Blood sugar (glucose) ranges
Blood glucose is measured in mg/dL (US) or mmol/L (most of the world). Fasting and 2-hour post-meal values diagnose prediabetes and diabetes; HbA1c reflects the average over about three months.
| State | mg/dL | mmol/L | Notes |
|---|---|---|---|
| Severe hypoglycemia | <54 | <3.0 | Seek medical help immediately |
| Hypoglycemia | 54–69 | 3.0–3.8 | Treat with 15 g fast-acting carbs |
| Normal (fasting) | 70–99 | 3.9–5.5 | Healthy fasting range |
| Normal (2h post-meal) | <140 | <7.8 | Standard postprandial target |
| Prediabetes (fasting) | 100–125 | 5.6–6.9 | Impaired fasting glucose |
| Prediabetes (2h post-meal) | 140–199 | 7.8–11.0 | Impaired glucose tolerance |
| Diabetes (fasting) | ≥126 | ≥7.0 | Confirm with a repeat test |
| Diabetes (2h post-meal) | ≥200 | ≥11.1 | Diagnosed via glucose tolerance test |
| HbA1c normal | <5.7% | <39 mmol/mol | 3-month average glucose |
| HbA1c diabetes | ≥6.5% | ≥48 mmol/mol | Diagnostic threshold |
Notes
- This is a general reference, not medical advice — always confirm results with a clinician.
- mmol/L = mg/dL x 0.0555; to convert back, mg/dL = mmol/L x 18.0.
- A single high reading is not a diagnosis; diabetes needs two abnormal tests or an HbA1c >= 6.5%.
- Symptoms of very low sugar (shaking, confusion) need fast carbs and medical care if severe.
Three tests, three windows in time
A fasting glucose shows the body at rest after 8-12 hours without food; a 2-hour post-meal or oral glucose tolerance test shows how well a carbohydrate load is cleared; HbA1c reflects average glucose over the previous two to three months and needs no fasting. The three can disagree: someone with normal fasting numbers may still spike after meals, and a single high fasting reading is not a diagnosis — diabetes needs two abnormal tests or an HbA1c of 6.5% or above.
Converting mg/dL and mmol/L
The two units differ by a factor of about 18. To go from mg/dL (US) to mmol/L (most countries), multiply by 0.0555; to go back, multiply by 18.0. So a fasting 100 mg/dL is 5.6 mmol/L, the diabetes cutoff of 126 mg/dL is 7.0 mmol/L, and the 200 mg/dL post-meal line is 11.1 mmol/L. Remembering that 90, 100, 126 and 200 mg/dL map to 5.0, 5.6, 7.0 and 11.1 mmol/L covers almost every threshold you will meet.
Handling low blood sugar
Below 70 mg/dL (3.9 mmol/L) is hypoglycemia; below 54 mg/dL (3.0 mmol/L) is a severe episode. Symptoms — shaking, sweating, confusion, a fast heartbeat — come on quickly and need fast-acting carbohydrate: about 15 g, such as half a glass of juice or three glucose tablets. Wait 15 minutes and retest, repeating if still low, then eat a slower carbohydrate to stabilise. Anyone with repeated lows, or a low they cannot explain, should review medication and activity with a clinician.
What a prediabetes reading really means
A fasting value of 100-125 mg/dL (5.6-6.9 mmol/L) or a 2-hour value of 140-199 mg/dL (7.8-11.0 mmol/L) marks prediabetes — a higher-risk zone, not a sentence. Roughly a quarter to a third of people in this band develop type 2 diabetes within five years, but losing 5-7% of body weight, moving more and modest diet changes cut that risk sharply. Retest in three to six months rather than immediately, because glucose varies from day to day.
What pushes readings up and down
Glucose moves with more than food. A meal raises it, and so can stress, illness, infection, poor sleep, some steroids and even the dawn phenomenon, where hormone release lifts early-morning levels. Exercise usually lowers glucose, but very intense sessions can raise it briefly through adrenaline. That variability is why diagnosis relies on standardised conditions — fasting, or exactly two hours after a measured glucose drink — rather than a spot check. When a reading surprises you, note what you ate, how you slept and whether you were ill before drawing conclusions.
Frequently asked questions
- What is a normal fasting blood sugar?
- A healthy fasting level is 70–99 mg/dL (3.9–5.5 mmol/L). Values of 100–125 mg/dL are prediabetes and 126 or above suggest diabetes.
- What is the 2-hour post-meal target?
- Under 140 mg/dL (7.8 mmol/L) two hours after eating is normal; 140–199 is prediabetes and 200 or more points to diabetes.
- How do I convert mg/dL to mmol/L?
- Multiply mg/dL by 0.0555 to get mmol/L (for example 100 mg/dL ≈ 5.6 mmol/L). To go back, multiply mmol/L by 18.0.
- What does HbA1c 6.5% mean?
- HbA1c of 6.5% or higher (>= 48 mmol/mol) is the diagnostic cutoff for diabetes and reflects average glucose over the past 2–3 months.