# BMI Calculator - Body Mass Index

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. A tall, skinny man may easily weigh more than a short but rotund woman. Your body mass index overcomes this problem by comparing your weight to your height, and returning a single number. This number is then compared on the scale of BMI ranges, which define the ranges of 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:

- How to calculate BMI.
- What is normal BMI.
- What the various BMI ranges are.

## How to calculate BMI

First, let's cover the calculation:

- Divide your weight in kilograms by your height in meters.
- Divide the answer by your height again to get your BMI.

So the body mass index formula is as follows:

`BMI = weight / height²`

.

## Normal BMI

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

If you are unsure whether you fall within the "normal" range of muscle mass, consider also using our more specialized calculators. The body fat calculator can zero-in on your percentage of body fat, and our lean body mass calculator can tell you how much you would weigh without any fat.

## 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 more

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 your height and weight, naturally. Plug in those values, and you'll instantly have your BMI, and your BMI Prime (see above).

So you are... "underweight", "awesome!", or "overweight".

Right at the bottom of the calculator are the BMI ranges, and an explanation of the BMI Prime number.

If you're overweight and want to know how much you'll have to lose exactly to get back into a 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.