BMI: Common Mistakes, Tips and FAQs
Introduction
Health numbers are useful when you understand what they measure and what they don't. Many popular formulas are estimates with real limits.
This article covers the most common mistakes people make with bMI, what really moves the result, and answers to the questions we hear most often.
Why it matters
Understanding your figures can help you set sensible fitness goals, track change over time and know when to ask a health professional for advice.
How it's calculated
BMI is weight in kg divided by height in metres squared. The NHS adult ranges are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight and 30 or more obese. BMI doesn't account for muscle, age or ethnicity.
What changes the result most
We raised each input by 10% on its own, starting from a typical example where your BMI is 24.1. Here's what happened:
- Raising height (feet) from 5 to 5.5 changes your BMI to 20.4 (-15.2%).
- Raising weight (stone) from 12 to 13.2 changes your BMI to 26.5 (+10.0%).
- Raising height (inches) from 10 to 11 changes your BMI to 23.4 (-2.8%).
The result is especially sensitive to height (feet) – a 10% change there moves it by more than 10%, so get that figure right first.
Common mistakes to avoid
- Treating a single number as a complete picture of health.
- Using formulas designed for adults to assess children.
- Measuring inconsistently, such as at different times of day.
- Changing diet or exercise drastically based on one estimate.
A quick example
With the inputs below, your BMI comes out at 24.1.
| Input | Value |
|---|---|
| Height (feet) | 5 |
| Height (inches) | 10 |
| Weight (stone) | 12 |
| Weight (pounds) | 0 |
Tips
- The NHS uses BMI as a screening tool, but waist measurement also matters.
- Estimates like BMR and body fat can vary by 5–10% from lab measurements.
- Track trends over weeks rather than day-to-day changes.
- Speak to your GP or midwife about any results that worry you.
Frequently asked questions
What's the quickest way to work out bMI?
Use our free bMI Calculator (Stone and Pounds). It applies this method automatically: BMI is weight in kg divided by height in metres squared. The NHS adult ranges are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight and 30 or more obese. BMI doesn't account for muscle, age or ethnicity.
Is this medical advice?
No. These tools give general estimates. For medical advice, speak to your GP, pharmacist or NHS 111.
Can I use these for children?
No. Children's BMI and health measures use age- and sex-specific charts. Use the NHS healthy weight calculator for children.
Why use stone and pounds?
Many people in the UK still think in stone and pounds, so we accept those units and show metric too.
Related calculators
- BMR Calculator
- Calorie Calculator
- Body Fat Calculator
- Healthy Weight Range Calculator
- Water Intake Calculator
- Alcohol Units Calculator
Try the bMI Calculator (Stone and Pounds)
Enter your own figures and get an instant answer – free, no sign-up.
Open the bMI Calculator (Stone and Pounds)For the full method, read How to Calculate BMI in the UK: Step-by-Step Guide.
More health and fitness guides
- BMR: Common Mistakes, Tips and FAQs
- Daily Calorie Needs: Common Mistakes, Tips and FAQs
- Body Fat Percentage: Common Mistakes, Tips and FAQs
- A Healthy Weight Range: Common Mistakes, Tips and FAQs
- Daily Water Intake: Common Mistakes, Tips and FAQs
This guide is general information, not financial, tax, legal or medical advice. Figures use 2025/26 UK rates where relevant. Always check GOV.UK or NHS.uk for official guidance.