BMI Calculator: Check Your Body Mass Index and Healthy Weight Range

A BMI calculator is the quickest way to see whether your weight sits in a healthy range for your height. Enter your height and weight in centimetres and kilograms, or in feet, inches and pounds, and you will instantly see your body mass index, your WHO weight category and the weight range that counts as healthy for you.

Most people check their BMI for a simple reason: they want to know where they stand and what the number actually means. This page answers both questions. Below you will find a clear explanation of each category, the formula behind the result and the situations where BMI needs a second opinion.

How to Use the BMI Calculator

Using the tool takes less than a minute.

1. Select metric or imperial units.
2. Type in your height. In imperial mode, feet and inches go in separate boxes.
3. Type in your weight.
4. Add your waist measurement if you know it. It is optional but gives a fuller picture.
5. Click Calculate BMI.

You can use it as a BMI calculator with kg and cm or as a BMI calculator in lbs, and it handles every conversion for you. Along with your BMI, you will see a colour gauge marking your position, your healthy weight range and, if you fall outside it, roughly how much weight you would need to gain or lose to reach it.

This BMI calculator is designed for adults aged 20 and older. If you are younger, your result needs to be read against growth charts, which are explained further down the page.

Understanding Your BMI Result

Once you have your number, the next step is knowing which group it falls into. Health organizations around the world, including the WHO, the CDC and the NIH, use the same adult BMI chart.

These categories describe average risk across large groups of people. They are a starting point for a conversation about your health, not a verdict on it.

Underweight: Below 18.5

A low BMI can mean your body has limited energy reserves. It is sometimes linked with poor nutrition, low bone density, weaker immunity or an underlying illness. Plenty of people are naturally lean and healthy, but sudden or unexplained weight loss should always be checked by a doctor.

Healthy Weight: 18.5 to 24.9

This is the normal BMI range for adults, and it is associated with the lowest average risk of weight-related disease. A healthy BMI is encouraging, but it does not replace routine checks such as blood pressure, blood sugar and cholesterol.

Overweight: 25 to 29.9

Your weight is above the recommended range for your height. The risk of high blood pressure, raised cholesterol and type 2 diabetes begins to climb here, especially when the extra weight is carried around the waist.

Obesity: 30 and Above

Obesity is split into three classes, and health risk increases with each step. If your result is in this range, a doctor can help you look beyond the number and put together a realistic plan.

Healthy Weight Range for Your Height

One of the most common reasons people use a BMI calculator is to find their ideal weight for height. BMI does not produce a single perfect weight, but it does give a healthy range. The figures below are based on a BMI of 18.5 to 24.9.

Any weight inside your range is considered healthy. Frame size, muscle and age explain why two people of the same height can both be healthy at noticeably different weights.

What Is BMI (Body Mass Index)?

Body mass index is a simple measure that relates your weight to your height. The idea dates back to the 1830s, when the Belgian mathematician Adolphe Quetelet noticed that adult weight tends to rise with the square of height. In 1972, the physiologist Ancel Keys named it the body mass index and showed it was a practical way to study weight across large populations.

Doctors still rely on it because it is fast, free and needs nothing more than a scale and a tape measure. A body mass index calculator simply removes the math. The key thing to remember is that BMI estimates weight status. It does not measure body fat directly, so it works best as a screening tool rather than a diagnosis.

How to Calculate BMI

You do not need a BMI calculator to work out your number, although it certainly saves time. The BMI formula comes in two forms, and both give identical results.

Metric Formula (kg/m²)

Example: a person who weighs 70 kg and is 175 cm (1.75 m) tall.

• 1.75 × 1.75 = 3.0625
• 70 ÷ 3.0625 = 22.9

Imperial Formula (lb/in² × 703)

Example: a person who weighs 160 lb and is 5 ft 9 in (69 inches) tall.

• 69 × 69 = 4,761
• 160 ÷ 4,761 × 703 = 23.6

The figure 703 is a conversion factor that brings pounds and inches onto the standard kg/m² scale.

