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Body composition

Body fat from a tape measure, and the fat-free mass index that actually tracks your training.

The equation takes different terms for each; it is an estimate either way.

cm
kg
cm

Just below the larynx, tape level all the way round, without pulling it tight.

cm

At the navel for men, at the narrowest point for women. Measure after breathing out, not while holding it in.

cm

At the widest point. Used by the equation for women only.

Your fat-free mass index

18.4FFMI

Average muscle for your height. Your body fat estimate is 28.5% — average range.

Body fat
28.5%Average range
Fat mass
20.5 kgWhat the estimate says you are carrying
Fat-free mass
51.5 kgMuscle, bone, organs and water
FFMI, unadjusted
17.8Fat-free mass divided by height squared
FFMI, normalised to 1.8 m
18.4The comparable figure — height alone would otherwise move it by two points

Worth knowing. The tape method sits within about 3–4 points of a DEXA scan, which makes it good for a direction over months and poor for a single reading. Measure at the same time of day, in the same state, and only compare readings taken the same way. If a number here worries you, it is worth a conversation rather than a diet.

Train with a coachFat-free mass is the number that moves with structured resistance work, and it is the one worth watching.

How this is worked out

US Navy circumference method

Body fat is estimated from height, neck and waist — plus hips for women — using the Navy equation in its original inch form, with centimetres converted on the way in. It needs no calipers and no scan, which is why it is the one most people can actually repeat.

Fat-free mass index, normalised

Fat-free mass divided by height in metres squared, then adjusted to a 1.8 m standard. Without the adjustment, height alone shifts the index by around two points and buries the thing you are trying to see.

Why there is no BMI here

BMI cannot tell muscle from fat, so it is wrong in exactly the direction that matters for anyone lifting — and it has nothing left to say once fat-free mass is on the screen.

How wrong it can be

About 3–4 percentage points against a DEXA scan, and more if the tape is pulled tight or held at an angle. Track the trend across months, not the reading on a Tuesday.

Other tools

  • Trek readinessWhat the trek asks against what you are doing now — the gap, named, and the weeks to close it.
  • Hike timeNaismith's rule, corrected for descent, pack and the stops nobody plans for.
  • Heart-rate training zonesFive zones from your own resting rate, not a formula that ignores your fitness.
  • Daily energy and proteinWhat a day actually costs you, and how much protein supports the training on top.
  • One-rep maxYour working maximum as a range, from two formulas that disagree — plus the percentages to train off.
  • Interval timerTabata, EMOM, hill repeats — with sound, a wake lock, and a clock that stays right in your pocket.

What would you do with this?

That is the part worth talking through, and the first conversation is free and takes half an hour.

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