By If you have had a diabetes check, you have probably seen a result labelled HbA1c, A1c, or glycated haemoglobin. Unlike a finger-prick sugar test, which shows your blood sugar at one moment, HbA1c gives a picture of your average blood sugar over the past two to three months.
This guide explains what HbA1c measures, what the ranges mean, how to convert between percentage and mmol/mol, and what can make the result less accurate.
What Is HbA1c?
Haemoglobin is the protein in red blood cells that carries oxygen. When sugar (glucose) circulates in your blood, some of it sticks to haemoglobin. This sugar-coated haemoglobin is called glycated haemoglobin, or HbA1c.
The higher your average blood sugar, the more haemoglobin becomes glycated. Because red blood cells live for about three months, HbA1c reflects your average blood sugar over roughly the past 8 to 12 weeks, with recent weeks counting the most.
Why HbA1c Matters
- Diagnosing diabetes and prediabetes without needing to fast
- Monitoring diabetes control over time
- Guiding treatment decisions
- Estimating long-term risk of complications such as eye, kidney, and nerve damage
Two Ways to Report HbA1c
HbA1c is reported in two units:
- Percentage (%): used in the USA, India, and many other countries
- mmol/mol: the international (IFCC) unit, used in the UK and much of Europe
The formula to convert is: mmol/mol = (% − 2.15) × 10.929. You do not need to calculate it yourself; use our free Blood Sugar & HbA1c Converter.
HbA1c Ranges
Organisations use slightly different thresholds for prediabetes:
| Category | ADA (USA) % | WHO / NHS (UK) mmol/mol |
|---|---|---|
| Normal | Below 5.7% | Below 42 (below 6.0%) |
| Prediabetes / higher risk | 5.7% to 6.4% | 42 to 47 (6.0% to 6.4%) |
| Diabetes | 6.5% or higher | 48 or higher (6.5% or higher) |
The threshold for diabetes, 6.5% (48 mmol/mol), is consistent across the American Diabetes Association and the World Health Organization. In people without symptoms, a diabetes diagnosis is usually confirmed with a repeat test.
HbA1c and Average Blood Sugar
HbA1c can be converted into an estimated average glucose (eAG), which uses the same units as a glucose meter. The commonly used formula is: eAG (mg/dL) = 28.7 × HbA1c − 46.7.
| HbA1c (%) | HbA1c (mmol/mol) | Estimated Average Glucose (mg/dL) | Estimated Average Glucose (mmol/L) |
|---|---|---|---|
| 5.7 | 39 | 117 | 6.5 |
| 6.0 | 42 | 126 | 7.0 |
| 6.5 | 48 | 140 | 7.8 |
| 7.0 | 53 | 154 | 8.6 |
| 8.0 | 64 | 183 | 10.2 |
| 9.0 | 75 | 212 | 11.8 |
| 10.0 | 86 | 240 | 13.3 |
Values are rounded. eAG is an estimate; actual daily sugar levels can vary widely around this average.
HbA1c Targets for People With Diabetes
For many adults with diabetes, a common target is below 7% (53 mmol/mol), and some guidelines suggest 6.5% (48 mmol/mol) where it can be achieved safely. However, targets should be personalised. Older adults, people with frequent low blood sugar, or those with other health conditions may have higher targets. Your doctor will set the right target for you.
What Can Affect HbA1c Accuracy?
Because HbA1c depends on red blood cells, anything that changes red cell lifespan or haemoglobin can make it misleading:
- Anaemia, including iron deficiency, which can raise HbA1c, or anaemias where red cells are destroyed faster, which can lower it
- Haemoglobin variants such as sickle cell trait or thalassaemia, depending on the lab method
- Recent blood loss or blood transfusion
- Pregnancy
- Chronic kidney disease or liver disease
- Some medicines
In these situations, doctors may rely more on blood glucose tests or continuous glucose monitoring. HbA1c is also generally not used to diagnose type 1 diabetes in children or diabetes that develops very quickly.
HbA1c vs Other Sugar Tests
| Test | What It Shows | Fasting Needed? |
|---|---|---|
| HbA1c | Average over 2–3 months | No |
| Fasting plasma glucose | Sugar after 8+ hours without food | Yes |
| Oral glucose tolerance test (OGTT) | How your body handles a sugar drink | Yes |
| Random glucose / finger prick | Sugar at that moment | No |
Learn more in normal blood sugar levels: fasting, after meals, and HbA1c.
How Often Should HbA1c Be Checked?
- Diabetes, stable and at target: often about twice a year
- Diabetes, after treatment changes or not at target: often every three months
- Prediabetes: commonly once a year
- Screening: based on age and risk factors, as advised by your doctor
How to Lower HbA1c
- Eat balanced meals with vegetables, protein, whole grains, and pulses; limit sugary drinks and refined carbohydrates. See our nutrition guides.
- Be active. Regular exercise helps muscles use glucose. Try our Heart Rate Zone Calculator to plan workouts.
- Lose weight if needed. Even modest weight loss can improve blood sugar.
- Sleep well and manage stress, which affect blood sugar through hormones like cortisol. See cortisol explained.
- Take medicines as prescribed and attend regular reviews.
Because HbA1c reflects about three months, give changes time before expecting a big difference in your result.
Questions to Ask Your Doctor About Your Result
- What is my personal HbA1c target, and why?
- Do I have anything, such as anaemia or kidney disease, that could affect this test?
- When should I repeat the test?
- Should I also check my blood sugar at home?
Frequently Asked Questions
Do I need to fast for an HbA1c test?
No. HbA1c can be measured at any time of day without fasting.
Is 6.0% HbA1c bad?
6.0% (42 mmol/mol) falls in the prediabetes or higher-risk range under most guidelines. It is a useful warning sign and a good time to make lifestyle changes.
Can HbA1c go back to normal?
For many people with prediabetes, and some with type 2 diabetes, lifestyle changes and treatment can bring HbA1c back into a lower range.
The Bottom Line
HbA1c shows your average blood sugar over the past two to three months. Below 5.7% (or below 42 mmol/mol under UK criteria) is generally normal, and 6.5% (48 mmol/mol) or above suggests diabetes. Use our converter to switch between units, discuss your result with your doctor, and remember that steady lifestyle changes can make a real difference. Explore more in our health section.
Sources
- American Diabetes Association
- NIDDK: Diabetes, US National Institutes of Health
- WHO: Diabetes fact sheet
This article is for general education and is not medical advice. Always speak to a qualified healthcare professional about your health. Read our editorial policy.
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