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Why Are There Two Different Units for Blood Sugar?
If you have ever read a British diabetes forum, used an imported glucose app, or compared notes with someone in a different country, you will have noticed that blood sugar readings can look completely different even when describing the same level. A reading of 99 in the US becomes 5.5 in the UK. Both are correct. They are simply two different ways of measuring the same thing.
The Unit Used in the United States: mg/dL
Milligrams per deciliter (mg/dL) measures the weight of glucose in a small volume of blood. The United States uses this system, as do a small number of other countries. American glucose meters, CGM devices sold in the US, lab results from your doctor, and resources from organizations like the American Diabetes Association all use mg/dL.
The Unit Used in the UK, Europe, Canada and Australia: mmol/L
Millimoles per liter (mmol/L) measures the concentration of glucose molecules in a liter of blood. This is the standard unit used across the United Kingdom, Europe, Australia, Canada, and most of the rest of the world. International research papers, British and European diabetes resources, and devices configured for international markets display readings in mmol/L.
Why Does This Cause Confusion?
The growing international nature of diabetes communities online means people regularly encounter readings in the other unit. Someone in the US reading a British diabetes forum, consulting a study published in a European journal, or using a budget glucose meter that shipped configured for international markets may see mmol/L values they cannot intuitively interpret. Many inexpensive meters and CGM apps sold online default to mmol/L, so a US user can end up staring at a reading of 5.5 and wondering whether to panic. This converter translates between the two instantly.
A Quick Reference: Common Readings in Both Units
| mg/dL | mmol/L | General Context |
|---|---|---|
| 36 | 2.0 | Low (hypoglycemia) |
| 63 | 3.5 | Below normal fasting range |
| 72 | 4.0 | Normal fasting (lower end) |
| 90 | 5.0 | Normal fasting |
| 99 | 5.5 | Normal fasting (upper end) |
| 108 | 6.0 | Prediabetes range |
| 126 | 7.0 | Diabetes threshold (fasting) |
| 140 | 7.8 | Normal 2hr post-meal upper limit |
| 180 | 10.0 | High post-meal reading |
| 252 | 14.0 | Very high, seek advice |
Understanding What Your Blood Sugar Reading Means
Converting your reading is only the first step. Understanding what the number actually means in the context of your health is equally important. Blood sugar levels are interpreted differently depending on when the reading was taken, whether you have been diagnosed with diabetes, and what your individual target range is. The categories below follow American Diabetes Association (ADA) guidelines.
Fasting Blood Sugar (Before Eating)
A fasting reading is taken after at least eight hours without food or drink other than water. This is the most commonly used measurement for diagnosing diabetes and prediabetes, and is what most standard blood tests measure.
| Category | mg/dL | mmol/L |
|---|---|---|
| Low (Hypoglycemia) | Below 70 | Below 3.9 |
| Normal | 70 to 99 | 3.9 to 5.5 |
| Prediabetes | 100 to 125 | 5.6 to 6.9 |
| Diabetes (diagnostic threshold) | 126 or above | 7.0 or above |
Post-Meal Blood Sugar (1 to 2 Hours After Eating)
Blood sugar naturally rises after eating as carbohydrates are broken down into glucose. A post-meal reading taken one to two hours after finishing a meal is normal at a higher level than a fasting reading.
| Category | mg/dL | mmol/L |
|---|---|---|
| Normal (non-diabetic) | Below 140 | Below 7.8 |
| Impaired glucose tolerance | 140 to 199 | 7.8 to 11.0 |
| Diabetic range | 200 or above | 11.1 or above |
Target Ranges for People with Diabetes
If you have been diagnosed with Type 1 or Type 2 diabetes, your doctor or diabetes care team will have set personalized target ranges for you. General ADA targets for most non-pregnant adults with diabetes are:
| When Measured | Target (mg/dL) | Target (mmol/L) |
|---|---|---|
| Before meals (fasting) | 80 to 130 | 4.4 to 7.2 |
| 1 to 2 hours after meals | Below 180 | Below 10.0 |
What is Hypoglycemia?
Hypoglycemia (a "low") occurs when blood sugar drops below 70 mg/dL (3.9 mmol/L). Symptoms include shakiness, sweating, confusion, dizziness, and hunger. For people managing diabetes with insulin or certain medications, hypoglycemia requires immediate treatment with fast-acting carbohydrates such as glucose tablets, fruit juice, or candy. If untreated, severe hypoglycemia can be dangerous.
What is Hyperglycemia?
Hyperglycemia refers to blood sugar that is too high, typically above 180 mg/dL (10 mmol/L) for people with diabetes, though thresholds vary individually. Mild hyperglycemia may cause increased thirst, frequent urination, and tiredness. Chronic high blood sugar damages blood vessels and nerves over time, leading to complications affecting the eyes, kidneys, feet, and heart. Persistent hyperglycemia should be discussed with your diabetes care team.
