Relative Fat Mass (RFM) Calculator

By Rick Campbell · Updated 12 Sept 2026 · Sourced to primary literature · Not medical advice

Relative Fat Mass estimates what percentage of your body is fat using two measurements you almost certainly have to hand: your height and your waist. No neck, no hips, no scale. It was published in 2018 by Orison Woolcott and Richard Bergman, who searched more than 350 possible anthropometric combinations against DEXA scans in the US national health survey and found this one worked best.

The equation is deliberately plain: 64 minus twenty times your height divided by your waist for men, and 76 minus the same term for women. That is the whole thing. What makes it interesting is that it beat body mass index against DEXA in the validation sample, particularly for women, and produced fewer false negatives, people whose body fat was genuinely high while BMI called them normal.

This page gives you the estimate with its honest ±5 point range, the interpretation band it falls in, and the waist-to-height ratio that the same two numbers produce, because the two screens answer slightly different questions.

In brief

Relative Fat Mass estimates body fat percentage from height and waist only: 64 − 20 × (height ÷ waist) for men, 76 − 20 × (height ÷ waist) for women.
It was validated against DEXA scans in NHANES data and predicted whole-body fat percentage better than BMI, with fewer missed cases of high body fat.
The accuracy is roughly ±5 percentage points for an individual, so it is a screening estimate, not a measurement.
Because it sees only a waist, RFM underestimates very muscular people and cannot tell where fat sits beyond the waistline.
The same two measurements also give your waist-to-height ratio, which has a single well-established threshold at 0.5.

What you'll see here

Your estimated body fat percentage with its ±5 point range, the ACE band it falls in, the waist that sits on every band boundary at your height, the centimetres that would move the estimate by one point, your waist-to-height and WHO waist readings, and, as you add weight, neck or age, the kilograms of fat and lean tissue and a second opinion from up to three independent methods.

ACE body fat categories (men)
Category	Range
Essential fat	2 – 5.9%
Athletes	6 – 13.9%
Fitness	14 – 17.9%
Average	18 – 24.9%
Obese	25 – 60+%

These interpretation bands come from the American Council on Exercise and are the same ones used on our tape-method page, so the two estimates can be read against a single scale. Essential fat is the floor the body needs for hormones and organ function, not a target.

What Relative Fat Mass measures

RFM answers the composition question (how much of your mass is fat) using a shape proxy. The ratio of height to waist captures something BMI cannot: two people of identical height and weight can have waists fifteen centimetres apart, and the one with the larger waist is carrying more fat and less muscle almost every time. By anchoring on that ratio rather than on total mass, RFM sidesteps BMI's biggest blind spot without needing calipers, a tape around the neck, or an appointment.

The trade-off is that it sees only one circumference. A tape-based method like the US Navy formula subtracts a neck measurement, which partly corrects for frame and muscularity; RFM does not. In exchange it needs one fewer measurement, and neck circumference is the one most people get wrong.

How to measure your waist for this

Everything rests on the waist measurement, so take it properly. Stand relaxed, find the midpoint between your lowest rib and the top of your hip bone (for most people that is at or just above the navel), and wrap a flexible tape level all the way around, checking in a mirror that it is not riding up at the back. Breathe out normally and read the tape at the end of that exhale, with the tape snug but not denting the skin.

Measure first thing in the morning, after the bathroom and before eating, and take two or three readings to average. Waist circumference swings two to three centimetres through an ordinary day on food and fluid alone, which is more than a month of genuine fat loss usually shows.

A centimetre of tape error moves an RFM result by about a third of a percentage point.
Sucking in can shave three to five centimetres, the single most common way people fool themselves.
Height matters too: measure without shoes, standing tall, heels together.

How to read your result

The bands shown with your result come from the American Council on Exercise, the same scale our tape-method page uses, so you can compare the two estimates without switching rulers. For men, roughly 6–13% is athletic, 14–17% fitness, 18–24% average and 25% or more falls in the obese range; for women each boundary sits about eight points higher. Essential fat, the minimum the body needs for hormones and organ function, is around 2–5% in men and 10–13% in women, and nobody should be aiming there.

