A1C Calculator,
understand your blood sugar.

Clinical guide

What is A1C and why does it matter?

The A1C test — also called HbA1c, glycated haemoglobin, or glycosylated haemoglobin — measures the percentage of haemoglobin molecules in your red blood cells that have glucose attached to them. Because red blood cells live for approximately 2–3 months, the test provides a picture of your average blood sugar control over that period, unlike a fasting glucose test, which only captures a single moment in time.

A1C is central to diagnosing diabetes and prediabetes, monitoring how well treatment is working, and predicting the risk of long-term complications such as kidney disease, nerve damage, retinopathy, and cardiovascular disease.

ADA diagnostic thresholds

The American Diabetes Association (ADA) uses the following A1C thresholds for diagnosis:

• Normal: A1C below 5.7%. Blood sugar regulation is healthy. No action required beyond routine monitoring and a healthy lifestyle.
• Prediabetes: A1C between 5.7% and 6.4% (inclusive). Blood sugar is elevated above normal but does not yet meet the diagnostic threshold for diabetes. Lifestyle intervention at this stage can prevent or significantly delay progression.
• Diabetes: A1C of 6.5% or higher on two separate tests (unless symptoms are present and a single test confirms it). A confirmed diagnosis requires clinical interpretation.

The World Health Organization (WHO) uses a slightly different threshold for prediabetes (6.0%–6.4%), but the diabetes threshold of ≥ 6.5% is consistent with the ADA.

The ADAG formula: A1C to estimated average glucose

The ADAG (A1C-Derived Average Glucose) formula was developed by Nathan et al. and published in Diabetes Care in 2008. It converts an A1C percentage to an estimated average glucose (eAG) value expressed in mg/dL or mmol/L:

The formula was derived from a study of 507 participants who wore continuous glucose monitors for 3 months while having regular A1C measurements. The correlation coefficient was 0.92, indicating a strong but imperfect relationship. Individual variation exists: two people with the same A1C may have different average glucose values due to differences in red blood cell lifespan and haemoglobin variants.

To convert eAG from mg/dL to mmol/L, divide by 18.0182:

IFCC units: mmol/mol (HbA1c)

In many European countries, A1C is reported in IFCC units (mmol/mol) rather than the NGSP percentage used in the United States and Canada. The conversion formula is:

For example, an A1C of 6.5% (NGSP) equals approximately 48 mmol/mol (IFCC), which is the WHO/ADA diabetes diagnostic threshold.

Management targets for people with diabetes

For people already diagnosed with type 2 diabetes, the ADA recommends an A1C target of below 7.0% for most non-pregnant adults. This target is associated with a substantial reduction in microvascular complications. Different targets may apply:

• Below 6.5% may be appropriate for some individuals — typically younger patients with short disease duration and no significant hypoglycaemia risk — if it can be achieved without excessive burden.
• Below 8.0% may be reasonable for older adults with limited life expectancy, extensive comorbidities, or a history of severe hypoglycaemia.
• At or above 9.0% is considered poorly controlled even within a diabetes diagnosis, and is associated with significantly increased risk of complications.

How often should A1C be tested?

The ADA recommends:

• At least twice per year for people with stable, well-controlled diabetes who are meeting treatment goals.
• Every 3 months (quarterly) for people whose therapy has recently changed or who are not meeting glycaemic targets.
• As part of a routine physical exam for adults at risk of diabetes (overweight/obese, family history, gestational diabetes history, etc.).

Factors that can affect A1C accuracy

The A1C test is very reliable in most people, but certain conditions can give falsely high or falsely low results:

• Haemoglobin variants (HbS, HbC, HbE): Common in people of African, Mediterranean, or Southeast Asian ancestry; some variants interfere with certain assay methods.
• Haemolytic anaemia: Shortened red blood cell lifespan means less time for glycation, producing falsely low A1C.
• Iron-deficiency anaemia: Can produce falsely elevated A1C by increasing red blood cell lifespan.
• Recent blood transfusion: Introduces new red blood cells that lower A1C artificially.
• Pregnancy: Especially the second and third trimesters, where increased red blood cell turnover may lower A1C independent of glucose control.
• Vitamin B12 or folate deficiency: May cause falsely elevated A1C.

In any of these situations, a fasting plasma glucose, 2-hour oral glucose tolerance test, or continuous glucose monitoring may be a more reliable measure of blood sugar control.

Medical disclaimer

This calculator is provided for educational and informational purposes only. It does not constitute medical advice, diagnosis, or treatment. The ADAG formula produces an estimate, not a direct measurement of blood glucose. Individual A1C results depend on multiple biological factors and must be interpreted by a qualified healthcare provider in the context of a patient’s complete clinical picture. Do not use this tool to self-diagnose diabetes or prediabetes, and do not adjust medications or insulin doses based on this calculator. Always consult your doctor, endocrinologist, or certified diabetes educator for personalised medical guidance.