--- title: "Body Fat Calculator UK: Tape Measure in Centimetres | AumnI tools" description: "Estimate body fat with the US Navy method from neck, waist and height in centimetres, with an age range and a BMI to read against NHS bands." url: https://aumnitools.com/uk/tool/body-fat-calculator category: "Health" country: "United Kingdom" updated: 2026-11-08 --- # Body Fat Calculator Most UK tape measures and bathroom scales are metric, so this body fat calculator opens in centimetres and kilograms. It runs the US Navy circumference equation on neck, waist and height, adds hips for women, and shows a category and an age-based range beside the BMI. It estimates only, so ask a GP if the figure worries you. ## United Kingdom rules and figures - The UK page opens on Metric, so weight is typed in kilograms and height, neck, waist and hip in centimetres rather than in stones or inches. - The BMI beside the percentage can be read against NICE and NHS adult bands: Healthy weight from 18.5 to 24.9, Overweight from 25 and Obesity class 1 from 30. - For some family backgrounds NICE advises a lower BMI trigger, with overweight beginning at 23 and obesity at 27.5, so a BMI near 24 may be read differently. - Children and teenagers aged 2 to 17 have their own NHS BMI calculator, and this page does not check age before it shows a percentage. Sources: [NICE NG246: Classifying overweight and obesity in adults](https://www.nice.org.uk/guidance/ng246/chapter/identifying-and-assessing-overweight-obesity-and-central-adiposity) (checked 2026-11-08); [NHS: Obesity diagnosis](https://www.nhs.uk/conditions/obesity/diagnosis/) (checked 2026-11-08); [NHS: BMI calculator](https://www.nhs.uk/health-assessment-tools/calculate-your-body-mass-index/) (checked 2026-11-08) ## How the body fat calculator works Centimetres go straight into the Navy equation with base-10 logarithms, and kilograms are used only to work out the BMI printed beside the percentage. Formula: `Men: 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450. For women the log term uses waist + hip − neck, with the constants 1.29579, 0.35004 and 0.22100` - The Unit System box opens on Metric for the UK. Switching it to Imperial changes the labels to pounds and inches, and the choice is kept. - A result appears once weight, height, neck and waist are filled in, plus hip when Female is picked. Without a hip the women's equation cannot run. - Age picks only the range text, so a 34-year-old man sees 11-17% and a 41-year-old woman sees 16-25%. It never moves the percentage. - If the tape readings make the log input zero or negative, the page asks you to re-measure rather than printing a number. ## How to use the body fat calculator 1. **Pick gender and metric units.** Choose Male or Female, and leave Unit System on Metric. Female brings up a hip box. 2. **Enter age, weight and height.** Use kilograms for weight and centimetres for height. Age is optional and only decides which range is quoted. 3. **Measure with a soft tape.** Record neck, waist and, for women, hips in centimetres. Keep the tape level and snug without pressing into the skin. 4. **Check the bands.** The Categories tab lists the bands for each sex, and the Guide tab explains where to place the tape. 5. **Copy what you found.** Copy Results puts the figures in plain text so you can paste them into a note or an email to a GP. ## Example: 178 cm, 85 cm waist, 34 years old Take a 34-year-old man of 178 cm and 80 kg, neck 38 cm, waist 85 cm. The page returns 16.4% body fat, Fitness, an 11-17% age range and a BMI of 25.2. For a 41-year-old woman, 165 cm and 62 kg, neck 32, waist 78 and hips 98 cm, the figures are 30.4%, Average, a 16-25% range and a BMI of 22.8. ## When to use the body fat calculator - **Adding a second view to BMI.** Enter your neck and waist to see a body fat estimate next to the BMI you got from the NHS bands. - **Tracking waist change.** Re-enter the same neck and height with a new waist reading to see how the estimate moves. - **Preparing questions for a GP.** Bring the percentage, the category and the BMI, and ask which of them matters for your situation. ## What the body fat calculator does not do - The Navy equation estimates from tape readings. It cannot see muscle, bone or where fat sits. - The categories and age ranges are the same in every country, and none of them comes from NICE or the NHS. - There is no body fat guidance from the sources used for this page, so the figure should not be read as an NHS category. - Height is one box, so a height given as feet and inches has to be changed to centimetres first. ## When something else is a better fit - **When you want dated tape readings kept for comparison**, use the [Body Measurement Tracker](https://aumnitools.com/tool/body-measurement-tracker.md), which stores dated entries. - **When you only want height and weight set against the NICE and NHS bands**, use the [BMI Calculator](https://aumnitools.com/tool/bmi-calculator.md). - **When you want a measured figure, or advice on what yours means**, use a GP, or a clinic that offers a DEXA scan, which measures rather than estimates. ## Questions about the body fat calculator ### Why does the page ask me to re-measure? The equation needs waist minus neck, or waist plus hip minus neck, to be above zero. If the tape readings break that, no percentage is shown. Check each reading is in centimetres and the tape is level. ### Does my weight in stones matter? No. Weight is only used for the BMI shown beside the result, and the percentage does not change with it. If you weigh yourself in stones, convert to kilograms before typing, or switch the unit system. ### Why is 16.4% called Fitness when the range says 11-17%? The label comes from the sex-specific bands alone, while the range is a separate lookup by age. A 34-year-old man at 16.4% sits inside 11-17% and under the 18% Fitness limit. ### Is this an NHS measure? No. The NHS and NICE publish BMI bands, not body fat bands. This page uses the US Navy method with its own categories, so treat the number as a rough guide and ask a GP what it means. ## Related tools - [BMI Calculator](https://aumnitools.com/tool/bmi-calculator.md): Works out BMI and its category from height and weight, with the healthy weight range - [Body Measurement Tracker](https://aumnitools.com/tool/body-measurement-tracker.md): Logs weight, body fat and tape measurements by date, with change and waist ratios - [Ideal Weight Calculator](https://aumnitools.com/tool/ideal-weight-calculator.md): Runs six ideal weight formulas for your height and shows the BMI 18.5 to 24.9 weight band ## About this page Runs in your browser: what you enter is processed on this device and is not uploaded. General information only, not medical advice. Talk to a doctor or other health professional about decisions that affect your health. Last checked against the tool's code on 2026-11-08. Interactive version: https://aumnitools.com/uk/tool/body-fat-calculator