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Calculates

BMI Calculator

Calculate your body mass index and see your healthy weight range.

Calculated & verified for mathematical accuracy·Reviewed by the Calculates Editorial Team

Your measurements

Unit system

BMI is a screening ratio, not a diagnosis. It cannot tell muscle from fat, and it is not valid for children, during pregnancy, or for highly trained athletes. Nothing here is medical advice.

Body mass index

22.9

Normal weight — based on the adult bands below.

Category
Normal weight
Healthy weight range
56.7 – 76.3 kg

Adult bands

  • UnderweightBelow 18.5
  • Normal18.5 – 24.9
  • Overweight25 – 29.9
  • Obese30 and above

Working

70 kg ÷ (1.75 m)² = 22.9

The healthy weight range is simply the span that produces a BMI between 18.5 and 24.9 at your height. It widens with the square of height, so a taller frame has a wider range in absolute weight.

About this calculator

Body mass index compares your weight against your height using a single ratio, kilograms divided by metres squared. It was designed to sort large populations into broad bands quickly, and it remains the standard first-pass screening figure used by clinicians and insurers. This calculator works in metric or imperial units and also shows the weight span that would place you inside the 18.5 to 24.9 band at your height.

How to use this calculator

  1. Choose metric or imperial

    Metric takes kilograms and centimetres; imperial takes pounds plus feet and inches. Both routes convert to the same underlying kilogram and metre figures, so the index is identical either way.

  2. Enter your height

    Height carries more weight in the formula than mass does, because it is squared. A 2 cm measurement error moves the index roughly twice as far as a 1 kg weighing error, so measure without shoes and stand straight.

  3. Enter your weight

    Use a recent figure taken at a consistent time of day. Body mass swings by one to two kilograms across a normal day through food and fluid, which is enough to nudge the index by a few tenths.

  4. Read the band and the healthy weight span

    The result shows your index, the band it falls in, and the kilogram range corresponding to 18.5 to 24.9 at your height. Treat the span as context for the number, not as a target.

Formula

BMI = weight (kg) / height (m)^2 Imperial inputs are converted first: kg = lb x 0.45359237 cm = (feet x 12 + inches) x 2.54 Healthy weight span at a given height: minimum kg = 18.5 x height (m)^2 maximum kg = 24.9 x height (m)^2
  • Height must be in metres, not centimetres. Dividing by 175 instead of 1.75 produces a number roughly ten thousand times too small, which is the single most common hand-calculation slip.
  • Because height is squared, the index rises faster for shorter people at the same absolute weight, and the healthy weight span widens with the square of stature rather than in proportion to it.
  • This calculator converts imperial entries to kilograms and metres rather than using the 703 x lb / inch^2 shortcut. The shortcut carries a rounded constant, so the two methods can disagree in the first decimal place.
  • The bands are continuous: 18.5 starts the normal band, 25 starts overweight and 30 starts obese. Published tables write the upper edges as 24.9 and 29.9 purely as a display convention.
  • The adult bands assume a grown, non-pregnant body. Children and teenagers are assessed against age-and-sex percentile charts instead, so a raw index means something entirely different for them.

Worked example

Inputs

  • Unit system: Metric
  • Weight: 70 kg
  • Height: 175 cm

Calculation

height in metres = 175 / 100 = 1.75 1.75^2 = 3.0625 BMI = 70 / 3.0625 = 22.86, shown as 22.9 Healthy weight span at 1.75 m: 18.5 x 3.0625 = 56.7 kg 24.9 x 3.0625 = 76.3 kg

A 70 kg adult standing 1.75 m has a BMI of 22.9, which sits inside the 18.5 to 24.9 band. At that height the band corresponds to roughly 56.7 kg to 76.3 kg, so there is about 6 kg of headroom above the current figure before the band boundary is reached.

