⚖️ BMI Calculator
By ToolNimba Health Team · Reviewed by ToolNimba Editorial Review, health content · Updated 2026-06-19
BMI is a screening tool, not a diagnosis, and it does not measure body fat directly. It is not suitable for assessing children, pregnant people or elite athletes the same way. Use it as a starting point and speak to a doctor before making decisions about your health.
Body Mass Index (BMI) estimates whether your weight is in a healthy range for your height. Enter your height and weight below, in metric or imperial, to get your BMI, the category it falls in, and the healthy-weight range for your height. BMI is a quick first screen, not a diagnosis, so read the notes below to understand what your number does and does not tell you.
What is the BMI Calculator?
BMI is a single number calculated from your weight and height: weight in kilograms divided by height in metres squared. It was developed in the 1830s by Belgian statistician Adolphe Quetelet to describe populations, not individuals, which is the root of both its usefulness and its limits. Because it is cheap, fast and needs no equipment, health bodies like the WHO and CDC use it as a population-level screening tool and as a first step before more detailed checks.
The adult categories are fixed worldwide: below 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 and above is obese, which is further split into Class I, II and III. These bands apply the same way to men and women and to most adults aged 20 and over. A BMI in the healthy band does not guarantee good health, and a BMI just outside it is not an emergency, but the further you sit from the middle of the range the more it is worth a closer look.
What BMI cannot do is tell muscle from fat, or show where fat sits on your body. A muscular athlete and a sedentary person can share a BMI of 27 with very different health profiles. Fat around the abdomen carries more risk than fat on the hips, and BMI is blind to that, which is why a waist measurement is often used alongside it. As a rough guide, health risk rises when waist circumference passes about 94 cm (37 in) for men and 80 cm (31.5 in) for women, with higher-risk thresholds around 102 cm (40 in) and 88 cm (34.5 in).
BMI is also read differently across groups. Children and teens are assessed against age- and sex-specific percentiles rather than the adult bands. Many health bodies use lower thresholds for people of South, Southeast and East Asian descent, treating 23 and above as increased risk and 27.5 and above as high risk, because these groups can carry more visceral fat at a given BMI. Older adults sometimes do better at the upper end of the healthy range, and BMI is not used the same way during pregnancy.
A useful companion figure is BMI Prime, your BMI divided by 25 (the upper limit of the healthy range). A BMI Prime below 0.74 maps to underweight, 0.74 to 1.00 to a healthy weight, and above 1.00 to overweight. Because it is a simple ratio, BMI Prime tells you at a glance how far above or below the healthy ceiling you are: a value of 1.20, for example, means you are 20 percent over the upper healthy limit.
When to use it
- Checking where you fall before a routine check-up, a fitness assessment or an insurance form.
- Tracking the direction of change over months as you adjust diet or activity.
- Setting a rough healthy-weight target range for your height.
- Working out how many kilograms or pounds you would need to lose or gain to reach the healthy band.
- Comparing your BMI with your waist measurement to get a fuller picture of risk.
- Understanding your BMI Prime, the simple ratio that shows how far you are above or below the healthy ceiling.
How to use the BMI Calculator
- Choose metric (cm/kg) or imperial (ft-in/lb) units.
- Enter your height and your current weight.
- Read your BMI value, your category, and the healthy weight range for your height.
- Compare your number with the BMI chart below and note your BMI Prime.
- If you are near a category boundary, recheck with a fresh weight and consider a waist measurement before drawing conclusions.
Formula & method
Worked examples
You weigh 70 kg and are 1.75 m tall.
- height² = 1.75 × 1.75 = 3.0625
- BMI = 70 ÷ 3.0625 = 22.9
Result: BMI 22.9, Healthy weight
You weigh 154 lb and are 5 ft 7 in (67 in).
- BMI = 703 × 154 ÷ (67 × 67)
- = 108,262 ÷ 4,489 = 24.1
Result: BMI 24.1, Healthy weight
You weigh 95 kg and are 1.78 m tall, and you want your BMI Prime.
