What is a healthy BMI?

For most adults, a BMI between 18.5 and 24.9 is considered the healthy range. It's a screening range, not a target — someone at BMI 26 with good fitness and a normal waist can be healthier than someone at 23 who is inactive.

Is BMI accurate if I'm muscular?

No — BMI counts muscle as if it were fat. Strength athletes routinely land in the 'overweight' category at low body fat. Pair BMI with a waist measurement for a more honest picture.

Should I track BMI or weight?

Track weight — BMI is just your weight rescaled by your fixed height, so day to day it adds nothing. BMI is useful occasionally as a rough check on where your weight sits for your height.

How do I calculate my BMI?

Divide your weight in kilograms by your height in metres squared. For example: 80 kg ÷ (1.8 m)² = 24.7. Or use a calculator that also shows your healthy weight range.

Weightnote · Basics

What Is BMI — and What It Can't Tell You

Guide · 3 min read

BMI is everywhere — doctor's offices, insurance forms, fitness apps. It's useful, crude, and widely misread. Here's what the number actually means and where it stops being helpful.

Quick answer: BMI is your weight in kilograms divided by your height in metres squared. 18.5–24.9 counts as the 'healthy' range. It's a decent population-level screening number but ignores muscle, so treat it as a rough guide, not a verdict.

The formula

BMI = weight (kg) ÷ height (m)². Someone who is 1.75 m and 72 kg has a BMI of 72 ÷ (1.75 × 1.75) ≈ 23.5. In pounds and inches, it's weight × 703 ÷ height².

The standard categories

Under 18.5: underweight.
18.5–24.9: healthy range.
25–29.9: overweight.
30 and above: obesity.

Where BMI falls short

BMI can't tell muscle from fat. A muscular athlete can be 'overweight' by BMI while carrying very little fat; an inactive person can sit in the 'healthy' range with high body fat. It also ignores where fat sits — and waist-level fat matters more for health than hip-level fat.

How to actually use it

Treat BMI as a starting point, not a diagnosis. If your BMI and your waist measurement both point the same way, that's meaningful. And whatever your starting BMI, the number that tells you whether things are improving is your weight trend over weeks — not the category label.

Check your BMI with the free BMI calculator →

Common questions

What is a healthy BMI?: For most adults, a BMI between 18.5 and 24.9 is considered the healthy range. It's a screening range, not a target — someone at BMI 26 with good fitness and a normal waist can be healthier than someone at 23 who is inactive.
Is BMI accurate if I'm muscular?: No — BMI counts muscle as if it were fat. Strength athletes routinely land in the 'overweight' category at low body fat. Pair BMI with a waist measurement for a more honest picture.
Should I track BMI or weight?: Track weight — BMI is just your weight rescaled by your fixed height, so day to day it adds nothing. BMI is useful occasionally as a rough check on where your weight sits for your height.
How do I calculate my BMI?: Divide your weight in kilograms by your height in metres squared. For example: 80 kg ÷ (1.8 m)² = 24.7. Or use a calculator that also shows your healthy weight range.

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