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BMI Calculator – Body Mass Index

Created by Aleksandra Zając, MD and Mateusz Mucha
Reviewed by Małgorzata Koperska, MD and Jack Bowater
Based on research by
Nuttall FQ. Body Mass Index: Obesity, BMI, and Health: A Critical Review.; Nutrition Today; May 2015See 1 more source
Aune D, Sen A, Prasad M, Norat T, Janszky I, Tonstad S, Romundstad P, Vatten LJ. BMI and all cause mortality: systematic review and non-linear dose-response meta-analysis of 230 cohort studies with 3.74 million deaths among 30.3 million participants.; BMJ; May 2016
Last updated: Jul 17, 2024

The BMI calculator is a useful tool that measures whether you are overweight, underweight, or just right. Your weight alone is not enough to tell, as a tall, skinny man may easily weigh more than a short but rotund woman. The body mass index, or BMI, overcomes this problem by finding a ratio of your weight to your height and returning a single number. This number will fit into a category on the scale of BMI ranges, which are defined as underweight, normal, overweight, and obese.

To work out your body mass index on your own and what the number means for you, you'll need to know the following:

  • How to calculate BMI.
  • What is a normal BMI?
  • What the various BMI ranges are.

How do I calculate BMI?

To compute BMI, you need to:

  1. Divide your weight (in kilograms) by your height (in meters).

  2. Divide the answer by your height again to get your BMI.

  3. That is the body mass index formula. Expressed differently, it is:

    BMI = weight / height²

Normal BMI

Our BMI calculator makes it a piece of cake for you to find your number. However, please remember that BMI is a rough estimation. The result can be somewhat misleading for individuals who are well-muscled (such as bodybuilders) or for those who have lost a significant amount of muscle (such as the elderly).

If you are unsure whether or not you fall within the "normal" range of muscle mass, please consider using one of our more specialized calculators. If you want to determine your body composition that adjusts for skeletal mass, you may want to use our body frame size calculator. Additionally, check out our skinfold body fat calculator to figure out what percentage of your body is fat.

What are the BMI ranges?

There are five basic ranges within the BMI scale:

  • Underweight = less than 18.5;
  • Normal weight = 18.5-24.9;
  • Overweight = 25-29.9;
  • Obesity = 30-35;
  • Severe obesity = 35 and over.

The BMI Prime is a nifty modification to our own BMI calculator. It's a decimal number where 1.0 = the upper limit to the "normal BMI" range. It's a super easy way to see if you're overweight or not. If your BMI Prime is more than 1, then you've got some weight to lose.

Using our body mass index calculator

At the top of our BMI calculator are fields for you to plug in your height and weight. Plug in these values, and you'll instantly have your BMI, your BMI Prime (see above), and a little message telling you what range you fall into.

If you're overweight and want to know exactly how much you'll have to lose to get into the healthy range, you can do that too. Leave your height and weight in the boxes where they are, and type "0.9" into the BMI Prime box. Your weight and BMI will adjust themselves.

Tip: If your height box adjusts itself when you change other boxes, you'll need to lock it. Click the right-hand panel of the box and select the lock icon to hold that figure in place while you play with the values in other boxes. That is... unless you plan on growing taller!

Other considerations

If you want to know if your metabolism is on your side in your quest towards a healthy weight, try our basal metabolic rate calculator. We often hear about things raising or lowering our metabolism (which one does skipping breakfast do again?), but we don't hear much about tracking those changes. That's what this calculator is for.

Your basal metabolic rate is the activity level of your body's metabolism when you wake up in the morning. This is the most reliable time of the day to measure metabolism since the day hasn't had time to heat you up yet!

Once you know how to calculate BMI, the next logical figure to know is your ideal weight. Use our ideal weight calculator to figure this out in a jiffy. You can also use our maintenance calorie calculator to calculate how many calories your body needs to maintain your current weight.


Does normal BMI depend on age?

No for adults, the range of normal BMI is 18.5-24.9, and it does not depend on a person's age. For children, however, age is essential in assessing their weight.

What is my BMI if I am a man and weigh 75 kilos?

The answer depends on your height! Assume it is 180 cm. Then your BMI is:

BMI = weight / height²

where the weight should be in kg and height should be in meters.

Therefore, convert height in cm to meters: 180/100 = 1.8 m.

So the BMI calculation is weight / height² = 75/1.8² = 23.148. So, it is in the normal (healthy) range. Note that this result does not depend on your gender!

Aleksandra Zając, MD and Mateusz Mucha
BMI Prime
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