Health📅 18 March 2025⏱ 5 min read
How to Calculate Body Fat Percentage Without a Scan
Body fat scanners cost hundreds of pounds, but several reliable formulas estimate body fat percentage from simple measurements. Here's how to use the most accurate methods at home.
JW
James WhitfieldPersonal Finance & Maths WriterJames has written about personal finance, health metrics, and everyday mathematics for over six years. He holds a BSc in Mathematics from the University of Leeds.
DEXA scans are the gold standard for body composition measurement — but at £50–100 per scan, most people rely on estimates. Several validated formulas produce surprisingly accurate results from measurements you can take at home.
Method 1: Navy Body Fat Formula
Developed by the US Navy, validated across large populations, and considered the most practical tape-measure method.
Men:
Body Fat % = 495 / (1.0324 − 0.19077×log(waist−neck) + 0.15456×log(height)) − 450
Women:
Body Fat % = 495 / (1.29579 − 0.35004×log(waist+hip−neck) + 0.22100×log(height)) − 450
All measurements in centimetres, log = log base 10.
Measurement guide:
- Waist: at the narrowest point (typically at navel level for men)
- Neck: just below the larynx (Adam's apple), sloping slightly forward
- Hip (women only): at the widest point
- Height: without shoes
Worked Example (Male)
Height: 178cm, Waist: 88cm, Neck: 38cm
log(waist−neck) = log(88−38) = log(50) = 1.699
log(height) = log(178) = 2.2504
BF% = 495 / (1.0324 − 0.19077×1.699 + 0.15456×2.2504) − 450
= 495 / (1.0324 − 0.3242 + 0.3477) − 450
= 495 / 1.0559 − 450
= 468.8 − 450
= 18.8%
Method 2: BMI-Based Estimate (Deurenberg Formula)
Less accurate than tape measurements but only requires height, weight, and age:
BF% = (1.20 × BMI) + (0.23 × Age) − (10.8 × Sex) − 5.4
Sex: 1 for male, 0 for female
Example: 35-year-old male, BMI 24
BF% = (1.20 × 24) + (0.23 × 35) − (10.8 × 1) − 5.4
= 28.8 + 8.05 − 10.8 − 5.4 = 20.65%
What Your Body Fat Percentage Means
CategoryMenWomen
Essential fat (minimum)2–5%10–13%
Athletic6–13%14–20%
Fitness14–17%21–24%
Acceptable / healthy18–24%25–31%
Obese25%+32%+
Accuracy Limitations
Tape measure methods have a margin of error of ±3–4% compared to DEXA. They're best used for tracking trends over time rather than point-in-time accuracy. A consistent measuring technique is more important than the formula used — measure at the same time of day (morning is best), take three measurements and average them, and use the same measuring tape each time.
Bioelectrical Impedance Scales
Consumer body fat scales (including most Fitbit, Garmin, and smart scales) use bioelectrical impedance — they send a small current through your body and estimate fat from resistance. Accuracy is genuinely poor (±5–8%) and varies enormously with hydration, recent exercise, and food consumption. Tape measure methods are more reliable for most people.