BMI Calculator For All (Body Mass Index Calculator)

What BMI is

Body mass index is weight in kilograms divided by height in metres squared. A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 divided by 3.0625, or about 22.9. The number is a ratio, not a percentage of body fat.

BMI was designed as a population-level screening tool. It is cheap, quick and consistent, which is why it is used in clinics and public health surveys. It is less useful for an individual with an unusual body composition, because muscle and bone weigh more than fat for the same volume.

The page uses the standard adult thresholds. It does not apply child or adolescent BMI percentiles, pregnancy-specific ranges or condition-specific targets. Those need a clinician and a different chart.

Inputs and units

Weight can be entered in kilograms or pounds. Height can be entered in centimetres, inches, feet, or feet and inches. The page converts pounds by dividing by 2.205 and inches by dividing by 39.37, then calculates the score.

Feet and inches are entered separately. If you choose feet and inches, fill both fields; leaving the inches blank may produce an error or an unexpected result. For centimetres, enter the full height, such as 175, not 1.75.

The result is rounded to two decimal places. The healthy weight range is calculated from the same height using the 18.5 and 24.9 BMI thresholds. It is an estimate, not a target prescribed for you.

How to calculate your BMI

1. Choose your weight unit and enter your weight.
2. Choose your height unit. For feet and inches, use the two-field option.
3. Press Calculate BMI.
4. Read the score and the category label.
5. Look at the healthy weight range for your height, if you want a reference point.
6. Repeat the measurement at the same time of day if you are tracking a trend.

Use a consistent scale and a reliable height measurement. A shoe-height error or a scale on an uneven floor can shift the result. For a trend, compare like with like rather than alternating units and times.

The categories used by the page

• Below 16.4: severely underweight.
• 16.4 to 18.4: underweight.
• 18.5 to 24.9: normal.
• 25.0 to 29.9: overweight.
• 30.0 to 34.9: obese.
• 35.0 to 39.9: severe obesity.
• Above 40: morbid or extreme obesity.

These labels are broad bands. A score just above or below a boundary is not a meaningful change on its own. The label also does not describe fitness, blood pressure, blood sugar or mental health.

Some health organizations use slightly different labels or cut-offs for different populations. The page uses one fixed set. If a clinician uses a different chart, follow the clinician's guidance.

Limitations to keep in mind

BMI can overstate risk for a muscular athlete and understate it for someone with low muscle mass and excess fat around the organs. Older adults may have a normal BMI with low muscle strength. Pregnancy changes weight and fluid distribution, so a standard BMI is not appropriate.

Ethnicity can affect the relationship between BMI and metabolic risk. Some guidelines use lower thresholds for people of South Asian descent, for example. The page does not adjust for ethnicity.

A single BMI is a snapshot. A trend over months, combined with waist circumference, fitness and clinical markers, is more informative than one reading. Do not make major diet or exercise decisions from one number.

What to do with the result

If the score is outside the normal band, do not panic. Look at the trend, your waist measurement and how you feel. If you have concerns, speak with a qualified health professional who can assess the whole picture.

If you want to move toward a healthier weight, focus on sustainable habits: adequate protein, vegetables, regular movement, sleep and stress management. Extreme restriction can reduce muscle and bone as well as fat.

The healthy range shown on the page is a reference, not a prescription. A realistic target may be different for your age, body type, medical history and goals.

Privacy and limits

The calculator runs locally in the browser. Your weight and height are not sent to Newisty and are not stored in an account. Refreshing the page clears the result.

The calculation is a mathematical estimate. It does not diagnose a condition, assess body fat, or replace a clinical examination. It also does not provide a personalised meal plan or exercise programme.

If you have an eating disorder or a medical condition affected by weight, use the tool with care and involve a clinician. A number on a screen is not a complete health assessment.

BMI across life stages

Adult BMI thresholds are not appropriate for children and teenagers. Children grow at different rates, so clinicians use age- and sex-specific percentiles rather than fixed bands. The page does not offer a child mode.

Pregnancy changes weight, blood volume and fluid distribution. A pre-pregnancy BMI may be used for some clinical assessments, but a standard BMI during pregnancy is not a meaningful screening tool. Ask a clinician if you need guidance.

Older adults may have a normal BMI with low muscle mass. Sarcopenia can affect strength and function even when the weight-to-height ratio looks fine. A clinician may consider grip strength, walking speed and body composition alongside BMI.

Waist and other measures

Waist circumference adds information about where fat is stored. Fat around the organs is more strongly associated with metabolic risk than fat on the hips and thighs. A tape measure at the midpoint between the lowest rib and the top of the hip bone is a common method.

Waist-to-height ratio is another simple screen: keep the waist under about half your height for a rough reference. It is not a diagnosis, and it has its own limitations.

Body composition scales and calipers estimate fat percentage, but they also have error. A DEXA scan is more precise but is not necessary for most people. Use the measure that fits the question you are asking.

Tracking changes

Weight fluctuates with hydration, salt, hormones and food in the gut. A daily change of a kilogram is usually water, not fat. Compare weekly averages rather than single readings.

Measure at the same time of day, with the same scale, and under similar conditions. Keep a simple log. A trend over four to eight weeks is more informative than a single BMI.

If your goal is health rather than a number, track behaviours you can control: steps, strength sessions, vegetables, sleep and stress. Those changes can improve health even when the scale moves slowly.

Frequently Asked Questions