The BMI chart for adults, explained

Body Mass Index (BMI) is a simple screening measure — your weight divided by the square of your height — used by the NHS and the WHO to flag whether someone’s weight is likely to sit in a healthy range for their height. It isn’t a diagnosis and it has real limitations, which this guide covers too, but the chart below uses the same standard adult ranges the NHS uses.

The adult BMI chart

These are the standard adult BMI categories, using the same 18.5 / 25 / 30 boundaries as the NHS and WHO:

The WHO calls the 18.5–24.9 band “normal weight”; the NHS calls it “healthy weight”. The numbers are the same. This scale is for adults only — see the section on children below.

BMI chart by height and weight

Find your height on the left and your weight along the top: the number where they meet is your BMI, and the colour shows its category. If the chart is wider than your screen, scroll it sideways to see heavier weights.

• Underweight (below 18.5)
• Healthy weight (18.5–24.9)
• Overweight (25–29.9)
• Obese, Class I (30–34.9)
• Obese, Class II and above (35+)

Using feet and stone? This version covers 4′10″ to 6′4″ and 7 to 18 stone. For weights in between, such as 11st 7lb, your BMI sits between the two neighbouring columns.

Values are rounded to one decimal place, so a result right on a boundary (such as 24.96, shown as 25.0) is coloured by the rounded figure. The colours use the standard adult ranges; if the lower NHS cut-offs for some ethnic groups apply to you (see above), read 23 and over as overweight and 27.5 and over as obese. For your exact figure, use the BMI Calculator.

Healthy weight range by height

Rather than a single BMI number, it’s often more useful to know the actual weight range that counts as healthy for your height. This table shows the weights that correspond to a BMI of 18.5–24.9 at common heights:

Feet-and-inches heights are rounded to the nearest inch. Don’t see your exact height? Altto’s BMI Calculator works out the range for any height, in metric or imperial.

How to calculate BMI by hand

The formula behind every BMI calculator, including Altto’s:

Metric:   BMI = weight (kg) ÷ height (m)²
Imperial: BMI = 703 × weight (lb) ÷ height (in)²

Worked example, metric: someone who is 1.70m tall and weighs 70kg has a BMI of 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2 — inside the 18.5–24.9 range.

Worked example, imperial: someone who is 5′8″ (68 inches) and weighs 154lb has a BMI of 703 × 154 ÷ (68 × 68) = 108,262 ÷ 4,624 = 23.4.

Weigh yourself in stones and pounds? Multiply the stones by 14 and add the pounds to get a total in pounds first — 11st 0lb is 154lb. The 703 in the imperial formula simply converts pounds and inches into the same kg/m² units the metric formula gives directly, so both formulas give the same answer.

Is BMI accurate? What it can’t tell you

BMI is quick, free and needs no equipment, which is why it’s so widely used. But it only looks at height and weight, so it can be misleading for some people:

• Muscle vs. fat: BMI can’t tell the two apart, so very muscular people can be classed as overweight or obese despite having little body fat.
• Ethnic background: the standard ranges were developed mostly from data on white European populations. The NHS notes that people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background can face increased health risks at a lower BMI, and uses lower thresholds for these groups: 23 to 27.4 counts as overweight and 27.5 or above as obese.
• Age: people tend to lose muscle as they get older, so an older adult can carry more body fat than a younger one at the same BMI.
• Pregnancy: BMI isn’t used to judge weight during pregnancy.
• Where weight is carried: BMI says nothing about whether weight sits around the middle, which matters for health (see waist size below).

Altto’s calculator uses the standard WHO scale for everyone. If the lower thresholds apply to you, keep them in mind when reading your result.

Waist size matters too

Because BMI can’t tell where weight is carried, UK guidance pairs it with a second, even simpler check: keep your waist measurement to less than half your height — a waist-to-height ratio below 0.5. Fat stored around the middle is more closely linked to problems like type 2 diabetes and heart disease than weight carried elsewhere.

For example, someone who is 170cm tall should aim for a waist under 85cm; someone who is 5′10″ (70 inches) should aim for under 35 inches. To measure, find the bottom of your ribs and the top of your hips, and wrap a tape measure around your middle halfway between them — usually just above the belly button.

BMI for children and teenagers

The adult chart isn’t used for anyone under 18. Children’s body shape changes as they grow, so their BMI is compared with other children of the same age and sex and given as a centile instead — for example, “on the 60th centile”. The NHS has a separate BMI calculator for children for this. Altto’s calculator is designed for adults only.

How to check your BMI

1. Open Altto’s BMI Calculator and choose metric (cm and kg) or imperial (feet, inches and pounds).
2. Enter your height and weight.
3. Your BMI, category and healthy weight range for your height update instantly, and the chart shows where your result sits.

Your height and weight are about as personal as numbers get, so the calculation runs entirely in your browser tab — nothing you type is sent anywhere or stored.

What to do with your result

A BMI outside the 18.5–24.9 range isn’t a diagnosis, and a result inside it doesn’t guarantee good health either. It’s a prompt to look at the bigger picture. If your result is lower or higher than you expected, or you’re worried about your weight for any reason, a GP or pharmacist can look at it alongside things BMI can’t see, such as your waist size, blood pressure, family history and general health. The NHS website also has free information and support for gaining or losing weight safely.

Common questions

Sources

The categories, thresholds and advice in this guide follow current UK and international guidance:

• NHS — Obesity: diagnosis (BMI ranges, including the lower thresholds for some ethnic groups, and waist size)
• NHS — Calculate your BMI (adults)
• NICE — Overweight and obesity management (NG246, 2025) (BMI and waist-to-height ratio)
• World Health Organization — Obesity and overweight fact sheet

This guide is general information, not medical advice. BMI is a screening measure, not a diagnosis. If you’re worried about your weight or health — or you’re under 18, pregnant, or have a medical condition — speak to your GP or a pharmacist, who can take your full circumstances into account.