BMI Prime Explained

BMI Prime compares your BMI with the upper limit of the healthy range. To find it, divide your BMI by 25. A score between 0.74 and 0.99 is healthy, while anything from 1.0 upward means you are above the range. A BMI of 22.9, for instance, gives a BMI Prime of 0.92. Some people find this ratio easier to follow over time than the BMI number itself.

Is BMI the Same for Everyone?

The formula is the same for every adult, but the meaning of the result can shift depending on who you are.

BMI for Men and Women

There is no separate formula, so a BMI calculator for women and a BMI calculator for men will return the same number for the same height and weight. The difference lies in body composition. At an equal BMI, women generally carry more body fat and men more muscle. Waist size helps fill this gap: the NIH links higher risk with a waist above 102 cm (40 in) in men and 88 cm (35 in) in women. BMI is also not used to assess weight during pregnancy.

BMI and Age

Adult categories apply from age 20 onward, so a BMI calculator by age gives adults the same result at 25 as at 65. What changes is what the number represents. Muscle mass tends to fall with age while body fat rises, so an older adult may carry more fat than a younger person with an identical BMI. Some studies also suggest that a BMI slightly above 25 may carry less risk in later life, which is one more reason for older adults to discuss their result with a doctor.

BMI for Children and Teens

Because children are still growing, their BMI is compared with others of the same age and sex using percentiles. The CDC uses these groups for ages 2 to 19:

• Below the 5th percentile: underweight
• 5th to below the 85th percentile: healthy weight
• 85th to below the 95th percentile: overweight
• 95th percentile and above: obesity

Adult cut-offs should never be applied to children, so an adult BMI calculator is not the right tool for this age group.

BMI for Asian Populations

Many people of Asian heritage develop type 2 diabetes and heart disease at lower BMI levels than people of European descent. A 2004 WHO expert consultation therefore proposed extra public health action points for Asian populations, with increased risk from a BMI of 23 and high risk from 27.5. A number of national guidelines across Asia now use these lower thresholds.

BMI for Athletes

BMI cannot tell muscle from fat. Rugby players, bodybuilders and anyone who does regular strength training can land in the overweight range while carrying very little body fat. For these people, body fat percentage is a far better measure.

Health Risks Linked to BMI

A BMI above the healthy range is associated with a higher chance of:

• Type 2 diabetes
• High blood pressure and cardiovascular disease
• Stroke
• Sleep apnea
• Osteoarthritis
• Certain types of cancer

A BMI below the healthy range is associated with:

• Nutritional deficiencies and anemia
• Reduced immune function
• Osteoporosis and fractures
• Fertility problems

These links come from population studies. They show patterns across large groups, not what will happen to any one person.

Limitations of BMI

Every BMI calculator, including this one, works with only two inputs. That makes it fast, but it also leaves gaps. BMI ignores muscle mass, bone density and where fat is stored, and it places adults of all ages, sexes and backgrounds into the same categories.

BMI vs Body Fat Percentage

Body fat percentage measures the share of your weight that is fat. It is more precise than BMI but requires tools such as skinfold calipers, a bioimpedance scale or a DEXA scan. Many people use BMI as a quick check and body fat percentage when they want more detail.

BMI vs Waist Circumference

Fat stored around the abdomen is more strongly linked to heart disease and diabetes than fat on the hips or thighs. A tape measure captures what BMI misses. A simple rule of thumb is the waist-to-height ratio: keep your waist measurement below half your height.

Tips to Reach a Healthy BMI

Small, steady changes tend to last longer than drastic ones.

• Fill most of your plate with vegetables, fruit, whole grains, pulses and lean protein.
• Cut back on sugary drinks, fried food and heavily processed snacks.
• Aim for at least 150 minutes of moderate activity a week, plus two strength sessions.
• Get 7 to 9 hours of sleep, as poor sleep can increase hunger.
• If you need to lose weight, target about 0.5 to 1 kg (1 to 2 lb) per week.
• If you are underweight, eat regular meals built around nutrient-rich foods.
• Check your progress with the BMI calculator every few weeks rather than every day.

Speak to your doctor before making major changes, particularly if you have a medical condition.

Frequently Asked Questions