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BMI calculator: BMI, FFMI and waist-to-height ratio
By Setonomy · published
Body mass index (BMI) relates your weight to your height: weight in kilograms divided by height in meters squared[1]. It’s simple and commonly used to classify underweight and overweight in adults[1]. What it can’t see is what your weight is made of. So if you like, this calculator adds two more numbers: your waist-to-height ratio and your FFMI, which counts only fat-free mass.
Calculate your BMI
Enter your weight and height. Add your waist size to see your waist-to-height ratio, and your body fat percentage to see your FFMI. The calculator is for adults 18 and over. For children and teens aged 5 to 19, the WHO compares BMI with growth charts by age; for younger children, it uses weight-for-height[2].
BMI · Underweight · Normal range · Overweight (pre-obese) · Obesity class I · Obesity class II · Obesity class III
Waist-to-height ratio · below 0.4, which NICE doesn’t classify · belly fat in the healthy range · increased belly fat · high belly fat
FFMI normalized to 1.80 m (5 ft 11 in)
Fat-free mass . How accurate this is depends on how your body fat was measured.
With a lot of muscle mass, your BMI can be 25 or more without excess fat being the reason; NICE advises interpreting it with caution then. With a BMI under 35, the waist-to-height ratio applies even with a lot of muscle mass.
For people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, NICE uses lower thresholds: overweight from 23, obesity from 27.5.
From age 65, NICE advises interpreting BMI with caution: other health conditions matter, and a slightly higher BMI may be protective in older age.
From a BMI of 35, NICE no longer classifies the waist-to-height ratio.
How the calculator works
BMI is your weight in kilograms divided by your height in meters squared. The WHO’s example is 70 kg at 1.75 m: 70 / 1.75² = 22.9[1]. The calculator converts pounds and inches first: 154 lb at 5 ft 9 in comes to 22.7. It shows one decimal place and classifies by that number, so 24.98 shows as 25.0 and then counts as 25.0.
| BMI | Category |
|---|---|
| under 18.5 | Underweight |
| 18.5 to 24.9 | Normal range |
| 25.0 to 29.9 | Overweight (pre-obese) |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40.0 and over | Obesity class III |
The cutoffs are the same for women and men and don’t depend on age. According to the WHO, the risk of related health problems rises steadily from a BMI above 25[1].
What BMI can’t see
BMI doesn’t tell muscle from fat[1]. That shows clearly in athletes: in a study of 439 college students, many with a BMI of 25 or more counted as overweight without having excess body fat. The best cutoffs there were 27.9 for male athletes and 27.7 for female athletes[3]. The UK’s National Institute for Health and Care Excellence (NICE) therefore advises interpreting BMI with caution in people with a lot of muscle mass[4]. It advises caution from age 65 as well, for other reasons: other health conditions and limits on physical function count too, and a slightly higher BMI may be protective in older age[4].
Background matters too. For people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, health risks start at a lower BMI; NICE uses overweight from 23 and obesity from 27.5 for them[4]. In 2004, a WHO expert consultation named additional action points for Asia at 23, 27.5, 32.5 and 37.5, but kept the WHO cutoffs as the international standard[5].
A commission in the journal The Lancet Diabetes & Endocrinology recommends using BMI only for populations and for screening, not as a measure of one person’s health. Excess fat should be confirmed with a second measurement, such as the waist-to-height ratio; above a BMI of 40, that isn’t necessary[6].
Waist-to-height ratio
Your waist shows how much fat sits around your middle better than your weight does. So NICE recommends that adults with a BMI under 35 also work out their waist-to-height ratio: waist divided by height, both in the same unit. NICE’s example: 38 inches divided by 67 inches is 0.57, as is 96.5 cm divided by 170 cm. Measure midway between your lowest rib and the top of your hip bone, usually just above your belly button, after breathing out normally[4].