Treat the number as a band rather than a point. A published error of ±5 percentage points means an RFM of 24% is honestly saying 'probably somewhere between 19 and 29'. That is still useful: it tells you which half of the scale you are on, and measured the same way each month it tracks change reliably even if the absolute level is a few points out.

Where the equation comes from and how accurate it is

Woolcott and Bergman developed RFM on 12,581 adults from the 1999–2004 US National Health and Nutrition Examination Survey, all of whom had whole-body DEXA scans, then validated it on a separate 3,456-person sample from 2005–2006. They tested hundreds of candidate combinations of height, weight, waist, hip and calf measurements; the height-to-waist form won.

In that validation, RFM's error against DEXA was smaller than BMI's, and it misclassified fewer people with genuinely high body fat as normal. Independent external validations have been more mixed (a Mexican cohort study found RFM performed similarly to, rather than better than, other anthropometric estimates), which is the usual pattern when an equation fitted in one population meets another. The honest summary: RFM is at least as good as BMI at estimating fatness, from measurements that are easier to take than the alternatives, with an individual error around five points.

Who RFM misreads

Muscular people are the main group it gets wrong, and it gets them wrong in the opposite direction to BMI. A powerlifter with a thick trunk reads high on RFM because the equation assumes a wide waist means fat; a lean sprinter with a narrow waist and heavy legs reads low because RFM never looks below the navel. Pregnancy invalidates it outright, as does significant abdominal swelling from any cause.

It was also fitted on American adults aged 20 and over, so it should not be used for children or teenagers (BMI-for-age percentile charts are the right tool there), and its behaviour at the extremes of height is not well characterised. If you are very short or very tall and the answer looks implausible, a tape-based or skinfold estimate will serve you better.

What to do with the number

If your estimate sits in the average or obese band and your waist-to-height ratio is over 0.5, the two screens agree, and the useful response is the ordinary one: a moderate calorie deficit planned from your maintenance calories, enough protein to hold onto muscle while you lose fat, and resistance training. If the two disagree (a high fat estimate with a waist under half your height, or the reverse), the disagreement is itself information, and a second method is worth the effort.

Either way, re-measure monthly rather than weekly, in the same conditions, and judge by the direction of travel across three or four readings. The absolute number carries a five-point band; the trend does not.

How it's calculated

Relative Fat Mass (Woolcott & Bergman, 2018)

Men: RFM = 64 − 20 × (height ÷ waist) · Women: RFM = 76 − 20 × (height ÷ waist)

Height and waist in the same unit: centimetres or inches, it cancels. The published single-equation form is 64 − 20 × (height/waist) + 12 × sex, with sex = 1 for women.

Fat and lean mass from the percentage

Fat mass = weight × RFM ÷ 100 · Lean mass = weight − fat mass

Arithmetic, not a second estimate; it carries the same ±5 point uncertainty as the percentage it comes from.

Waist-to-height ratio, from the same two measurements

WHtR = waist ÷ height

Shown alongside because it has a single evidence-backed threshold at 0.5, whereas RFM's bands are interpretation ranges.

Worked example: a man of 175 cm with an 86 cm waist

Divide height by waist: 175 ÷ 86 = 2.035.
Multiply by 20: 2.035 × 20 = 40.70.
Subtract from 64 (the male constant): 64 − 40.70 = 23.3% body fat.
Read the band: 23.3% falls in the ACE 'average' range for men, which runs 18% to just under 25%.
Apply the error band: the honest statement is 'probably between 18 and 28 per cent', most likely near 23.
Cross-check with the other screen: 86 ÷ 175 = 0.49 waist-to-height, just inside the healthy band. The two measures agree that this is an unremarkable result, sitting close to the line on both.

Where this number is used in the real world

Population screening, where taking a height and a waist from thousands of people is practical and DEXA scans are not.
Primary care and community health checks, as a fast second opinion when a BMI reading looks out of step with the person in front of you.
Self-tracking between measurements, because a tape is free and the trend over months is what actually matters.
Research settings using existing survey data, where waist and height were recorded but body composition was not.
Remote and telehealth consultations, where the patient can take both measurements at home with a tape measure and no scale.
Cross-checking a tape-method or smart-scale reading: two independent estimates that disagree tell you more than either alone.