What the result means

  • The index is a ratio of mass to squared height and nothing more. It carries no information about body composition, fitness, blood pressure, blood sugar or any other marker of health.
  • A figure inside the 18.5 to 24.9 band means your mass-to-height ratio matches the range where population-level studies observe the lowest average risk. It is a statement about a large group, not a verdict on one person.
  • A figure outside the band is a prompt to look further, not a diagnosis. Muscular builds, dense bone structure and differing torso-to-leg proportions all push the number around without any change in body fat.
  • The bands were derived largely from European-ancestry populations. Several health bodies apply lower thresholds for people of South Asian, Chinese and other Asian ancestry, so the same index can be read differently depending on the reference used.

Related calculations

  • Work out the weight change that would move you from one band edge to another by taking the difference between your current weight and the span shown for your height.
  • Convert between stones, pounds and kilograms before entering a figure if your scales report in stones, since the imperial mode here expects plain pounds.
  • Recompute the healthy span for a different height to see how sharply it widens: the square term means a 10 cm difference shifts the whole band by several kilograms.

Frequently asked questions

What is a good BMI for an adult?

The conventional adult bands put 18.5 to 24.9 in the normal range, below 18.5 in underweight, 25 to 29.9 in overweight and 30 or above in obese. These cut-offs come from population studies showing where average mortality and disease rates are lowest, so they describe a group trend rather than an individual target. Where you sit in the band matters far less than what the rest of your clinical picture looks like.

Is BMI accurate for everyone?

No. BMI is a screening ratio, not a measurement of body fat, and it misreads several groups systematically. Muscular athletes are routinely classified as overweight or obese despite low body fat, because muscle is denser than fat and the formula cannot tell them apart. Older adults who have lost muscle can show a normal index while carrying a high proportion of fat. It is also not valid during pregnancy or for children and teenagers.

Why does BMI not distinguish muscle from fat?

The formula has only two inputs, total body mass and height, so every kilogram counts identically regardless of whether it is muscle, fat, bone or water. A rugby forward and a sedentary person of the same height and weight produce the same index even though their body composition is completely different. Methods such as DEXA scanning, skinfold measurement or bioimpedance are needed to separate the tissue types.

How do I calculate BMI from pounds and inches?

Convert to metric first for the cleanest result: multiply pounds by 0.45359237 to get kilograms, and multiply total inches by 2.54 to get centimetres before dividing by 100 for metres. The widely quoted alternative is to multiply pounds by 703 and divide by inches squared, which approximates the same thing but carries a rounded constant that can shift the answer by a tenth of a point.

Can I use this calculator for a child?

No. Children and adolescents are assessed against age-and-sex-specific percentile charts, because body proportions change continuously through growth and the fixed adult cut-offs have no meaning before development finishes. A raw index that looks alarming on the adult scale can be entirely ordinary for a child of that age, and the reverse is equally possible. Paediatric assessment belongs with a clinician who has the right growth reference.

Is BMI valid during pregnancy?

It is not. Pregnancy adds weight from the foetus, placenta, amniotic fluid and expanded blood volume, none of which the formula can account for, so the resulting index describes nothing meaningful about body composition. Pre-pregnancy BMI is sometimes recorded by maternity services as a baseline for planning, but that is a different figure from one calculated during pregnancy.

What is the healthy weight range for my height?

Multiply your height in metres squared by 18.5 for the lower edge and by 24.9 for the upper edge. At 1.75 m that gives roughly 56.7 kg to 76.3 kg; at 1.60 m it narrows to about 47.4 kg to 63.7 kg. The span widens with the square of height, which is why taller people have a noticeably wider range in absolute kilograms. This calculator shows the span for whatever height you enter.

Does BMI replace advice from a doctor?

Absolutely not, and nothing shown here is medical advice. BMI is one crude input among many that a clinician would weigh alongside waist measurement, blood pressure, blood test results, family history, activity level and how you actually feel. If your result concerns you, or if you are planning any change to diet, exercise or medication, that conversation belongs with a qualified healthcare professional who can examine your full situation.

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