---
title: "BMI Calculator (Metric & Imperial) | Boost Fitness"
canonical_url: "https://boostfitness.io/tools/bmi-calculator"
last_updated: "2026-09-02T05:56:17.792Z"
meta:
description: "Free BMI calculator in metric or imperial units. Get your body mass index, see the NHS weight categories, and learn what BMI can and cannot tell you."
"og:description": "Free BMI calculator in metric or imperial units. Get your body mass index, see the NHS weight categories, and learn what BMI can and cannot tell you."
"og:title": "BMI Calculator (Metric & Imperial) | Boost Fitness"
---
Boost Fitness
# BMI calculator
**Calculate your body mass index in metric or imperial units and see where you sit against the NHS weight categories — along with an honest explanation of what BMI misses, especially for athletes and muscular builds.**
**Height (cm)**
**Weight (kg)**
Your BMI
**24.5**
NHS category
**Healthy weight**
**NHS adult BMI categories **
| **BMI ** | **Category ** |
| --- | --- |
| Below 18.5 | Underweight |
| 18.5 – 25 | Healthy weight |
| 25 – 30 | Overweight |
| 30 – 40 | Obese |
| 40 and above | Severely obese |
**General guidance, not medical advice**
This calculator gives population-level estimates for healthy adults. It is not a diagnosis or a prescription — speak to your GP or a registered professional before making significant changes, especially if you are pregnant, under 18, or managing a medical condition.
## How BMI is calculated
Body mass index is your weight in kilograms divided by your height in metres squared (kg/m²). Imperial inputs are converted first — feet and inches to centimetres, stones and pounds to kilograms — so both unit systems produce exactly the same result. A 1.75 m person weighing 75 kg has a BMI of 75 ÷ 1.75² = 24.5, which sits inside the NHS healthy-weight band of 18.5–24.9.
BMI was designed as a quick population-level screening measure, and at that job it is genuinely useful: across large groups, higher BMI correlates with higher risk of type 2 diabetes, cardiovascular disease, and several cancers. That is why your GP records it and why the NHS builds guidance around the bands above.
## The honest limitations
For individuals — especially people who train — BMI has well-known blind spots, and it would be dishonest for a fitness company to pretend otherwise:
- **It cannot see body composition.** Muscle is denser than fat, so a lean 85 kg lifter and an untrained 85 kg person of the same height get identical scores. Most rugby players and many dedicated gym-goers are "overweight" by BMI while being metabolically healthier than average.
- **It ignores where fat is stored.** Visceral (abdominal) fat drives most of the health risk, which is why waist-to-height ratio — keep your waist under half your height — is a valuable companion measure.
- **The standard bands don't fit everyone.** The NHS applies lower thresholds (23 and 27.5) for people of South Asian, Chinese, other Asian, Middle Eastern, Black African, and African-Caribbean backgrounds, and the adult bands are not valid for children, during pregnancy, or without caveats in older adults.
If you want the quick companion check now: measure your waist at the midpoint between the bottom of your ribs and the top of your hips, against bare skin, after a relaxed breath out. If that number is more than half your height, it is worth acting on regardless of what the BMI band says — and if it is comfortably under half, a mildly "overweight" BMI is rarely a cause for concern on its own.
## How a coach should actually use it
Treat BMI as one data point at intake, never as a target. For clients far from a healthy weight it is a reasonable starting marker; for trained clients it is close to meaningless, and coaches should lean on waist measurements, progress photos, body-weight trends, and performance instead. What moves those markers is consistent training and nutrition — if that is the next step, our [macro calculator](https://boostfitness.io/tools/macro-calculator) and [protein intake calculator](https://boostfitness.io/tools/protein-intake-calculator) will give you practical daily targets, and [Boost Fitness](https://boostfitness.io/features/nutrition) lets coaches track all of these measures for every client in one place.
**Useful detail **
## Frequently asked questions
For most adults, the NHS defines a healthy BMI as 18.5 to 24.9. Below 18.5 is classed as underweight, 25 to 29.9 as overweight, 30 to 39.9 as obese, and 40 or above as severely obese. For people of South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean backgrounds, the NHS uses lower thresholds — 23 for increased risk and 27.5 for high risk — because health risks appear at lower BMI values in these groups.
BMI only knows your height and weight — it cannot tell muscle from fat. Muscle is denser than fat, so lean, well-trained people (rugby players, sprinters, dedicated lifters) routinely land in the "overweight" band while carrying very little body fat. If that describes you, waist measurement, body-fat estimates, and performance markers are far more informative.
No — it is a screening tool for populations, not a diagnosis for individuals. It is least reliable for athletes and very muscular people, adults over about 65 (where slightly higher BMIs are associated with better outcomes), pregnant women (it is not valid in pregnancy), and children, who use age- and sex-adjusted centile charts instead of the adult bands.
Waist-to-height ratio is a simple, evidence-backed companion: the NHS suggests keeping your waist below half your height, because it captures the abdominal fat that BMI misses. Coaches typically combine several markers — waist measurement, progress photos, body-weight trend over weeks, and training performance — rather than judging anything from a single number.
Rarely — it moves slowly and day-to-day weight fluctuations make frequent checks misleading. Monthly, or at natural review points in a training programme, is plenty. The trend across months matters far more than any single reading.
**Keep calculating **
## More free tools
[**One Rep Max Calculator**
Estimate your 1RM from any set with the Epley and Brzycki formulas, plus a 50–95% loading table.](https://boostfitness.io/tools/one-rep-max-calculator) [**Macro Calculator**
Calories and macros for any goal — BMR (Mifflin-St Jeor), TDEE, and a protein/fat/carb split.](https://boostfitness.io/tools/macro-calculator) [**Protein Intake Calculator**
An evidence-based daily protein range (1.2–2.2 g/kg) for your body weight and goal.](https://boostfitness.io/tools/protein-intake-calculator)
From Boost Fitness — see [**nutrition coaching features **](https://boostfitness.io/features/nutrition), [**pricing **](https://boostfitness.io/pricing), or [**how we compare **](https://boostfitness.io/compare).
**Ready when you are**
## Prescribe targets to clients automatically
**Boost Fitness turns numbers like these into daily calorie, macro, and training targets on each client's calendar — with a food diary, programme builder, and progress tracking built in. Start your 30-day free trial.**
[Start your 30-day free trial](https://boostfitness.io/register) [Explore nutrition coaching](https://boostfitness.io/features/nutrition)
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