How it works
What "body fat percentage" actually measures
Your total body mass is the sum of fat mass (subcutaneous + visceral fat) and lean mass (muscle, bone, organs, water). Body fat percentage is simply fat mass divided by total mass. It's a more useful indicator of metabolic health than BMI, because two people of identical BMI can carry very different amounts of muscle.
This body fat calculadora uses the US Navy tape-measure formula, which a RAND Corporation study validated against hydrostatic weighing with a standard error of around 3–4 percentage points — not as tight as a DEXA scan, but accessible to anyone with a soft tape measure.
The US Navy formulas, exact
All measurements in centimetres. If you prefer inches, multiply your cm figure by 0.3937.
- Men: %BF = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
- Women: %BF = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
How to measure correctly
Waist — horizontal, at the narrowest point above the navel, breathing out normally. Don't suck in.
Neck — just below the larynx, with the tape perpendicular to the long axis of the neck.
Hip (women only) — the widest point across the buttocks, feet together.
Height — without shoes, heels and shoulders to a wall.
Measure three times and average. First-time users typically over-tighten the tape, which shows up as artificially low readings.
A worked example
Example: a 180 cm tall man with a waist of 84 cm and neck of 39 cm.
waist − neck = 84 − 39 = 45 cm.
log₁₀(45) = 1.6532; log₁₀(180) = 2.2553.
%BF = 86.010 × 1.6532 − 70.041 × 2.2553 + 36.76 = 142.17 − 157.95 + 36.76 = 20.98 %.
This sits at the upper edge of the "acceptable" range for adult men, just above what is often called "average fitness".
Healthy ranges by age and sex
The American Council on Exercise categories below are the most widely quoted. Age adjusts the bars: at 50+ even an active adult typically sits 4–5 points higher than at 25.
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Acceptable | 18–24% | 25–31% |
| Obese | ≥ 25% | ≥ 32% |
Limitations of any single reading
- The US Navy formula assumes typical fat distribution — it over-estimates in very lean athletes with small waists and under-estimates in abdominal-obese adults.
- Hydration swings skew impedance-based scales (BIA) by ± 3% between morning and evening. Tape-measure methods are steadier.
- DEXA is the gold standard but costs £60–£200 in the UK and delivers a very small radiation dose.
- Trend matters more than the absolute number. Track the same method, same tape, same time of day across weeks.
Works well with
Body-fat figures are most useful alongside other data points — these companion calculadoras give you the rest of the picture.
- **BMI calculadora** — sense-check against height-weight.
- **BMR calculadora** — resting metabolic rate.
- **TDEE calculadora** — total daily calories for your activity.
- **Waist-to-hip ratio** — complementary cardiometabolic marker.
How we keep this accurate
The calculadora uses the exact US Navy regression coefficients as published in the Department of Defense Directive 1308.3. See our editorial policy and corrections policy, and please email us if you spot a bug — we publish a change log of every material update.
Comparing the main body-fat measurement methods
Five main methods are used in the UK. Each sits at a different trade-off between accuracy, cost and convenience.
| Method | Typical error | Cost | Notes |
|---|---|---|---|
| US Navy tape measure | ± 3–4 percentage points | Free | Steady if done consistently; validated against hydrostatic weighing |
| Bioimpedance (smart scales) | ± 3–6 percentage points | £30–£200 for the scale | Swings with hydration; good for daily trend |
| Skinfold calipers | ± 2–4 percentage points | £15–£40 for calipers | Requires trained hands; 3-site and 7-site Jackson–Pollock protocols |
| DEXA scan | ± 1–2 percentage points | £60–£200 per scan | Gold standard; small radiation dose |
| BodPod (air displacement) | ± 2–3 percentage points | £40–£80 per session | Less common; university sport-science labs |
Five common pitfalls when tracking body fat
A badly-run body-fat programme frustrates people into abandoning it. Five traps to avoid.
- Measuring too often — daily readings fluctuate with hydration and bloating. Weekly is plenty.
- Changing methods mid-stream — swapping from tape to scales to DEXA yields apparent jumps that aren't real body changes.
- Chasing a number that doesn't match your goals — an endurance runner won't and shouldn't aim for a bodybuilder's 8 %.
- Ignoring the bigger picture — sleep, stress and training load matter more than any specific reading.
- Starving for quick drops — extreme deficits burn muscle too, which lowers metabolism and makes the number harder to maintain.
Essential fat, storage fat and why both matter
Body fat splits into two functional categories.
Essential fat
Bone marrow, central nervous system, cell membranes, breasts, pelvis, hips and thighs. It's critical for hormone production (including oestrogen and testosterone), insulation and protection of internal organs. Men can't sustainably go below ~3 % body fat; women below ~10 % often lose their menstrual cycle and suffer bone-density drops.
