// Basic Fitness Calculators

BMI Calculator

Enter your height and weight to get your Body Mass Index and the healthy weight range for your height.

Your details

Your result

Your BMI
24.5
Category
Normal weight
Healthy weight range
56.7–76.3kg
  • BMI is a screening tool, not a body-fat measure. Very muscular people can read high without excess fat.

Estimates are for general guidance only and are not medical advice. For a personalised plan, book a free guided tour at RPM Gym.

What your result means

Your BMI is your weight in kilograms divided by your height in metres squared. It places you on a population-level scale. It's a screening number, not a measurement of your body.

The healthy weight range shown alongside it is more actionable than the BMI itself: it converts the 18.5–24.9 band back into kilograms at your specific height, so you can see how far you're from the range rather than just which side of it you sit on.

How to read the categories

UnderweightBelow 18.5Below the WHO reference band. Worth raising with a doctor, particularly alongside fatigue or unintentional weight loss.
Normal weight18.5 – 24.9Within the WHO reference band for adults. Body composition still varies widely inside this range.
Overweight25.0 – 29.9Above the reference band. In muscular people this often reflects lean mass rather than fat.
Obese (class I)30.0 – 34.9WHO class I. A clinician can interpret this alongside waist measurement and medical history.
Obese (class II)35.0 – 39.9WHO class II.
Obese (class III)40.0 and aboveWHO class III.

How it is calculated

Body Mass Index (Quetelet index), WHO adult categories

BMI = weight (kg) ÷ height (m)²

What it assumes

  • The WHO categories are defined for adults aged 20 and over.
  • The same cut-offs are applied to men and women; BMI itself doesn't take sex into account.
  • No adjustment is made for muscle mass, bone density, pregnancy or ethnicity.

Units and rounding

  • Height is entered in centimetres and divided by 100 to give metres.
  • Weight is entered in kilograms.
  • A height or weight of zero returns no result rather than an error value.

Worked example

Someone 178 cm tall weighing 72 kg.

  1. Convert height to metres: 178 ÷ 100 = 1.78 m
  2. Square it: 1.78 × 1.78 = 3.1684
  3. Divide weight by that: 72 ÷ 3.1684 = 22.7
  4. Healthy range at 1.78 m: 18.5 × 3.1684 = 58.6 kg, and 24.9 × 3.1684 = 78.9 kg

BMI 22.7, inside the 18.5–24.9 band, with a healthy weight range of roughly 58.6–78.9 kg at that height.

Limitations

  • BMI doesn't measure body fat. It can't tell muscle from fat, so a heavily trained lifter and someone carrying excess fat can share a BMI.
  • It says nothing about where weight sits on the body. Waist measurement carries information BMI doesn't, so the waist-to-height ratio tool is a better companion than a second weight-based number.
  • The WHO cut-offs are population reference points, not individual thresholds. Some health bodies apply lower cut-offs for South Asian populations.
  • It isn't valid during pregnancy, and it isn't designed for children or adolescents, who are assessed on age-and-sex percentile charts instead.
  • A BMI figure isn't a diagnosis. Only a doctor can interpret it against your history and other measurements.

What to do next

  • Note the healthy weight range for your height rather than the BMI number. That's the figure you can actually train and eat towards.
  • Pair it with a waist measurement. Two people at BMI 27 with very different waist measurements aren't in the same position.
  • If you're trying to change the number, work out your maintenance calories first. A target without a calorie baseline is a guess.
  • If your BMI sits outside the reference band and you have any related symptoms or history, take it to a doctor rather than to a training plan.

Sources

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Body FatWaist-to-Height RatioIdeal WeightBMRTDEECalorie
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// Frequently Asked

Questions, answered.

What is a healthy BMI?

The WHO reference band for adults is 18.5 to 24.9. It's a population reference rather than a personal target, and where you sit inside or just outside it matters less than your waist measurement, training history and how your weight has changed over time.

Is BMI accurate for people who lift weights?

Often not. BMI uses only height and weight, and muscle is denser than fat, so a lifter carrying significant lean mass can read 'overweight' with low body fat. If you train regularly, a body-fat estimate is more informative than your BMI.

Does BMI differ for men and women?

The formula and the WHO cut-offs are identical. What differs is body composition. Women typically carry a higher proportion of essential body fat at the same BMI, which is one reason BMI alone is a weak comparison between individuals.

Can I use BMI while pregnant?

No. BMI isn't valid during pregnancy, because weight gain is expected and is monitored differently. Pre-pregnancy BMI is sometimes used by clinicians as a starting reference, but that's a conversation for your doctor or midwife.

What should I measure instead of BMI?

Waist circumference, waist-to-height ratio and a body-fat estimate each tell you something BMI can't. Tracking the same measurement over weeks is usually more useful than any single reading.

My BMI says overweight but my clothes fit the same. What changed?

Weight moves for reasons that have nothing to do with fat — hydration, food volume, glycogen and, over months, added muscle. A single reading is noise. Look at the direction of travel across several weeks alongside how your measurements and training are going.

A number is a starting point, not a plan.

Book a free guided tour at RPM Gym in Al Manhal and talk through what your measurements actually mean for how you should be training.

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