- 0.4 to 0.49: belly fat in the healthy range, no increased risk
- 0.5 to 0.59: increased belly fat, increased risk
- 0.6 and over: high belly fat, further increased risk
With a BMI under 35, these categories apply to women and men, any background, and people with a lot of muscle mass. NICE’s rule of thumb: keep your waist to less than half your height[4].
FFMI: fat-free mass for your height
FFMI, the fat-free mass index, works like BMI but counts only fat-free mass: your weight minus your body fat, divided by your height squared. VanItallie and colleagues proposed it in 1990 to assess nutritional status regardless of height[7].
In 1995, Kouri and colleagues also normalized it to a height of 1.80 m (5 ft 11 in): FFMI + 6.3 × (1.80 − height in m). In 74 athletes who didn’t use anabolic steroids, the normalized values went up to 25.0; 20 Mr. America winners from 1939 to 1959 averaged an estimated 25.4. The authors call their findings preliminary[8]. The calculator shows both numbers and doesn’t rate them.
For comparison: in a Swiss sample of 5,635 adults, the median FFMI at ages 18 to 34 was 18.9 for men and 15.4 for women[9]. How accurate an FFMI is depends on your body fat percentage, and so on how it was measured. NICE doesn’t accept bioimpedance as a substitute for BMI as a measure of overall body fat[4].
Your weight in Setonomy
Setonomy doesn’t calculate BMI and doesn’t ask for your height. But you can log your weight there or bring it in from Apple Health: the trend weight smooths out day-to-day swings and shows where you’re heading, along with the change in 30 days. It’s free. More in the help on body weight.
Frequently asked questions
What BMI counts as overweight?
Does BMI apply to children?
The cutoffs are for adults only. For children and teens aged 5 to 19, the WHO compares BMI with growth charts by age; for children under 5, it uses weight-for-height[2].
Why is my BMI high with a lot of muscle?
What FFMI values are typical?
The calculator doesn’t rate your FFMI. For comparison: in Swiss adults aged 18 to 34, the median was 18.9 for men and 15.4 for women[9].
How do I measure my waist?
Wrap the tape around your waist midway between your lowest rib and the top of your hip bone, usually just above your belly button, and measure after breathing out normally[4].
Does Setonomy calculate my BMI?
No. Setonomy doesn’t ask for your height. The app shows your current weight, along with the trend weight and the change in 30 days.
Sources
- World Health Organization (2000): Obesity: preventing and managing the global epidemic. Report of a WHO Consultation. WHO Technical Report Series 894, section 2.3 and table 2.1, retrieved October 25, 2026
- World Health Organization: Obesity and overweight, fact sheet, October 7, 2026, retrieved October 25, 2026
- Ode JJ, Pivarnik JM, Reeves MJ, Knous JL (2007): Body mass index as a predictor of percent fat in college athletes and nonathletes. Medicine & Science in Sports & Exercise 39(3), 403–409, retrieved October 24, 2026
- National Institute for Health and Care Excellence (NICE) (2025, updated 2026): Overweight and obesity management, NICE guideline NG246, recommendations 1.9.5–1.9.15, retrieved October 25, 2026
- WHO Expert Consultation (2004): Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet 363(9403), 157–163, retrieved October 24, 2026
- Rubino F et al. (2025): Definition and diagnostic criteria of clinical obesity. The Lancet Diabetes & Endocrinology 13(3), 221–262, retrieved October 24, 2026
- VanItallie TB, Yang MU, Heymsfield SB, Funk RC, Boileau RA (1990): Height-normalized indices of the body’s fat-free mass and fat mass: potentially useful indicators of nutritional status. American Journal of Clinical Nutrition 52(6), 953–959, retrieved October 24, 2026
- Kouri EM, Pope HG Jr, Katz DL, Oliva P (1995): Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clinical Journal of Sport Medicine 5(4), 223–228, retrieved October 24, 2026
- Schutz Y, Kyle UU, Pichard C (2002): Fat-free mass index and fat mass index percentiles in Caucasians aged 18–98 y. International Journal of Obesity 26(7), 953–960, retrieved October 24, 2026