Body Fat and Lean Mass Calculator

What the tape measure can and cannot tell you

The US Navy developed this circumference method in 1984 to assess sailors without laboratory equipment. Against reference scans it estimates body fat to within about 3 to 4 percentage points for most adults, which is good enough to follow a trend but not to compare yourself with someone else to the decimal.

It assumes a typical distribution of fat and muscle. Swelling from lymphoedema, fluid retention during chemotherapy, pregnancy or very high body weights all distort the girths and the result. Bioelectrical impedance scales have similar limits and are also affected by hydration.

Measuring consistently

• Use a flexible, non-stretch tape, snug against the skin but not compressing it.
• Measure in the morning, before eating, standing upright and relaxed.
• Neck: just below the larynx, tape sloping slightly downwards to the front.
• Waist: at the navel for men and at the narrowest point for women, after breathing out normally.
• Hips (women): around the widest point of the buttocks.
• Repeat each measurement twice and use the average. Track monthly rather than daily.

Muscle loss during cancer treatment

Chemotherapy, hormone therapy, steroids, inactivity and the tumour itself all cause muscle to be lost faster than fat. Because muscle is heavy, the scales can stay the same while body composition changes underneath: this is sarcopenia, and when it comes with weight loss and inflammation it is called cachexia.

Low muscle mass predicts worse tolerance of chemotherapy, more complications after surgery and lower survival across many cancers. Oncologists increasingly measure it on the CT scans you already have. At home, a rising body fat percentage with a stable weight, or a falling lean mass, is the signal to raise with your team.

Protecting muscle: what works

ESPEN recommends that people with cancer maintain or increase physical activity during and after treatment to support muscle mass and function, and eat more than 1 g of protein per kilogram of body weight per day. Resistance training two or three times a week, adapted to your energy and any surgical restrictions, is the most effective single measure; many cancer centres run supervised exercise programmes.

Our nutrition calculator gives energy and protein targets for treatment and recovery.

Body fat and cancer risk after treatment

Excess body fat, and abdominal fat in particular, produces hormones and inflammatory signals linked to at least 13 cancers, including breast after the menopause, bowel, endometrial, kidney, oesophageal and pancreatic cancer. For survivors, keeping body fat in a healthy range while rebuilding muscle lowers the risk of recurrence and of the heart disease and diabetes that follow treatment.

Aim for slow change, about half a kilogram a week, with enough protein so that the weight lost is fat rather than the muscle you worked to keep.

Questions to ask your team

Bring the result with you and ask:

• Have my scans shown muscle loss, and does it change my treatment plan?
• Is resistance exercise safe for me now, and is there a supervised programme?
• How much protein should I aim for each day?
• Should I be referred to a dietitian or physiotherapist?