FFMI: What It Measures and What It Does Not Tell You

BMI has a known flaw: it relates total weight to height without separating muscle from fat. A lifter at 85 kg and 1.75 m is classed as overweight while carrying 12 % body fat. FFMI, the fat free mass index, fixes exactly that by considering only non-fat mass.

It is a more relevant indicator, but it rests on a shaky measurement, and it is routinely used to claim something it cannot establish.

The calculation

Three steps, from your weight, your height and your body fat percentage.

1. Fat free mass = weight × (1 − body fat percentage)
2. FFMI = fat free mass ÷ height²
3. Normalised FFMI = FFMI + 6.1 × (1.80 − height)

The third step brings the result back to a reference height of 1.80 m, so that people of different builds can be compared. It is this normalised version that most tables use.

For example, at 80 kg, 1.75 m and 15 % body fat: fat free mass is 68 kg, FFMI is 68 ÷ (1.75 × 1.75) = 22.2, and normalised FFMI is 22.2 + 6.1 × 0.05 = 22.5.

Reference values

These figures are indicative and describe observations rather than targets.

One useful scale: gaining a single FFMI point represents roughly 3 kg of fat free mass for someone 1.80 m tall. Going from 20 to 22 therefore corresponds to several years of work for an already trained lifter.

The threshold of 25, and why it is shaky

This is the most widespread use of FFMI, and the most questionable. The idea circulates that an FFMI above 25 betrays the use of performance-enhancing drugs. It comes from work in the 1990s that compared bodybuilders who used them with those who did not, and found that the latter rarely exceeded that value.

The problem is not the observation, which is sound, but the conclusion drawn from it. A statistical threshold describes a population, it does not constitute an individual test. Three concrete limits:

• The input measurement is imprecise. The whole calculation depends on body fat percentage, whose estimate varies by several points depending on method. Two points of difference in body fat moves FFMI by around one point.
• Natural outliers exist. Someone very tall, heavily built in the skeleton, or genetically favoured can exceed 25 unassisted, exactly as some exceed so-called elite strength standards.
• FFMI does not measure muscle. It measures everything that is not fat: muscle, but also bone, water, organs and glycogen. A shift in hydration alone moves the result.

A reasonable conclusion: FFMI is a good descriptor of muscular development, and a very poor index of suspicion.

What it is genuinely good for

Tracking a recomposition. Its most interesting use. During a period when weight does not move, a rising FFMI means fat free mass is increasing while fat decreases, something the scale alone never shows. Our article on body recomposition covers the phenomenon.

Checking a bulk. If you gain 4 kg and your FFMI stays flat, most of the gain is fat. The correction then belongs in calories, not in training.

Setting a realistic goal. Knowing that one FFMI point represents three kilos of fat free mass puts eight-week transformation expectations back in proportion.

How to track it without fooling yourself

Since body fat is the weak link in the calculation, the rule is simple: always keep the same estimation method, at the same time, under the same conditions. Skinfold calipers used by the same person, or a bioimpedance scale used in the morning fasted, will give debatable absolute values but consistent ones over time.

It is the trend across three to six months that informs, not today's number. An FFMI calculated once tells you almost nothing, and it is in any case less useful than the progression of your loads. For measurement tracking, our article on body measurements gives the protocol.

Frequently asked questions

Does an FFMI above 25 prove doping?

No. It is a statistical threshold observed across a population, not a test applicable to an individual. The imprecision of body fat measurement and normal variation in build are enough to explain high values in drug-free lifters.

Which method should you use to estimate body fat?

The one you can repeat identically. Skinfold calipers and bioimpedance scales both carry a notable margin of error, but their consistency lets you follow a trend, which is what matters here.

Is FFMI useful for a beginner?

Not much. During the first year, load progression and arm or thigh measurements say far more. FFMI becomes interesting once gains slow and you need to separate fat free mass from fat.

Should you aim for a specific FFMI?

No, and aiming for one mostly leads to massaging the number through the body fat estimate. It is a tracking indicator, not a training goal.

The calculation itself takes ten seconds, provided you have both figures to hand. In Musku, body measurements record your weight and your body fat percentage with their dates, which gives you the history needed to recalculate your FFMI months apart and compare two comparable periods. The calculation stays on your side, Musku does not produce it, but the inputs are already there, next to the load progression that remains the better measure of the work done.