Frequently asked questions

Is RFM more accurate than BMI?

In the study that introduced it, yes: against DEXA scans in a large US national sample, Relative Fat Mass estimated whole-body fat percentage with less error than BMI and produced fewer false negatives, meaning fewer people with genuinely high body fat being classed as normal. Independent validations in other populations have found it comparable rather than clearly superior. The fair summary is that RFM is at least as good as BMI at the thing BMI is usually asked to do, using measurements that are just as easy to take.

How accurate is the Relative Fat Mass equation for one person?

About five percentage points either side of a DEXA scan for most adults. That means a result of 24% is honestly saying 'probably between 19 and 29 per cent'. It is a screening estimate rather than a measurement, and like every anthropometric formula its error is not random; it runs in a predictable direction for particular body types, underestimating muscular people with narrow waists and overestimating people who carry mass around the middle without much fat.

Why does RFM use a different constant for men and women?

Because at any given height-to-waist ratio, women carry a higher percentage of body fat than men: more of it subcutaneous, distributed around the hips and thighs where a waist measurement cannot see it. The published equation handles this with a single term, adding 12 for women, which is why the male constant of 64 becomes 76. The slope on the height-to-waist term is identical for both sexes; only the intercept moves.

Can I use RFM if I am very muscular?

With caution, and preferably alongside a second method. RFM assumes a wide waist means fat, so a lifter with a thick, muscular trunk will read higher than their true body fat, while a lean athlete with a narrow waist and heavy legs may read lower. The fat-free mass index is the better tool for tracking muscle, and the tape-based Navy method, which subtracts a neck measurement, corrects partly for frame. Use RFM as one input rather than the verdict.

Does RFM work for children and teenagers?

No. The equation was fitted on adults aged 20 and over, and body composition changes continuously through childhood and puberty in ways a single ratio cannot capture. Children and adolescents are assessed with BMI-for-age percentile charts specific to their age and sex, which is exactly what our child BMI calculator does. Running adult equations on a growing body produces a number with no defensible interpretation.

Should I use RFM or waist-to-height ratio?

They answer different questions from the same two measurements, so use both; this page shows them together. Waist-to-height asks whether your middle is large relative to your frame, and has a single memorable threshold at 0.5 backed by a large body of cardiometabolic evidence. RFM converts that same shape information into an estimated fat percentage, which is more intuitive to track and easier to compare against a DEXA or tape reading. Neither replaces the other.

Keep going

A single number rarely tells the whole story. Alongside the RFM result, the body fat calculator, the waist-to-height calculator, the BMI calculator and the lean body mass calculator each add a different angle on the same measurements. For the reasoning behind the numbers, read Body fat methods, Measure your waist and BMI vs body fat vs waist.

Sources

Woolcott OO, Bergman RN. Relative fat mass (RFM) as a new estimator of whole-body fat percentage: a cross-sectional study in American adult individuals. Sci Rep 2018;8:10980. doi.org/10.1038/s41598-018-29362-1
Corrêa MM, et al. External validation of the relative fat mass (RFM) index in adults from north-west Mexico using different reference methods. PLoS One 2019;14:e0226767. doi.org/10.1371/journal.pone.0226767
Ashwell M, Gibson S. Waist-to-height ratio as an indicator of early health risk. BMJ Open 2016;6:e010159. doi.org/10.1136/bmjopen-2015-010159
WHO. Waist circumference and waist–hip ratio: report of a WHO expert consultation. Geneva, 2008. www.who.int/publications/i/item/9789241501491

Cite this page

Quoting a figure from here in an article, a report or a piece of coursework? Use whichever of these your style guide asks for.

APA: Campbell, R. (2026). Relative Fat Mass (RFM) Calculator. Body Stats. https://bodystats.co/app/relative-fat-mass-calculator
Plain text: “Relative Fat Mass (RFM) Calculator”, Body Stats, last updated 12 September 2026, https://bodystats.co/app/relative-fat-mass-calculator

Every formula and threshold on this page is written out with its primary source on our methodology page. These results are informational and educational, not a diagnosis or a substitute for professional advice. See the medical disclaimer.

Last updated 12 September 2026. Written by Rick Campbell; not medically reviewed. See review status.