targets.
Advanced kidney or liver diseaseMay affect red-cell survival and create discordance.

If A1C and repeated glucose data do not match, the clinician may repeat the test, review the laboratory method, use plasma glucose criteria, or consider another marker such as fructosamine or glycated albumin in selected situations.

eAG, Meter Average, CGM Average, and GMI Are Not Identical

  • eAG: calculated from a laboratory A1C using the ADAG formula.
  • Meter average: arithmetic average of the finger-stick readings actually performed.
  • CGM mean glucose: average of available sensor glucose values during the selected period.
  • Glucose Management Indicator: calculated from CGM mean glucose to estimate the A1C commonly associated with that sensor average.

GMI is not a laboratory A1C. CGM also provides time in range, time below range, time above range, and variability—information A1C and eAG cannot show.

How Often Should A1C Be Checked?

Measurement approximately every three months helps determine whether goals have been reached and maintained.

  • About every three months is often appropriate when treatment changes, goals are not met, or glucose is unstable.
  • At least twice yearly may be sufficient when glucose management is stable and goals are being met.
  • Different timing may be needed during pregnancy, major illness, rapid treatment changes, or when A1C is unreliable.

What A1C Level Requires Hospitalization?

There is no single A1C level that automatically requires hospitalization. A1C reflects chronic glucose exposure and cannot show whether a person is currently dehydrated, acidotic, producing ketones, severely hypoglycemic, or mentally altered.

A very high A1C suggests prolonged hyperglycemia and needs prompt treatment review. Emergency or hospital care depends mainly on current glucose, ketones, vomiting, hydration, breathing, mental status, and the possibility of DKA or HHS.

Seek emergency medical care for vomiting, abdominal pain, deep or difficult breathing, fruity-smelling breath, confusion, marked drowsiness, severe dehydration, inability to keep fluids down, seizure, unconsciousness, or moderate-to-large ketones. Do not wait for an A1C test when a glucose emergency is possible.

Common Questions About A1C and eAG

Is eAG the same as fasting glucose?

No. Fasting glucose is one measurement after fasting. eAG estimates average glucose across day and night over a longer period.

Can I calculate A1C from one glucose reading?

No. Entering one reading into the reverse calculator produces a mathematically possible number but not a medically meaningful A1C estimate.

Does an A1C of 7% mean glucose stayed near 154 mg/dL?

No. Glucose may have moved above and below 154 mg/dL. The estimate does not reveal variability or hypoglycemia.

Can A1C change in one month?

Yes. Recent weeks have greater influence, so a major sustained glucose change can alter A1C before three months have passed.

Why does laboratory eAG differ from my CGM average?

The laboratory eAG is calculated from A1C, while the CGM average is calculated from sensor readings during a specific period. Biological and measurement differences can produce discordance.

Doctor’s Note: I use A1C to understand longer-term glucose exposure, but I do not interpret it alone. Meter or CGM patterns show whether the average hides hypoglycemia, post-meal spikes, overnight highs, or large variability. When A1C and glucose disagree, I investigate the difference before changing treatment.

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Related Resources

Medical disclaimer: This calculator and article provide general education. They do not diagnose diabetes, set an A1C goal, prescribe treatment, or determine whether emergency care is required. Do not change insulin or medicine based only on a calculated eAG or estimated A1C.

References

  1. American Diabetes Association: Diagnosis and Classification of Diabetes—Standards of Care in Diabetes 2026
  2. American Diabetes Association: Glycemic Goals, Hypoglycemia, and Hyperglycemic Crises—2026
  3. NIDDK: The A1C Test and Diabetes
  4. NGSP: HbA1c and Estimated Average Glucose
  5. NGSP: Factors That Interfere With HbA1c Test Results
  6. NGSP: IFCC Standardization of HbA1c
  7. Nathan and colleagues: Translating the A1C Assay Into Estimated Average Glucose