Blood Sugar (HbA1c) Conversion & Tracker
Convert between HbA1c percentages, mmol/mol, and average blood glucose levels. Interpret your reading against NHS thresholds.
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HbA1c Guide
What HbA1c Measures?
HbA1c (glycated haemoglobin): the percentage of red blood cells with glucose attached. Reflects average blood glucose over the past 8-12 weeks (the lifespan of red blood cells). Unlike a finger-prick spot check, HbA1c gives a long-term picture. Why it matters: high glucose damages blood vessels, kidneys, eyes, nerves throughout the body — cumulatively. HbA1c tracks the long-term exposure that matters. NHS units: changed from % to mmol/mol in 2011 in UK. Both still in use internationally. Conversion between the two scales is straightforward using a standard formula, and most UK labs now report both figures side by side.
What's the key thing to understand about NHS Thresholds (mmol/mol)?
Below 42 (6.0%): normal, no diabetes. 42-47 (6.0-6.4%): pre-diabetes (also called impaired glucose regulation). 48+ (6.5%+): diabetes diagnosis. Targets for treated diabetes (NICE guidance): Type 2 diabetes lifestyle only: target 48 mmol/mol (6.5%). T2D on one drug not causing hypos: 48 (6.5%). T2D on insulin or sulfonylureas: 53 (7.0%) — slightly higher to balance hypoglycaemia risk. Type 1 diabetes: 48 (6.5%) if achievable without disabling hypos. Above 58 mmol/mol (7.5%): treatment intensification is usually considered, meaning your GP or diabetes team may suggest a change in medication, dose, or lifestyle approach.
What's the key thing to understand about HbA1c to Average Glucose?
Approximate conversion: average glucose (mmol/L) = (HbA1c% × 1.59) − 2.59. Or: mg/dL = (HbA1c% × 28.7) − 46.7. Examples: HbA1c 5.0% (31 mmol/mol): avg glucose 5.4 mmol/L = 97 mg/dL (normal). HbA1c 6.5% (48): avg glucose 7.8 mmol/L = 140 mg/dL (diabetes threshold). HbA1c 7.0% (53): 8.6 mmol/L = 154 mg/dL (typical T2D target). HbA1c 8.0% (64): 10.1 mmol/L = 183 mg/dL (treatment intensification needed). HbA1c 10.0% (86): 13.3 mmol/L = 240 mg/dL (poor control, complications likely). Continuous glucose monitors (CGM) give real-time readings and can help identify patterns a single HbA1c figure can't show, such as which meals or times of day cause the biggest spikes.
What should I know about Lowering HbA1c?
Lifestyle changes can dramatically reduce HbA1c: weight loss (5-10% body weight): typically 5-10 mmol/mol reduction. Carbohydrate reduction: lower starchy carb intake reduces glucose peaks. Mediterranean or low-GI diets show strong evidence. Exercise: 150 min/week moderate activity reduces HbA1c 5-7 mmol/mol on average. Strength training adds further benefit. Sleep: chronic sleep deprivation worsens insulin resistance. 7-9 hours/night supports glucose control. T2D remission: the DiRECT trial showed that substantial, sustained weight loss — around 15kg — can put type 2 diabetes into remission for many people, though this requires structured support and isn't achievable or appropriate for everyone.
What's the difference between Fasting Glucose and HbA1c Testing?
These two tests measure related but different things. A fasting plasma glucose test is a single snapshot — your blood sugar level at one moment, after at least 8 hours without food — and is sensitive to what you ate the day before, stress, illness or even a recent poor night's sleep. HbA1c instead measures the percentage of haemoglobin (the oxygen-carrying protein in red blood cells) that has glucose attached to it, which happens gradually and doesn't reverse until the red blood cell is replaced — so it reflects average blood glucose over roughly the past 2-3 months, the typical lifespan of a red blood cell. This makes HbA1c more useful for diagnosing diabetes and monitoring long-term control, since it isn't thrown off by a single unusual day, while fasting glucose remains useful for day-to-day monitoring and for catching acute changes. NICE guidance uses both: fasting glucose and HbA1c thresholds for diabetes and prediabetes are defined separately because they're measuring the condition from two different angles.
What Affects Blood Glucose Readings Beyond Diet?
