BMI Calculator

About BMI Calculator

How Is BMI Calculated?

The metric formula is straightforward. Divide your weight in kilograms by the square of your height in metres.

Worked example (metric): Weight = 75 kg, Height = 1.78 m

BMI = 75 / (1.78 x 1.78) = 75 / 3.1684 = 23.7

Worked example (imperial): Weight = 165 lbs, Height = 5 ft 10 in (70 inches)

BMI = (165 x 703) / (70 x 70) = 115,995 / 4,900 = 23.7

Both formulas give the same result. Toggle between cm/kg and ft, in/lbs in this calculator and the conversion is handled automatically.

BMI Categories (WHO Classification)

The World Health Organization defines the following BMI ranges for adults aged 20 and older:

Category	BMI Range	Risk Level
Severe thinness	Below 16.0	High (malnutrition, organ damage)
Moderate thinness	16.0 - 16.9	Moderate
Mild thinness	17.0 - 18.4	Low
Normal weight	18.5 - 24.9	Low
Overweight (pre-obese)	25.0 - 29.9	Increased
Obese Class I	30.0 - 34.9	High
Obese Class II	35.0 - 39.9	Very high
Obese Class III	40.0 and above	Extremely high

The "normal" range of 18.5 to 24.9 is where population-level health risks are statistically lowest. Individual risk depends on many other factors. In England, 66% of adults aged 18 and over were estimated to be overweight or living with obesity in 2024, with 30% classified as obese, according to NHS Digital's Health Survey for England. In the US, 40.3% of adults aged 20 and over have obesity and 9.4% have severe obesity, according to CDC data. Prevalence is highest among adults aged 40-59 (46.4%) and in the most deprived communities.

What Is a Healthy BMI for My Height?

This table shows the approximate weight range for a "normal" BMI (18.5 - 24.9) at common heights:

Height	Min Weight (BMI 18.5)	Max Weight (BMI 24.9)
5'0" / 152 cm	95 lbs / 43 kg	128 lbs / 58 kg
5'4" / 163 cm	108 lbs / 49 kg	146 lbs / 66 kg
5'8" / 173 cm	122 lbs / 55 kg	164 lbs / 74 kg
6'0" / 183 cm	137 lbs / 62 kg	184 lbs / 84 kg
6'4" / 193 cm	152 lbs / 69 kg	205 lbs / 93 kg

What Are the Limitations of BMI?

BMI is a population-level screening tool, not a diagnostic measure. It has significant limitations for individuals:

Limitation	Who It Affects	Why It Matters
Does not distinguish muscle from fat	Athletes, bodybuilders	Muscular people may register as "overweight" despite low body fat
Does not account for fat distribution	Everyone	Abdominal fat carries higher health risk than fat elsewhere, but BMI cannot tell where fat is stored
Less accurate for older adults	People over 65	Muscle loss with age means BMI may underestimate body fat
Not designed for children	Under 20	Children need age-and-sex-specific BMI percentile charts
Ethnic variation	Asian, South Asian populations	Health risks may increase at lower BMI thresholds - WHO suggests BMI 23 as overweight cut-off for some Asian populations
Does not measure health directly	Everyone	Blood pressure, cholesterol, blood sugar, and fitness level matter more than BMI alone

Despite these limitations, BMI remains useful as a quick, zero-cost screening metric. Large epidemiological studies consistently show that BMI in the normal range correlates with lower risk of type 2 diabetes, cardiovascular disease, and certain cancers at the population level. The key is to treat it as one data point, not the whole picture. With obesity prevalence climbing in both the UK and US - up from 61.2% overweight/obese in England in 2015/16 to 66% in 2024 according to NHS Digital - population-level screening tools remain an important first step.

BMI vs Other Body Composition Measures

Measure	What It Assesses	Pros	Cons
BMI	Weight relative to height	Quick, free, no equipment	Cannot distinguish fat from muscle
Waist circumference	Abdominal fat	Simple, identifies visceral fat risk	Does not capture whole-body composition
Waist-to-hip ratio	Fat distribution	Better predictor of cardiovascular risk than BMI	Requires two measurements
Body fat percentage	Fat vs lean mass	Most accurate body composition measure	Methods vary in accuracy and cost
DEXA scan	Bone, fat, and lean mass	Gold standard accuracy	Expensive, requires clinical visit
Skinfold calipers	Subcutaneous fat	Low cost, portable	Technique-dependent, misses visceral fat

For estimating your body fat percentage without a clinic visit, the Body Fat Calculator uses the US Navy circumference method. It requires waist and neck measurements (plus hip for women) and gives a useful estimate alongside your BMI.

Waist Circumference Thresholds

Many health organisations now recommend measuring waist circumference alongside BMI. The NHS and WHO use these thresholds for increased health risk:

	Increased Risk	Substantially Increased Risk
Men	94 cm / 37 inches or more	102 cm / 40 inches or more
Women	80 cm / 31.5 inches or more	88 cm / 34.5 inches or more

Someone with a BMI of 26 (technically overweight) but a waist circumference well below these thresholds may have a lower actual health risk than someone with a BMI of 24 (normal) but a high waist circumference. Context matters.

