A1C Calculator & A1C Chart

A1C at a glance

A1C (also called HbA1c or glycated hemoglobin) is a blood test that shows your average blood sugar over the past two to three months. A normal A1C is below 5.7% (39 mmol/mol), 5.7–6.4% is prediabetes, and 6.5% (48 mmol/mol) or higher is in the diabetes range when confirmed. Use the A1C calculator above to convert your result to average glucose and mmol/mol, and the A1C chart below to see where any value falls.

A1C and average glucose: the short answer

Your A1C (also written HbA1c) can be translated into an estimated average glucose (eAG) with a simple formula from the A1C-Derived Average Glucose (ADAG) study: eAG (mg/dL) = 28.7 × A1C − 46.7. To get mmol/L, divide the result by 18. An A1C of 7% therefore corresponds to an eAG of about 154 mg/dL (8.6 mmol/L), and an A1C of 6.5% – the diagnostic threshold for diabetes – to about 140 mg/dL (7.8 mmol/L). The calculator above does this in both directions, and the chart below lists every common A1C value.

Translating A1C into a glucose number helps because most people with diabetes think in terms of the readings on their meter or continuous glucose monitor (CGM), not percentages. It also helps explain why an A1C of 8% is a meaningful difference from 7%: it means average glucose about 29 mg/dL (1.6 mmol/L) higher, around the clock, for months. This guide explains what A1C measures, where the formula comes from, how accurate eAG is for an individual, and why A1C and meter averages sometimes disagree.

What A1C measures

Glucose in the blood attaches slowly and irreversibly to hemoglobin inside red blood cells – a process called glycation. The higher the average glucose, the more hemoglobin becomes glycated. HbA1c is the specific glycated form measured by laboratories, expressed as a percentage of total hemoglobin (NGSP/DCCT units) or in mmol of HbA1c per mol of hemoglobin (IFCC units).

Red blood cells live for about 120 days, so A1C reflects average glucose over roughly the previous two to three months. It is weighted towards the most recent weeks: about half of the A1C value is determined by glucose levels in the last 30 days, and about a quarter by the 30 days before that.

A1C is used to:

• diagnose prediabetes and diabetes (with fasting glucose or an oral glucose tolerance test as alternatives)
• monitor long-term glucose control, usually every three to six months
• guide treatment targets, which are individualised

Where the eAG formula comes from

For many years, there was no agreed way to convert A1C into an average glucose figure. The ADAG study, published in 2008, addressed this. It enrolled 507 adults with type 1 diabetes, type 2 diabetes or no diabetes, in ten centres in the US, Europe, Africa and Asia. Over three months, participants wore continuous glucose monitors for at least two days every four weeks and performed frequent finger-stick tests, generating about 2,700 glucose readings per person. Their A1C at the end of the period was then compared with their average glucose.

The relationship was close to a straight line, described by:

• eAG (mg/dL) = 28.7 × A1C (%) − 46.7
• eAG (mmol/L) = 1.59 × A1C (%) − 2.59

The correlation was strong (r² = 0.84), and the American Diabetes Association and other organisations adopted eAG as a way of reporting A1C in glucose units.

Reversing the formula

If you know your average glucose and want to estimate A1C: A1C (%) = (average glucose in mg/dL + 46.7) ÷ 28.7. For an average of 130 mg/dL, that gives (130 + 46.7) ÷ 28.7 = 6.2%. The calculator above includes this reverse direction.

How to calculate eAG step by step

1. Take your A1C as a percentage (NGSP units). If your result is in mmol/mol, convert it first – see our HbA1c mmol/mol to % converter.
2. Multiply by 28.7.
3. Subtract 46.7. The result is eAG in mg/dL.
4. Divide by 18.016 (or simply 18) for mmol/L.

Worked examples

• A1C 5.0%: 28.7 × 5.0 − 46.7 = 97 mg/dL (5.4 mmol/L).
• A1C 5.7% (the lower limit of prediabetes): 28.7 × 5.7 − 46.7 = 117 mg/dL (6.5 mmol/L).
• A1C 6.5% (the diabetes threshold): 28.7 × 6.5 − 46.7 = 140 mg/dL (7.8 mmol/L).
• A1C 7.0% (a common treatment target): 28.7 × 7.0 − 46.7 = 154 mg/dL (8.6 mmol/L).
• A1C 8.0%: 28.7 × 8.0 − 46.7 = 183 mg/dL (10.2 mmol/L).
• A1C 10.0%: 28.7 × 10.0 − 46.7 = 240 mg/dL (13.3 mmol/L).

A simple rule of thumb follows from the formula: each 1 percentage point of A1C corresponds to about 29 mg/dL (1.6 mmol/L) of average glucose.

