Relative Fat Mass (RFM) Calculator

Use this calculator to estimate your body fat percentage from just your waist and height, using the Relative Fat Mass (RFM) formula. Checked against DXA body scans, RFM estimated body fat more accurately than BMI, and this calculator shows whether your result reaches the RFM cut-point for obesity.

What Is Relative Fat Mass?

Relative Fat Mass is a simple formula for estimating your whole-body fat percentage. It was published in 2018 by Orison Woolcott and Richard Bergman at Cedars-Sinai Medical Center, who set out to find a measure that anyone could take with a tape measure but that tracked body fat better than BMI.

They used data from more than 12,000 adults in the US National Health and Nutrition Examination Survey (NHANES), whose body fat had been measured by DXA scan, and tested 365 combinations of common body measurements against it. The simplest one that beat BMI needed only two: height and waist. They then checked it on a separate group of nearly 3,500 adults.

How RFM Is Calculated

Measure your height and waist in the same unit, inches or centimeters, and use the formula for your sex:

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

The result is an estimate of your body fat as a percentage of your weight. The women's formula is the same as the men's plus 12 points, because at the same waist and height women carry more of their weight as fat. Since only the ratio of height to waist matters, the larger your waist is for your height, the higher your RFM.

Example: A man who is 5'10" (177.8 cm) tall with a 38 inch (96.5 cm) waist has an RFM of 27.2%, in the Borderline range. He would reach the obesity cut-point at a waist of about 41.1 inches (104.4 cm). A woman who is 5'5" (165.1 cm) tall with a 34 inch (86.4 cm) waist has an RFM of 37.8%, also Borderline, and would reach the cut-point at about 36.1 inches (91.6 cm).

RFM Cut-Points for Obesity

The 2018 study only estimated body fat. In 2020, the same researchers set cut-points for obesity by linking RFM to death from any cause in more than 31,000 NHANES adults followed for a median of about 8 years. The levels that best separated those who died from those who did not were about 41% for women and 29% to 31% for men, which they rounded to 40% for women and 30% for men.

After adjusting for age, BMI, ethnicity, education and smoking, women with an RFM of 40% to 44.9% had a 41% higher risk of death than women under 35%, and men with an RFM of 30% to 34.9% had a 57% higher risk than men under 25%. The same pattern held in a second, older NHANES survey followed for more than 20 years.

This calculator places you in one of three ranges for your sex. Because RFM depends only on your waist-to-height ratio, each range also corresponds to a fixed ratio, the same at any height:

RFM Ranges

Range	Women	Men
Lower	Under 35% WHtR under about 0.49	Under 25% WHtR under about 0.51
Borderline	35% to 39.9%	25% to 29.9%
Obesity	40% or more WHtR about 0.56 or more	30% or more WHtR about 0.59 or more

The lower edge of the Borderline range, 35% for women and 25% for men, was the reference group in the 2020 study and matches a body fat threshold for obesity that some researchers have proposed, though there is no agreed definition. It falls at a waist-to-height ratio close to 0.5, the familiar "keep your waist under half your height" boundary.

RFM Compared With BMI

In the 2018 validation group, RFM estimated body fat within 20% of the DXA value for 91.5% of women and 88.9% of men, against 21.6% and 81.9% for a BMI-based estimate. BMI's weakness was mostly in women: it failed to flag 72% of the women the study classed as having obesity by DXA scan, while RFM missed 5%. The trade-off was more false alarms in women, so RFM leans toward flagging excess body fat rather than missing it.

Because it uses the waist rather than weight, RFM is also less likely than BMI to be misled by muscle, though it has not yet been tested in athletes. A muscular person with a BMI over 30 but a trim waist will usually have a modest RFM, while someone with a normal BMI but a large waist will not.

What Newer Research Shows

Obesity is more common than BMI suggests. A 2023 study using NHANES found that in 2017–2020, 64.7% of US women and 45.8% of US men had obesity by RFM, compared with about 42% of each by BMI.
Mortality. A 2024 study of 46,535 NHANES adults found RFM more strongly linked to diabetes-related deaths than BMI or waist circumference in both women and men, and somewhat better than either at predicting death, most clearly from diabetes.
Diabetes worldwide. A 2026 analysis in Diabetes Care of nearly 600,000 adults with a BMI of 18.5 to 29.9, from 82 low- and middle-income countries, found that RFM and waist circumference identified diabetes better than BMI in both men and women.

How to Measure

Use a flexible, non-stretchable measuring tape. Remove bulky clothing or measure over thin, form-fitting garments, and stand straight with your feet together.

For waist: Find the top of your hip bone (the iliac crest) at your side and wrap the tape around your waist just above it, keeping it level all the way around. Keep it snug without compressing the skin, don't pull your stomach in, and read the tape at the end of a normal breath out. This is the site used in the NHANES surveys RFM was developed from.

For height: Stand barefoot against a wall with your heels, buttocks, and upper back touching the wall. Look straight ahead and have someone mark the highest point of your head. Measure from the floor to this mark.

