Healthy Blood Sugar Levels: The Complete Reference Guide

"Healthy blood sugar" sounds like it should be a single number — but it actually depends on which test was used, when it was taken, and who's being tested. This guide pulls the major reference ranges into one place, in both US (mg/dL) and UK (mmol/L) units, and points you to more detail on the situations that change the picture.

What blood sugar readings actually measure

A blood sugar (glucose) reading measures the concentration of glucose in your bloodstream at the moment the sample was taken. Because glucose rises after eating and settles between meals, the same person can have very different readings depending on timing — which is why every reference range is tied to a specific test type.

The four common ways blood sugar is measured

• Fasting glucose: taken after at least 8 hours without food, usually first thing in the morning.
• Random glucose: taken at any time of day, regardless of when you last ate.
• 2-hour post-meal (postprandial), including the oral glucose tolerance test (OGTT): taken 2 hours after eating or after a standardized glucose drink.
• A1C (HbA1c): a blood test reflecting your average blood sugar over roughly the past 2–3 months, reported as a percentage (and, in the UK, also in mmol/mol).

Reference ranges at a glance

These are the widely published categories used by the American Diabetes Association (ADA) and, for the UK figures, guidance commonly cited by the NHS and Diabetes UK. They describe general population diagnostic categories — not personal treatment targets, which your doctor sets individually.

Fasting glucose

• Normal: under 100 mg/dL (under 5.6 mmol/L) — UK sources commonly cite 4.0–5.4 mmol/L for a normal fasting reading
• Prediabetes / impaired fasting glucose: 100–125 mg/dL (5.6–6.9 mmol/L)
• Diabetes range: 126 mg/dL (7.0 mmol/L) or above, confirmed on repeat testing

2-hour post-meal / OGTT

• Normal: under 140 mg/dL (under 7.8 mmol/L)
• Prediabetes: 140–199 mg/dL (7.8–11.0 mmol/L)
• Diabetes range: 200 mg/dL (11.1 mmol/L) or above

A1C

• Normal: below 5.7%
• Prediabetes: 5.7%–6.4%
• Diabetes range: 6.5% or above

For a closer look at what a single reading tells you (and what it doesn't), see What Is a Healthy Blood Sugar Reading?

Why "healthy" isn't one-size-fits-all

These diagnostic ranges apply to the general adult population, but the target a specific person aims for can look different:

• People already diagnosed with diabetes are usually given a personalized target range by their care team, often somewhat higher than "normal" ranges on purpose, to balance control against the risk of blood sugar dropping too low.
• Age can shift how targets are set, especially for older adults or children — see Healthy Blood Sugar Levels by Age.
• Pregnancy uses entirely different diagnostic thresholds for gestational diabetes — covered in Healthy Blood Sugar Levels for Women.
• Where you live can change which units and rounded reference numbers you see — see Healthy Blood Sugar Levels UK for the mmol/L-first version of this guide.

Supporting healthy levels day to day

Regardless of which range applies to you, the everyday levers are largely the same: what and how you eat, how active you are, how well you sleep, and how consistently you manage stress. We cover this in detail in How to Maintain Healthy Blood Sugar Levels.

When to talk to a doctor

A single unusual reading is rarely meaningful on its own — illness, stress, sleep, and meal timing all cause normal variation. Consistently elevated readings, or any reading alongside symptoms like unusual thirst, frequent urination, or unexplained fatigue, are worth discussing with a healthcare provider, who can order proper diagnostic testing and interpret it in the context of your health history.