A1C chart

The A1C chart below lists common A1C values with the matching IFCC value in mmol/mol, the estimated average glucose in mg/dL and mmol/L, and the American Diabetes Association category. All values are calculated from the NGSP–IFCC master equation and the ADAG formula.

What A1C levels mean

The American Diabetes Association uses these diagnostic categories for A1C:

The World Health Organization and the UK's NICE use 48 mmol/mol (6.5%) for diagnosing diabetes and, in the UK, 42–47 mmol/mol (6.0–6.4%) to identify people at high risk ("non-diabetic hyperglycaemia"). In the absence of symptoms, a diagnosis of diabetes requires two abnormal results, either from the same test repeated or from two different tests.

Normal A1C levels by age

There is no separate diagnostic cut-off by age: below 5.7% is normal at any age in adults, and the same thresholds are used for diagnosing diabetes. In practice, though, A1C drifts slightly upwards with age in people without diabetes. Studies of non-diabetic adults have found average A1C values around 5.2–5.4% in people in their 20s and 30s, rising to around 5.5–5.6% in people over 70 – an increase of roughly 0.1 percentage point per decade in the Framingham Offspring and NHANES data. This is one reason why more older adults fall into the prediabetes range.

Women and men have very similar normal A1C values. Some studies have found that people of Black, Hispanic and Asian ancestry have A1C values slightly higher, on average, than white people at the same glucose levels, although the clinical meaning of this difference is still debated.

A1C in mmol/mol

Outside the US, A1C is usually reported in mmol/mol (IFCC units). The two systems are linked by a fixed master equation: % = 0.09148 × mmol/mol + 2.152. Key equivalents are 39 mmol/mol = 5.7%, 42 mmol/mol = 6.0%, 48 mmol/mol = 6.5%, 53 mmol/mol = 7.0%, 64 mmol/mol = 8.0% and 75 mmol/mol = 9.0%. Our HbA1c mmol/mol to % converter explains the history of the two units in detail.

Prediabetes A1C range: what it means

An A1C of 5.7–6.4% (39–46 mmol/mol) means your blood sugar is higher than normal but not in the diabetes range. In the US, about one in three adults has prediabetes, and most do not know it. The risk of progressing to type 2 diabetes rises across the range – people at 6.0–6.4% have a substantially higher risk than those at 5.7–5.9%. The good news is that the risk can be cut: in the Diabetes Prevention Program, losing about 7% of body weight and doing 150 minutes of moderate activity a week reduced the development of diabetes by 58% over about three years. People with prediabetes should usually have their A1C checked every year.

Treatment targets

Targets are individualised. Common reference points are:

• ADA: an A1C below 7% (53 mmol/mol) for many non-pregnant adults, with a lower goal (such as 6.5%) if it can be achieved safely without significant hypoglycaemia, and a less strict goal (such as below 8%) for people with limited life expectancy, frequent hypoglycaemia or extensive other illnesses.
• NICE (type 2 diabetes): 48 mmol/mol (6.5%) for people managed with lifestyle and a single medicine not associated with hypoglycaemia, and 53 mmol/mol (7.0%) for those on medicines that can cause hypoglycaemia.
• NICE (type 1 diabetes): 48 mmol/mol (6.5%) or lower, while minimising hypoglycaemia.
• Pregnancy: stricter targets, often below about 6–6.5% (42–48 mmol/mol), if achievable without hypoglycaemia.

How accurate is eAG for an individual?

The ADAG formula describes the average relationship across many people. For any individual, actual average glucose can differ considerably. In the ADAG data, for an A1C of 7%, the 95% range of measured average glucose ran from about 123 to 185 mg/dL (6.8–10.3 mmol/L). Put simply, two people with the same A1C can have average glucose levels that differ by 30 mg/dL or more.

Some of this variation is due to individual differences in red cell lifespan and in how readily hemoglobin is glycated (sometimes called the "glycation gap"). It means eAG is best used as a helpful translation of A1C, not a precise measurement.

Why your meter or CGM average may differ from eAG

It is common for the average on a glucose meter to differ from eAG. Reasons include:

• Testing pattern. Finger-stick averages over-represent the times you test – often before meals and in the morning – and miss highs after meals and lows overnight. A CGM average is much more representative.
• Time window. Meter averages often cover 7, 14 or 30 days, while A1C reflects about three months, weighted towards the recent weeks.
• Red cell factors. Anything that shortens red cell lifespan lowers A1C relative to true glucose: haemolysis, recent blood loss, blood transfusion, treatment of iron deficiency with iron, pregnancy, and erythropoietin therapy. Anything that prolongs it – such as untreated iron deficiency or B12 deficiency – can raise A1C.
• Hemoglobin variants. Sickle cell trait, hemoglobin C, E and others can interfere with some A1C methods. The NGSP publishes information on which methods are affected.
• Kidney disease. Advanced CKD, and especially dialysis, can make A1C unreliable.
• Ethnicity. Some studies have found slightly higher A1C for the same glucose levels in Black and some Asian populations, though the clinical significance is debated.