Limitations of RFM

It is an estimate. Even in the validation group, about 1 in 10 estimates was off by more than 20% of the measured value.
It was developed and tested mainly in Mexican American, European American and African American adults. Its authors note that it still needs to be evaluated in other groups, including Asian and Native American populations, athletes, and people with specific diseases.
It predicted body fat less well in older adults, as it did for every equation the study tested.
DXA, the reference method, tends to underestimate body fat at the lower end and in men, and overestimate it at the higher end and in women.
The obesity cut-points are for adults aged 20 and over.

Frequently Asked Questions about RFM

What is Relative Fat Mass (RFM)?

Relative Fat Mass is a formula that estimates your whole-body fat percentage from just your height and waist. It was developed in 2018 by Orison Woolcott and Richard Bergman at Cedars-Sinai, who tested 365 combinations of body measurements against DXA body scans of more than 12,000 US adults and chose the simplest one that beat BMI. The result is read as a body fat percentage.

How is RFM calculated?

For men, RFM = 64 − (20 × height ÷ waist). For women, RFM = 76 − (20 × height ÷ waist): the same formula plus 12 points, because women carry more body fat than men at the same waist size. Height and waist must be in the same unit, inches or centimeters, since only their ratio matters.

What is a good RFM?

The cut-points for obesity are an RFM of 40% for women and 30% for men. They were set in a 2020 study of more than 31,000 US adults as the levels that best separated people who died during follow-up from those who did not. Below 35% for women and 25% for men was the reference group in that study; above those levels matches a body fat threshold for obesity that some researchers have proposed, though there is no agreed definition.

Is RFM more accurate than BMI?

In the original validation, yes. Checked against DXA scans, RFM estimated body fat within 20% of the measured value for 91.5% of women and 88.9% of men, compared with 21.6% and 81.9% for a BMI-based estimate. It also missed far fewer cases of excess body fat in women: BMI failed to flag 72% of the women the study classed as having obesity by DXA scan, while RFM missed 5%. The trade-off was more false alarms in women.

Why does RFM label more people as having obesity than BMI?

Because BMI misses many people, particularly women, who have a high body fat percentage at a normal or overweight BMI. A 2023 study of US adults found that 64.7% of women and 45.8% of men had obesity by RFM in 2017–2020, compared with about 42% of each by BMI. RFM is measuring body fat rather than weight, and by that measure obesity is more common than BMI suggests.

Do I need to enter my age or weight?

No. The formula uses only height, waist and sex. Its authors tested adding age and ethnicity and found that neither made the estimate more accurate. The obesity cut-points were set for adults aged 20 and over; a 2019 study found the adult formula can also be used for teenagers aged 15 to 19, but the cut-points are not meant for them.

How do I measure my waist for RFM?

Find the top of your hip bone (the iliac crest) at your side and wrap the tape around your waist just above it, level all the way around. Measure at the end of a normal breath out, keep the tape snug without compressing the skin, and don't pull your stomach in. This is the site used in the NHANES surveys RFM was developed from, and a waist measured elsewhere, such as at the navel, will shift the estimate.

How can I lower my RFM?

Because your height doesn't change, the only way to lower your RFM is to reduce your waist. For a man 5 ft 10 in tall, each inch off the waist lowers RFM by a little under one point near the obesity cut-point. There is no way to spot-reduce belly fat, but regular aerobic exercise, resistance training, a diet built on whole foods with fewer refined carbohydrates, adequate sleep and limiting alcohol all reduce body fat over time.

Caution: This calculator is intended for educational purposes only, and is not for medical use. A healthcare provider can put your result in the context of your wider health picture.

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References

O.O. Woolcott and R.N. Bergman introduced the Relative Fat Mass in Scientific Reports (2018), selecting it from 365 body-measurement indices tested against DXA scans of US adults in NHANES 1999–2006, and showing it estimated whole-body fat percentage more accurately than BMI. View
A 2020 study by O.O. Woolcott and R.N. Bergman in the International Journal of Obesity linked RFM to all-cause mortality in 31,008 NHANES adults (1999–2014) and proposed the obesity cut-points used in this calculator: 40% for women and 30% for men. View
A 2024 study by O.O. Woolcott, E. Samarasundera and A.K. Heath in Scientific Reports followed 46,535 NHANES adults (1999–2018) and found RFM more strongly associated with diabetes-related mortality than BMI or waist circumference, and better at predicting mortality. View
A 2026 analysis by J.A. Seiglie and colleagues in Diabetes Care of nearly 600,000 adults in 82 low- and middle-income countries found that RFM and waist circumference identified diabetes better than BMI among people with a BMI of 18.5 to 29.9. View
A 2023 study by O.O. Woolcott and T. Seuring in BMJ Open tracked obesity among US adults from 1999 to 2020 and found it far more common by RFM than by BMI, particularly among women. View
A 2019 study by O.O. Woolcott and R.N. Bergman in Scientific Reports found the adult RFM equation suitable for adolescents aged 15 to 19, and proposed a modified equation for children aged 8 to 14. View
The U.S. Centers for Disease Control and Prevention's NHANES Anthropometry Procedures Manual describes the waist measurement used in the surveys RFM was developed from: taken just above the top of the right hip bone (iliac crest), at the end of a normal breath out. View