---
title: "BMI Calculator (Metric & Imperial) | Boost Fitness"
canonical_url: "https://boostfitness.io/tools/bmi-calculator"
last_updated: "2026-07-11T17:40:47.642Z"
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.

## **Frequently asked questions **

## **More free tools **

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[One Rep Max Calculator](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.

[Macro Calculator](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.

[Protein Intake Calculator](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).

## **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)

### **Targets that update themselves**

Set it once — your clients see it every day