Storage fat
The subcutaneous fat under the skin and the visceral fat around organs. This is the fat you can reduce through diet and training without clinical harm. Visceral fat carries the greatest cardiometabolic risk — a large waist despite a "normal" BMI is often a red flag for pre-diabetes and dyslipidaemia.
How the split looks in practice
A 75 kg man at 15 % body fat carries about 11.25 kg of fat. Roughly 2.5 kg is essential; the remaining 8.75 kg is storage. A 65 kg woman at 23 % carries about 15 kg of fat, of which 7 kg is essential.
The science behind the US Navy formula
The US Navy formula was derived by Hodgdon and Beckett at the Naval Health Research Center in the 1980s, validated against hydrostatic weighing on 602 men and 214 women. The coefficients were deliberately chosen to work with the smallest practical set of tape measurements — waist, neck and (for women) hip — so sailors could self-measure on board.
Later studies reproduced the accuracy across civilian populations. A 2008 study in the journal *Military Medicine* compared tape-measure results with DEXA in 2,100 US Marines and found 87 % of readings within 4 percentage points of the DEXA value. The formula can under-estimate body fat in very muscular populations (rugby forwards, powerlifters) because a thick neck skews the subtraction.
Realistic body-fat journeys over 16 weeks
Three real-world scenarios to calibrate expectations.
Office worker going from 24 % to 20 % while keeping weight steady
Three strength sessions a week, protein at 1.8 g/kg, walking 8,000 steps daily. Over 16 weeks the scale barely moves, but the tape shows waist shrinking 4 cm and neck staying the same. This is textbook recomposition.
Marathon runner at 12 % aiming for 10 % ahead of a race
Already lean; a modest 200-calorie deficit for 8 weeks pre-race drops body fat by ~1.5 percentage points without muscle loss. Pushing harder risks underperformance on race day.
Post-pregnancy parent regaining fitness
Starts tape measurements 16 weeks postpartum. Combines pelvic-floor recovery work, gradual resistance training and walking. Body fat drops from 33 % to 28 % over six months with no aggressive dieting — slow, steady and sustainable.
The role of sleep, stress and alcohol
Research in the *British Journal of Nutrition* and *American Journal of Clinical Nutrition* shows three factors with material impact beyond diet and training.
- Sleep under 6 hours raises ghrelin (hunger) and lowers leptin (satiety), easily adding 250–400 kcal/day of appetite.
- Chronic stress raises cortisol, which preferentially deposits visceral fat even without total weight gain.
- Alcohol is calorie dense (7 kcal/g) and suppresses fat oxidation for up to 48 hours. Cutting alcohol typically drops body fat by 1–2 percentage points with nothing else changing.
- Resistance training remains the single biggest long-term lever — muscle burns more calories at rest and redistributes fat.
- Dietary fibre (25–35 g/day) feeds gut microbiota and smooths insulin spikes that drive abdominal fat storage.
When to consult a professional
The tape and calculator are for tracking trends. Three signals suggest professional support is worth seeking.
- Rapid swings (more than 3 points in 4 weeks) without an obvious change in lifestyle — may indicate a hormonal imbalance.
- Persistent inability to lose fat despite a consistent deficit — a registered dietitian can assess metabolism and eating patterns.
- Body fat below the "athlete" range without a sport-specific goal — can mask disordered eating and warrants medical review.
Specialist populations and adjustments
The US Navy formula assumes a typical adult body. Four populations need careful interpretation.
Highly muscular strength athletes
Olympic lifters, rugby forwards and powerlifters often have thick necks relative to their waists, which artificially lowers the formula's output. Pair with calipers or DEXA every 3 months for a cross-check.
Elderly adults
Sarcopenia (age-related muscle loss) means even a "healthy" tape reading can mask worsening body composition. NHS clinicians increasingly use grip strength alongside body fat as a dual marker.
Post-bariatric surgery
After sleeve gastrectomy or gastric bypass, rapid weight loss creates loose skin that can distort tape measurements. Wait until weight stabilises for 3 months before using the calculator as a reliable tracking tool.
Children and adolescents
The formula isn't validated under 18. The NHS recommends using UK90 BMI-for-age charts for paediatric assessment; a GP or paediatric dietitian is the right resource.
How body-fat data fits into wider NHS screening
NHS Health Checks (offered free every five years in England to adults aged 40–74) use a combination of BMI, waist circumference, blood pressure, lipids and HbA1c. Body fat percentage is not routinely measured, but waist circumference — which is part of the tape-measure method — is. Men with waist above 94 cm and women above 80 cm are flagged for weight-management advice, regardless of BMI.
If you're using this calculator alongside a Health Check, focus first on waist circumference for cardiometabolic risk, then use body fat as the longer-term tracking number once you've stabilised lifestyle changes.
