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Blood sugar explained
What fasting, one-hour, two-hour and HbA1c values mean, the WHO and ADA ranges in mmol/L and mg/dL, the signs of a hypo, and what a continuous monitor adds.
The readings
| Reading | Normal range starts at | What it is |
|---|---|---|
| Fasting blood sugar | 4.0-5.5 mmol/L | The expected fasting range in a healthy adult. |
| One hour after eating | under 7.8 mmol/L | In most people without diabetes, glucose stays below 7.8 mmol/L even at the peak. |
| Two hours after eating | under 7.8 mmol/L | Two hours after a meal (or after the 75 g oral glucose tolerance test), a healthy response is back under 7.8 mmol/L. |
| HbA1c: the three-month average | under 5.7% mmol/L | The expected range in someone without diabetes. |
| Low blood sugar (hypoglycaemia) | 3.0-3.8 mmol/L | The ADA's "alert" range: the body's counter-response is already active. Treat with fast sugar and re-check in 15 minutes. |
| Continuous glucose monitors (CGM) | 3.9-10.0 mmol/L mmol/L | The international consensus target: for most adults with diabetes, at least 70% of the day inside this band. |
The shape, not just the points
Glucose is a curve. After a meal it rises for 45-75 minutes and falls back over the next two to three hours; the fasting value is the flat line between meals. What matters for how you feel is the shape — a tall spike that crashes to a hypo feels worse than a flat modest bump from the same calories, which is why protein, fibre and fat change a meal's effect without changing its energy.
Different readings answer different questions: the fasting value shows the overnight baseline, the two-hour value shows how well the body clears a meal, and HbA1c shows the three-month average. They can disagree — a normal fasting value with a diabetic two-hour value happens — which is why clinicians look at more than one.
Reference only, not a diagnosis: these are population-level WHO and ADA thresholds. Your own numbers get interpreted by a clinician against your history, medicines and other tests. Symptoms always outrank a number — a value that does not look alarming on a chart but comes with symptoms still deserves a phone call.
About the figures
Diagnostic thresholds follow the WHO definition and diagnosis of diabetes mellitus; treatment targets follow the ADA Standards of Care. Values are given in both mmol/L and mg/dL (conversion factor 18.018).
Sources: WHO — diabetes fact sheet · WHO — definition and diagnosis of diabetes mellitus and intermediate hyperglycaemia · ADA — Standards of Care in Diabetes · International Diabetes Federation — facts and figures · checked 2026-09-07