When A1C and glucose readings disagree substantially, clinicians may use CGM data, fructosamine or glycated albumin to get a clearer picture.

eAG versus GMI

If you use a CGM, your software probably reports a Glucose Management Indicator (GMI) – you can calculate it with our GMI calculator. GMI works in the opposite direction from eAG: it estimates what your A1C would be, based on the average glucose recorded by the CGM. The formula is GMI (%) = 3.31 + 0.02392 × mean glucose (mg/dL).

GMI and laboratory A1C can differ by 0.5 percentage points or more in either direction for the reasons described above. The international consensus on CGM data recommends using GMI alongside, not instead of, laboratory A1C, and paying particular attention to time in range – the percentage of readings between 70 and 180 mg/dL (3.9–10.0 mmol/L), with a target of more than 70% for most adults with diabetes.

How much can A1C change, and how fast?

Because A1C reflects about three months of glucose, weighted to the most recent weeks, it changes gradually. After a major change in treatment, a meaningful fall is usually visible within six weeks, but the full effect takes about three months. This is why A1C is not normally rechecked more often than every three months.

Typical A1C reductions from common interventions, when starting from a raised level, are approximately:

Each 1 percentage point fall corresponds to about 29 mg/dL (1.6 mmol/L) lower average glucose. Large trials, such as the UK Prospective Diabetes Study, found that each 1 percentage point reduction in A1C was associated with substantially fewer small-blood-vessel complications affecting the eyes, kidneys and nerves.

A1C in children and older adults

In children and adolescents with type 1 diabetes, the ADA and the International Society for Pediatric and Adolescent Diabetes recommend an A1C target below 7% (53 mmol/mol) for most, and NICE recommends 48 mmol/mol (6.5%) or lower. CGM metrics are increasingly used alongside A1C.

In older adults, targets are relaxed when health is complex or life expectancy limited, because the risks of hypoglycaemia – falls, confusion, heart rhythm problems – may outweigh the long-term benefits of tight control. The ADA suggests an A1C below about 7.0–7.5% for healthy older adults and below 8.0% for those with complex health needs, avoiding reliance on A1C alone in very poor health.

A brief history of A1C standardisation

Different A1C methods once gave substantially different results. The National Glycohemoglobin Standardization Program (NGSP) was set up in 1996 to align US results with the method used in the Diabetes Control and Complications Trial (DCCT), which established the link between A1C and complications. Later, the IFCC developed a more specific reference method, which gives lower numbers and is reported in mmol/mol. Laboratories in many countries now report IFCC units, and the two are related by a fixed "master equation". This is why the same A1C may be written as 7.0% or 53 mmol/mol.

When A1C should not be relied on

Laboratory A1C may be misleading, and glucose-based tests are preferred, in:

• pregnancy (especially the second and third trimesters) and the postpartum period
• conditions with altered red cell turnover, such as haemolytic anaemia, recent blood loss or transfusion
• advanced chronic kidney disease, particularly on dialysis or with erythropoietin therapy
• HIV treated with some medicines
• rapidly developing diabetes, such as type 1 diabetes in children, where A1C lags behind glucose

Using eAG in conversations with your care team

eAG is most useful as a bridge between the laboratory report and daily glucose readings. If your A1C is 8.0% (eAG about 183 mg/dL, 10.2 mmol/L) but most of the readings you see on your meter are around 130 mg/dL (7.2 mmol/L), that gap is a signal worth discussing: it may mean that high readings are occurring at times you are not testing, such as after meals or overnight, or that a red-cell factor is affecting the A1C. Bringing both numbers to an appointment, ideally with CGM data or a structured testing record covering different times of day, helps your care team decide whether to change treatment or investigate further.

When to see a doctor

See a doctor if your A1C is 5.7% (39 mmol/mol) or higher and you have not been assessed for prediabetes or diabetes, or if it is 6.5% (48 mmol/mol) or higher. If you already have diabetes, discuss your results if your A1C is above your agreed target, if it has risen since your last test, or if your meter or CGM readings do not seem to match your A1C. Seek urgent care for symptoms of very high glucose – extreme thirst, frequent urination, weight loss, vomiting, abdominal pain or drowsiness – or for repeated hypoglycaemia.