Blood glucose levels respond to far more than the last meal eaten. Illness and infection trigger stress hormones that raise blood sugar even without eating, which is why people with diabetes are advised to monitor more closely when unwell ("sick day rules"). Physical and emotional stress has a similar effect through cortisol and adrenaline. Many people also experience the "dawn phenomenon" — a natural early-morning rise in blood glucose caused by a surge in hormones that counteract insulin, peaking in the hours before waking, which is a normal physiological pattern rather than a sign that evening habits were wrong. Poor sleep and some medications, including steroids and certain antidepressants, can raise readings too. For people monitoring their own glucose, a single reading is a snapshot influenced by all of these factors, which is one reason HbA1c and repeated fasting glucose tests are used for diagnosis and monitoring rather than a single number — they smooth out this day-to-day variability. Anyone with persistently unusual readings should discuss the pattern with their GP rather than reacting to one result.
What should I know about Continuous Glucose Monitoring?
Continuous glucose monitors (CGMs) — small sensors worn on the arm or abdomen that sample interstitial fluid every few minutes — have become widely available beyond people with diabetes, including over-the-counter in the UK and US. They show glucose trends and "time in range" rather than the single-point snapshot a finger-prick or lab HbA1c gives, which can reveal patterns a one-off reading misses, such as large post-meal spikes or overnight dips. However, CGM readings lag blood glucose by several minutes and are calibrated for people with diabetes, so the "optimal" ranges marketed to healthy users for wellness purposes aren't based on the same clinical evidence as HbA1c thresholds. NHS and NICE guidance still treats HbA1c and fasting/random venous glucose as the diagnostic standard for diabetes and prediabetes; CGM data is a useful personal trend tool but isn't currently used to diagnose or stage diabetes outside of specific research or insulin-management contexts.
How Diabetes and Prediabetes Are Actually Diagnosed
A single home blood glucose reading is never used on its own to diagnose diabetes — UK and WHO diagnostic criteria rely on either a fasting plasma glucose test, a random glucose test alongside symptoms, or an HbA1c test, usually confirmed with a second test on a different day unless symptoms are present and glucose is very high.
| Category | Fasting glucose | HbA1c | Random glucose |
|---|---|---|---|
| Normal | Below 6.0 mmol/L | Below 42 mmol/mol (6.0%) | Below 11.1 mmol/L |
| Prediabetes | 6.1-6.9 mmol/L | 42-47 mmol/mol (6.0-6.4%) | Not used alone |
| Diabetes | 7.0 mmol/L or above | 48 mmol/mol (6.5%) or above | 11.1 mmol/L or above with symptoms |
Worked example: a 52-year-old has a fasting glucose of 6.4 mmol/L and an HbA1c of 44 mmol/mol. Both values sit inside the prediabetes band (fasting 6.1-6.9; HbA1c 42-47), so the correct interpretation is 'non-diabetic hyperglycaemia' — raised risk, not yet diabetes. NICE guidance recommends this is used as a trigger for structured lifestyle advice (weight management, reduced refined carbohydrate and saturated fat intake, increased activity) and annual re-testing, since progression to type 2 diabetes is common but far from inevitable with early changes. HbA1c can be misleadingly low or high in certain conditions (sickle cell trait, anaemia, pregnancy, recent blood loss or transfusion), which is why fasting glucose is sometimes used alongside or instead of HbA1c to confirm a borderline result.
Recognising and Treating Hypoglycaemia
Hypoglycaemia (low blood sugar, generally below 4.0 mmol/L) is most relevant to people on insulin or certain diabetes medications (such as sulfonylureas), though it can occasionally occur in people without diabetes after prolonged fasting or intense exercise. Early warning symptoms include trembling, sweating, hunger, a fast heartbeat, irritability and difficulty concentrating; if untreated, blood glucose can fall further and cause confusion, slurred speech, and in severe cases loss of consciousness or seizure. The standard first-aid response for a conscious person is the '15-15 rule': take 15g of fast-acting carbohydrate (roughly 150ml of regular fizzy drink or fruit juice, 3-4 glucose tablets, or a small handful of jelly sweets), wait 15 minutes, then recheck blood glucose. If still below 4.0 mmol/L, repeat. Once levels recover, a longer-acting snack containing starch (such as a slice of toast or a few crackers) helps prevent a second dip, particularly before activity or sleep. Someone who is unconscious or unable to swallow safely should never be given food or drink by mouth — this is a medical emergency requiring an ambulance; a glucagon injection (if available and the person is trained to give it) or emergency glucose gel applied inside the cheek can be used while help is on the way. Night-time hypoglycaemia (sweating, nightmares, or waking with a headache) is a particular risk for people on insulin and should always be reported to the diabetes team, since overnight doses may need adjusting.