FFMI Calculator (Fat-Free Mass Index)

Fat-Free Mass Index is the muscular counterpart to BMI. Instead of dividing total body weight by height squared, it divides only the fat-free portion — your muscle, bone, organs and water — by height squared. That removes the biggest complaint about BMI: that a heavily muscled person gets the same label as someone carrying a lot of fat.

There is one refinement that makes FFMI fair across heights. Taller people naturally carry more lean mass, so a raw index quietly favours them. Researchers therefore apply a small height correction to produce a "normalised" FFMI: add 6.1 multiplied by 1.8 minus your height in metres. For someone exactly 1.8 m tall the correction is zero; shorter people gain a little, taller people lose a little, and the result is comparable between individuals.

The scale most often quoted comes from a study of experienced bodybuilders. In men, a normalised FFMI around 18 to 20 is average, 20 to 22 is above average, 22 to 23.5 is excellent, and values above 25 are rare enough that they historically prompted suspicion of drug use — though the original study's ceiling of 25 has been questioned, and well-trained natural athletes do exceed it, especially at taller heights. Women sit several points lower on average because of differences in muscle mass.

To use the calculator you need a reasonably accurate body-fat percentage, because a small error there is amplified in the result. If you have a tape-measure or caliper estimate, use that rather than guesswork. Track normalised FFMI over a training block to see whether a bulk is building muscle or merely adding fat: if weight rises but FFMI stays flat, the gain is mostly fat, and it may be time to adjust your calorie surplus.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.