What are the standard blood glucose levels for humans?

The standard reference ranges for blood glucose levels vary depending on the timing of measurement relative to meals and the clinical context. In healthy adults, fasting plasma glucose (measured after at least 8 hours without caloric intake) should typically fall between 70–99 mg/dL (3.9–5.5 mmol/L). A fasting glucose level of 100–125 mg/dL (5.6–6.9 mmol/L) is classified as prediabetes (impaired fasting glucose), while a value ≥126 mg/dL (≥7.0 mmol/L) on two separate occasions confirms a diagnosis of diabetes mellitus.

Postprandial (after-meal) glucose—measured 2 hours after the start of a meal—should generally remain below 140 mg/dL (7.8 mmol/L) in nondiabetic individuals. Levels between 140–199 mg/dL (7.8–11.0 mmol/L) indicate impaired glucose tolerance (another prediabetic state), and ≥200 mg/dL (≥11.1 mmol/L) during a standardized 75-g oral glucose tolerance test (OGTT) supports a diabetes diagnosis.

Random (nonfasting) plasma glucose ≥200 mg/dL (≥11.1 mmol/L), accompanied by classic hyperglycemic symptoms (e.g., polyuria, polydipsia, unexplained weight loss), is also diagnostic of diabetes. It is important to note that these thresholds apply to plasma glucose measured in a clinical laboratory; capillary (fingerstick) glucose readings may show slight variation and are not used for formal diagnosis.

Individualized targets may differ—for example, older adults or those with comorbidities may have less stringent goals to minimize hypoglycemia risk—so clinical interpretation should always consider the patient’s age, overall health, and treatment plan.