Blood Glucose Unit of Measurement: UK Guide

Blood glucose unit of measurement is a fundamental concept for anyone managing diabetes or interpreting health results in the UK. In the UK, blood glucose is expressed in millimoles per litre (mmol/L), the standard used across NHS clinical practice, NICE guidelines, and laboratory reports. Understanding this unit, how it differs from the milligrams per decilitre (mg/dL) system used in the United States, and how to convert between the two is essential for reading your own results accurately, using glucose monitoring devices safely, and communicating clearly with your healthcare team.

What units are used to measure blood glucose in the UK?

Blood glucose in the UK is measured in millimoles per litre (mmol/L), the standard unit used across NHS meters, CGMs, laboratory reports, and NICE guidelines.

In the UK, blood glucose is measured in millimoles per litre, written as mmol/L. This is the standard unit used across NHS clinical practice, including on NHS-issued glucose meters, continuous glucose monitors (CGMs), and in laboratory blood test reports.^[1][2] The unit reflects the concentration of glucose molecules dissolved in a litre of blood plasma.

The choice of mmol/L aligns the UK with most of Europe and with international scientific convention for reporting electrolytes and metabolites. It differs from the United States and several other countries, which use milligrams per decilitre (mg/dL). This distinction becomes practically important when patients read glucose targets from overseas sources, use imported devices, or review research published in American journals, as the numerical values look very different even when describing the same physiological state.

NICE guidelines, NHS diabetes care pathways, and clinical reference ranges published in the UK are expressed in mmol/L. When a GP, diabetes nurse or hospital team discusses your blood glucose results, they will use mmol/L. Understanding this unit is therefore fundamental to interpreting your own readings accurately and communicating safely with your healthcare team.

mmol/L vs mg/dL: what is the difference and why does it matter?

mmol/L counts glucose molecules per litre; mg/dL measures glucose mass per decilitre, producing values roughly 18 times higher for the same reading, which can cause dangerous misinterpretation.

The key difference between mmol/L and mg/dL is the way each unit expresses glucose concentration: mmol/L counts the number of glucose molecules (in millimoles) per litre of blood, while mg/dL measures the mass of glucose (in milligrams) per decilitre. Because glucose has a molecular weight of approximately 180 grams per mole, the same blood glucose level produces a number roughly 18 times larger in mg/dL than in mmol/L.^[2]

This matters clinically because a reading of 7.0 mmol/L (a commonly cited fasting threshold in UK guidelines) is equivalent to approximately 126 mg/dL in the American system. If a patient misreads a mg/dL value as mmol/L, they may believe their glucose is dangerously high when it is actually within a normal range, or vice versa. Such misinterpretation can lead to unnecessary anxiety, incorrect self-treatment, or failure to recognise a genuinely abnormal result.

The confusion is increasingly common as patients access health information online, use apps developed in the United States, or purchase glucose meters abroad. Some CGM apps allow users to toggle between units, which can compound the problem if the setting is changed inadvertently. Always confirm which unit your device or app is displaying before acting on a reading, and if in doubt, contact your GP or diabetes care team for clarification.

How do you convert blood glucose readings between units?

Multiply mmol/L by 18 to get mg/dL, or divide mg/dL by 18 to get mmol/L; for example, 7.0 mmol/L equals approximately 126 mg/dL.

Converting between mmol/L and mg/dL uses a straightforward calculation based on the molecular weight of glucose. To convert from mmol/L to mg/dL, multiply the mmol/L value by 18.02 (commonly rounded to 18). To convert from mg/dL to mmol/L, divide the mg/dL value by 18.02.

For example:

• A fasting glucose of 7.0 mmol/L equals approximately 126 mg/dL (7.0 x 18 = 126)

• A postprandial reading of 10.0 mmol/L equals approximately 180 mg/dL (10.0 x 18 = 180)

• A reading of 90 mg/dL equals approximately 5.0 mmol/L (90 / 18 = 5.0)

These conversions are approximate because the precise factor is 18.02, but rounding to 18 is acceptable for everyday use and is the approach used in most conversion tables. Diabetes UK publishes a conversion tool on its website if you need to check a specific value.

Many CGM apps and some glucose meters include a built-in unit conversion function. If you are using a device or app that displays mg/dL and you need to report results to an NHS clinician, convert the values before your appointment or bring the device so the team can review the raw data directly. If you are unsure whether a reading is in the correct unit, do not adjust your medication or insulin dose without first seeking advice from your GP or diabetes nurse.