Diabetes Management Essentials
A1C: The Other Key Diabetes Measure
Alongside day-to-day blood glucose readings, people with diabetes will be familiar with the A1C test (also written HbA1c), a blood test that gives a picture of average blood sugar control over the previous two to three months. Unlike a finger-stick glucose reading, which reflects your blood sugar at a single moment in time, A1C reveals how well controlled your levels have been overall.
What is A1C?
Hemoglobin is the protein in red blood cells that carries oxygen. When glucose is present in the blood, it attaches to hemoglobin, forming glycated hemoglobin or HbA1c. Because red blood cells live for around two to three months, the A1C test measures the proportion of hemoglobin that has glucose attached, reflecting average blood sugar over that period.
A1C Units: % vs. mmol/mol
Like blood glucose, A1C is reported in different units in different countries. The United States uses a percentage (%), while the UK and some other countries use mmol/mol following international standardization. Both measure the same thing, so if you encounter an A1C in mmol/mol in international resources, it is simply the other unit.
| Category | A1C (%) | A1C (mmol/mol) |
|---|---|---|
| Normal (non-diabetic) | Below 5.7% | Below 39 |
| Prediabetes | 5.7% to 6.4% | 39 to 47 |
| Diabetes diagnosis | 6.5% or above | 48 or above |
| Common treatment target | Below 7.0% | Below 53 |
How Often is A1C Tested?
For people with well-controlled Type 2 diabetes, A1C is typically tested every six to twelve months. For those with Type 1 diabetes, or where control is less stable, testing every three months is common. Newly diagnosed patients and those changing medication may be tested more frequently.
Managing Blood Sugar Levels: Practical Guidance
Whether you have been diagnosed with diabetes, are prediabetic, or simply want to understand and support your metabolic health, there are well-evidenced lifestyle approaches that can positively influence blood sugar levels.
Diet and Blood Sugar
Carbohydrates have the most direct effect on blood glucose because they are broken down into sugar during digestion. This does not mean carbohydrates must be avoided entirely, but the type, quantity, and timing of carbohydrate consumption all matter.
- Choose lower glycemic index (GI) foods where possible. Low-GI foods release glucose more slowly, producing a gentler rise in blood sugar rather than a sharp spike. Examples include oats, legumes, most vegetables, and whole-grain bread.
- Reduce refined carbohydrates and sugar. White bread, sugary drinks, candy, and processed foods cause rapid blood sugar spikes. Swapping these for whole food alternatives makes a measurable difference.
- Include protein and healthy fat with meals. Both slow the absorption of carbohydrates, reducing the post-meal glucose peak.
- Watch portion sizes. Even healthy carbohydrates can raise blood sugar significantly if eaten in large quantities.
- Consider meal timing. Spacing meals evenly throughout the day helps avoid large fluctuations in blood sugar.
Physical Activity
Exercise is one of the most powerful tools for improving blood sugar control. Muscle contractions during activity use glucose directly, lowering blood sugar in the short term. Regular exercise also improves insulin sensitivity, meaning the body uses insulin more efficiently over time.
- Aim for at least 150 minutes of moderate aerobic activity per week, such as brisk walking, cycling, or swimming
- Resistance training (weights or bodyweight exercises) is particularly effective at improving long-term insulin sensitivity
- Even a 10 to 15 minute walk after meals can significantly reduce post-meal blood sugar spikes
- People on insulin should be aware that exercise can lower blood sugar substantially and may need to adjust their approach with their diabetes team's guidance
Weight Management
Excess weight, particularly around the abdomen, is closely linked to insulin resistance and Type 2 diabetes. Even modest weight loss of 5 to 10% of body weight can produce significant improvements in blood sugar control and, in some cases of early Type 2 diabetes, can lead to remission. The DiRECT trial demonstrated that intensive dietary weight loss of 33 pounds (15kg) or more led to remission in around half of participants at two years.
Sleep and Stress
Both poor sleep and chronic stress raise cortisol levels, which in turn raises blood sugar. People who consistently sleep fewer than six hours per night show measurably worse blood sugar control than those sleeping seven to eight hours. Managing stress through techniques such as mindfulness, regular exercise, and adequate rest is a meaningful if often overlooked component of blood sugar management.
Medication and Monitoring
For people with diagnosed diabetes, medication plays a central role alongside lifestyle. Metformin remains the most commonly prescribed first-line medication for Type 2 diabetes. Insulin is essential for Type 1 and used in many cases of Type 2 where other medications are insufficient. Regular self-monitoring of blood glucose, as recommended by your diabetes care team, allows you to understand how food, activity, stress, and medication affect your individual readings.