- height² = 1.78 × 1.78 = 3.1684
- BMI = 95 ÷ 3.1684 = 30.0
- BMI Prime = 30.0 ÷ 25 = 1.20
Result: BMI 30.0 (Obese Class I), BMI Prime 1.20 (20 percent over the healthy ceiling)
BMI categories for adults (WHO)
| BMI range | Category | Health risk |
|---|---|---|
| Below 18.5 | Underweight | Increased |
| 18.5 to 24.9 | Healthy weight | Lowest |
| 25.0 to 29.9 | Overweight | Increased |
| 30.0 to 34.9 | Obese (Class I) | High |
| 35.0 to 39.9 | Obese (Class II) | Very high |
| 40.0 and above | Obese (Class III) | Extremely high |
Asian and South Asian BMI thresholds (WHO public-health action points)
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 22.9 | Healthy weight |
| 23.0 to 27.4 | Increased risk (overweight) |
| 27.5 and above | High risk (obese) |
Healthy weight range by height (BMI 18.5 to 24.9)
| Height | Healthy weight (metric) | Healthy weight (imperial) |
|---|---|---|
| 1.55 m / 5 ft 1 in | 45 to 60 kg | 99 to 132 lb |
| 1.60 m / 5 ft 3 in | 47 to 64 kg | 104 to 141 lb |
| 1.65 m / 5 ft 5 in | 50 to 68 kg | 111 to 150 lb |
| 1.70 m / 5 ft 7 in | 53 to 72 kg | 118 to 159 lb |
| 1.75 m / 5 ft 9 in | 57 to 76 kg | 125 to 168 lb |
| 1.80 m / 5 ft 11 in | 60 to 81 kg | 133 to 178 lb |
| 1.85 m / 6 ft 1 in | 63 to 85 kg | 140 to 188 lb |
Common mistakes to avoid
- Treating BMI as a body-fat percentage. It estimates a category from height and weight only; it never measures fat directly.
- Using adult thresholds for children. Kids and teens are assessed on age- and sex-specific percentiles, not the 18.5-24.9 adult band.
- Mixing units. Entering height in cm but weight in pounds (or vice versa) throws the result off by a wide margin.
- Squaring the wrong number. In the metric formula you square the height in metres, not the weight, and not the height in centimetres.
- Reading a single reading as the whole story. Weight swings with hydration, meals and time of day. Track the trend over weeks rather than reacting to one figure.
- Ignoring ethnicity-specific thresholds. For people of Asian descent, risk often rises from a BMI of 23, lower than the standard 25 cut-off.
Glossary
- BMI
- Weight (kg) divided by height (m) squared, a screening estimate of weight category.
- BMI Prime
- Your BMI divided by 25; a ratio above 1.0 means you are above the healthy ceiling.
- Obesity class
- Sub-bands (I, II, III) within the obese range used clinically to gauge risk.
- Healthy weight range
- The span of weights that put you between a BMI of 18.5 and 24.9 for your height.
- Waist circumference
- The measurement around your middle, used alongside BMI to gauge abdominal fat.
- Waist-to-height ratio
- A complementary measure that captures where fat is carried, which BMI ignores.
- Visceral fat
- Fat stored around the abdominal organs, which carries more health risk than fat under the skin.
- Quetelet index
- The original name for BMI, after Adolphe Quetelet who devised it in the 1830s.
Frequently asked questions
What is a healthy BMI range?
For most adults, a BMI of 18.5 to 24.9 is considered the healthy range. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is in the obese range.
What BMI is considered obese?
A BMI of 30 or above is classified as obese. It is split into Class I (30-34.9), Class II (35-39.9) and Class III (40+), which doctors use to gauge health risk.
Is BMI different for men and women?
The BMI formula and adult category thresholds are the same for men and women. However, at the same BMI women tend to have more body fat than men, so BMI is interpreted alongside other measures rather than in isolation.
Why is BMI inaccurate for athletes?
Muscle is denser than fat, so very muscular people can have a high BMI while carrying little fat. For them BMI overestimates health risk, which is why body-fat or waist measurements are used instead.
How do I calculate BMI by hand?
In metric, divide your weight in kilograms by your height in metres squared. In imperial, multiply your weight in pounds by 703 and divide by your height in inches squared.
How much should I weigh for my height?
Your healthy weight is the range that puts your BMI between 18.5 and 24.9. For example, at 1.70 m (5 ft 7 in) that is roughly 53 to 72 kg (118 to 159 lb). See the weight-by-height chart above for other heights.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A value below 1.0 means you are within or below the healthy band, and above 1.0 means you are above it. A BMI Prime of 1.2, for instance, means you are 20 percent over the healthy ceiling.
Should I use a lower BMI threshold if I am of Asian descent?
Many health bodies say yes. For people of South, Southeast and East Asian descent, increased risk often begins at a BMI of 23 and high risk at 27.5, rather than the standard 25 and 30 cut-offs, because they tend to carry more visceral fat at a given BMI.
Is BMI or waist size a better measure of health?
They measure different things and work best together. BMI flags overall weight relative to height, while waist circumference captures abdominal fat, which carries more risk. Risk rises when the waist passes about 94 cm (37 in) for men or 80 cm (31.5 in) for women.
Does this calculator store my data?
No. The calculation happens entirely in your browser and nothing is sent anywhere or saved.
Sources
- Body mass index (BMI) , World Health Organization
- About Adult BMI , U.S. CDC
- BMI healthy weight calculator , NHS