A Brief History of BMI

BMI was not invented by a doctor. It was created in the 1830s by Adolphe Quetelet, a Belgian mathematician and astronomer who was trying to define the "average man" using statistical methods. He never intended the formula for individual health screening. It was only in 1972 that physiologist Ancel Keys published a paper in the Journal of Chronic Diseases comparing different weight-for-height indices and concluded that Quetelet's formula was the best simple proxy for body fat at the population level. Keys coined the term "Body Mass Index" in that paper. By the 1980s, health organisations had adopted BMI as a standard screening tool because it requires nothing more than a scale and a tape measure.

Why BMI Fails for Athletes and the Elderly

Muscle is denser than fat. A kilogram of muscle takes up about 20% less space than a kilogram of fat. This means someone who strength trains regularly can carry significant muscle mass and register a BMI of 27 or 28 while having a body fat percentage of 12-15%, well within healthy range. Rugby players, sprinters, and CrossFit athletes are routinely classified as "overweight" or even "obese" by BMI despite being lean and fit. The Body Fat Calculator gives a better picture for people who carry more muscle than average.

At the other end of the spectrum, older adults lose muscle mass progressively after about age 50, a process called sarcopenia. An 80-year-old with a BMI of 23 may actually have a higher body fat percentage than a 30-year-old with the same BMI, because much of the older person's weight is fat rather than muscle. Some geriatric research, including a large meta-analysis published in the American Journal of Clinical Nutrition (Winter et al., 2014), suggests that the lowest mortality risk for people over 65 actually falls in the BMI 23-29 range, higher than the standard "normal" range.

Children's BMI: Percentile Charts, Not Fixed Ranges

The standard BMI categories (18.5-24.9 = normal) only apply to adults aged 20 and over. For children and teenagers aged 2-19, the CDC uses age-and-sex-specific growth charts that express BMI as a percentile. A child's BMI is calculated with the same formula, but instead of comparing it to fixed thresholds, it is plotted against what other children of the same age and sex weigh. The categories are:

Percentile Range	Category
Below 5th percentile	Underweight
5th to 84th percentile	Healthy weight
85th to 94th percentile	Overweight
95th percentile and above	Obese

These percentile charts are based on CDC growth data from the 1960s-1990s, before the childhood obesity epidemic took off. A child at the 90th percentile today is heavier relative to children in the reference population, not relative to today's children. Paediatricians track BMI percentile over time to spot trends rather than relying on a single reading.

BMI in Pregnancy

BMI is not a useful measure during pregnancy. Weight gain during pregnancy is expected and healthy, driven by the growing baby, placenta, amniotic fluid, increased blood volume, and breast tissue changes. The NHS and the American College of Obstetricians and Gynecologists (ACOG) both recommend using pre-pregnancy BMI to guide how much weight gain is appropriate during pregnancy, not measuring BMI during it. Their general guidelines are 11.5-16 kg (25-35 lbs) total gain for someone who started at a normal BMI, less for those who started overweight, and more for those who started underweight. If you are pregnant, this calculator is not the right tool. Talk to your midwife or doctor about healthy weight gain for your specific situation.

Ethnic-Specific BMI Thresholds

The standard BMI cutoffs were developed primarily from studies of European populations. Research has shown that health risks start increasing at lower BMI values in some ethnic groups. The WHO published a 2004 expert consultation report noting that South Asian, Chinese, and other Asian populations develop type 2 diabetes and cardiovascular disease at BMIs of 23-24, well below the standard "overweight" threshold of 25. Some key adjusted thresholds used in clinical practice:

Population	Overweight Cutoff	Obese Cutoff	Source
Standard (WHO)	25.0	30.0	WHO 1995/2000
South Asian	23.0	27.5	WHO Expert Consultation 2004, NICE PH46
Chinese	24.0	28.0	Working Group on Obesity in China, 2002
Japanese	23.0	25.0	Japan Society for the Study of Obesity

In the UK, NICE guideline PH46 specifically recommends using lower BMI thresholds for people of South Asian, Chinese, and other Asian family backgrounds. If you fall into one of these groups, a BMI of 23 may already indicate increased metabolic risk even though it sits comfortably in the "normal" range by standard classification. The Waist-to-Hip Ratio Calculator can provide an additional perspective on fat distribution risk regardless of ethnic background.

Using BMI as a Starting Point

If your BMI is outside the normal range and you want to understand your calorie needs, the Calorie Calculator estimates daily intake based on your stats and goals. For a breakdown of your total daily energy expenditure, use the TDEE Calculator. And if you want to estimate how much of your weight is lean tissue versus fat, the Lean Body Mass Calculator provides that breakdown.

Your height and weight data are processed entirely in your browser and are never sent to any server.

Sources

• WHO - Obesity and Overweight Fact Sheet
• NHS Digital - Health Survey for England 2024 (Adults' Overweight and Obesity)
• CDC NCHS Data Brief No. 508 - Adult Obesity in the US, 2021-2023
• NICE PH46 - BMI Thresholds for Black, Asian and Other Minority Ethnic Groups
• ACOG - Weight Gain During Pregnancy
• CDC - Children's BMI Growth Charts