What do normal blood glucose levels look like in mmol/L?

Many NHS laboratories report a fasting plasma glucose of 4.0 to 5.9 mmol/L as within the non-diabetic range; 7.0 mmol/L or above on two occasions meets a diagnostic criterion for diabetes.

Reference ranges for fasting blood glucose vary between laboratories, and you should always interpret your result against the reference interval provided by the laboratory or clinical team that tested you. As a general guide, many NHS laboratories report a fasting plasma glucose of around 4.0 to 5.9 mmol/L as within the non-diabetic range for adults, though the exact figures differ between sites.

According to WHO diagnostic criteria, a fasting plasma glucose of 6.1 to 6.9 mmol/L may indicate impaired fasting glucose, sometimes described in NHS prevention pathways as non-diabetic hyperglycaemia. A fasting plasma glucose of 7.0 mmol/L or above on two separate occasions is one of the diagnostic criteria for diabetes, as set out in WHO guidance and reflected in NICE guidance (NG28).

For people already diagnosed with diabetes, the NHS recommends individualised glucose targets agreed with a clinician. As a general guide for adults with type 2 diabetes, the NHS suggests a fasting glucose of 4.0 to 7.0 mmol/L before meals, and around 8.5 mmol/L or below two hours after meals, though targets vary depending on treatment, age, and individual circumstances. Your care team will agree the most appropriate targets for you.

For people using insulin or medicines that carry a risk of hypoglycaemia, a blood glucose below 4.0 mmol/L is considered hypoglycaemic and requires prompt treatment with fast-acting carbohydrate. Symptoms of hypoglycaemia include shakiness, sweating, confusion and palpitations. If a person is unconscious or unable to swallow safely, call 999 immediately. For urgent advice about a glucose reading that concerns you but does not involve loss of consciousness, contact NHS 111 or your GP.

HbA1c, which reflects average glucose over approximately three months, is reported in mmol/mol in the UK (not as a percentage), and is a separate but complementary measure to day-to-day glucose readings.^[2][3]

Which unit does your NHS glucose meter or CGM device use?

NHS-issued glucose meters and CGM devices are standardly set to mmol/L; privately purchased or imported devices may default to mg/dL and should be checked before use.

Glucose meters and CGM devices issued through the NHS in the UK are standardly set to display readings in mmol/L, in line with UK clinical practice.^[2][7] This applies to devices provided directly by GP practices, hospital diabetes teams, and those dispensed via NHS prescription. Devices used in structured education programmes are calibrated and labelled in mmol/L.

However, some devices purchased privately, imported from abroad, or downloaded as apps from international platforms may default to mg/dL. It is important to check the unit displayed on the screen before interpreting any reading. On most meters, the unit is shown alongside the numerical value (for example, "5.4 mmol/L"). On CGM apps, the unit setting is usually found in the device or app settings menu. If you are unsure, consult the device manual or contact the manufacturer's helpline.

Patients who self-fund glucose monitoring equipment or use consumer-grade CGM devices should confirm the unit setting with their diabetes care team before using the readings to guide treatment decisions. Bolt Pharmacy is a GPhC-registered online pharmacy in Great Britain offering online consultations, with prescriber review and pharmacist support, where clinically appropriate guidance on diabetes-related medicines may be provided alongside advice on monitoring.

If you suspect your device is displaying readings in the wrong unit, or if a reading seems implausibly high or low, do not act on it without verification. Contact your GP or diabetes nurse, or call NHS 111 for urgent advice. Suspected device faults or unexpected readings from a licensed medical device can be reported to the MHRA via the Yellow Card scheme at yellowcard.^[8]mhra.gov.uk.

Scientific References

1. Checking your own blood glucose, and target levels (NG17).
2. Diabetes mellitus, diagnostic and monitoring devices.
3. Blood glucose management – Type 2 diabetes in adults (NG28).
4. Hypoglycaemia – Treatment summaries.
5. Low blood sugar (hypoglycaemia).
6. Living with type 1 diabetes.
7. Continuous glucose monitoring and hybrid closed loop for diabetes.
8. The Yellow Card scheme: guidance for healthcare professionals, patients and the public.

Frequently Asked Questions