BMI / Body Mass Index Calculator
Calculate your BMI, adult weight category, and healthy weight range based on your height and weight.
Stepping on a scale is easy. The hard part is grasping what that number on the screen really tells you.
BMI, or body mass index, reduces the relationship between height and weight to a single number. That’s the practical side. You enter your height and weight, and the tool gives you a value: 22.6; 27.4; 31.1, for example. The number is short, but interpreting it requires a bit more attention.
This calculator finds your BMI value based on height and weight. It shows the weight range for adults. It calculates approximate normal weight limits for your height. If you like, you can also add your waist circumference; then the waist-to-height ratio appears as well.
So it’s not just a single scale result, but a slightly more readable table. But don’t be intimidated by the table; the point is to place a few numbers correctly.
Get the Numbers Right First
In such tools, the most common mistake is not in the formula but in the input fields. Height is entered in the wrong unit. Weight is left over from an old scale. Waist circumference is measured with a tightly pulled tape. Then the result comes out strange.
It’s normal for it to come out strange. The tool calculates exactly what you feed it.
Height and Weight Fields
The height field is usually filled in centimeters. If you are 1.70 m tall, entering 170 is sufficient. The tool converts it to meters for the calculation.
For weight, the most natural input is kilograms. If you weigh 70 kg, you enter 70. For those working with different units, there may be options like lb, stone, feet, or inches; but cm and kg are the least confusing pair.
Think of it this way: if you accidentally write 1.70 cm instead of 170 cm, the calculation collapses. If you write 170 m instead of 1.70 m, you’re measuring a tower, not a person. A small mistake, a big absurdity.
So, first the unit.
The Age Group Is Not Asked for No Reason
The adult option is used for ages 20 and over. The CDC’s adult BMI classification also bases on this age group. In this group, BMI values are placed into specific ranges: underweight, normal weight, overweight, and obesity classes, for example.
For children and adolescents, the same table is not used. Between ages 2-19, the BMI result is evaluated with percentile according to age and sex. The tool still calculates BMI in this age group, but it does not provide an adult category.
Good thing it doesn’t. Because it would not be right to interpret the same BMI value for a 12-year-old and a 35-year-old in the same way.
Where Does the BMI Number Come From?
The essence of BMI calculation is simple: you divide your weight by the square of your height.
In plain text, the formula is:
BMI = weight \/ (height x height)
Here weight is in kilograms and height is in meters. For 70 kg and 1.70 m, the calculation is:
70 \/ (1.70 x 1.70) = 70 \/ 2.89 = 24.2
You’ll see approximately 24.2 on the screen. Since the tool displays the result with one decimal place, small fractions are rounded.
That’s it. The adult ranges used on the page are as follows:
| BMI range | Adult classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25 - 29.9 | Overweight |
| 30 - 34.9 | Obesity class 1 |
| 35 - 39.9 | Obesity class 2 |
| 40 and above | Obesity class 3 |
Why is the number divided by the square of height?
Because the same weight does not mean the same thing at different heights. If one of two people who weigh 70 kg is 1.55 m and the other is 1.90 m, they look the same on the scale; but their body proportions are not the same. BMI tries to account for this difference.
Of course, it’s a crude measure. It doesn’t distinguish muscle from fat. If you put someone who never leaves the gym and has high muscle mass into this calculator, they may appear overweight. The formula has no eyes; it only looks at height and weight.
This warning is enough. There’s no need to write “not a diagnosis” in every corner of the text. BMI is a screening indicator, not a doctor’s examination.
How to Read Adult Ranges?
The ranges used for adults are roughly as follows: below 18.5 is considered underweight. 18.5 to 24.9 is the normal weight range. 25 to 29.9 falls into the overweight category. Values of 30 and above are evaluated under obesity; 30-34.9 is class 1, 35-39.9 is class 2, and 40 and above is class 3.
The CDC’s adult categories use these thresholds. The World Health Organization also defines overweight as BMI 25 and above and obesity as 30 and above for adults.
It’s important to stay calm at boundary values. There’s no dramatic line in life between 24.9 and 25.0. There is in classification, not in the body.
How to Read the Result on the Screen?
The first value you’ll see after the calculation is the BMI. For example, 24.2, 29.4, or 20.0.
Most people stop here. But the real benefit is a bit further down: which range this value falls into, where the approximate normal weight limit is for your height, and how far your current weight is from that limit.
This information alone doesn’t make decisions. But it takes a picture. Even if blurry, it works. The basic BMI ranges used for adults are given similarly in most official sources. The CDC adult BMI categories use the following ranges:
| BMI range | Adult weight category |
|---|---|
| \(BMI < 18.5\) | Underweight |
| \(18.5 \le BMI < 25\) | Normal weight |
| \(25 \le BMI < 30\) | Overweight |
| \(30 \le BMI < 35\) | Obesity class 1 |
| \(35 \le BMI < 40\) | Obesity class 2 |
| \(BMI \ge 40\) | Obesity class 3 |
Normal Weight Range
For adults, the normal BMI range is between 18.5 and 24.9. The tool applies these two limits to your height.
Let’s say your height is 1.70 m. The lower limit of the normal range is found as follows:
18.5 x 1.70 x 1.70 = 53.5 kg
The upper limit is:
24.9 x 1.70 x 1.70 = 72.0 kg
So, for an adult with a height of 1.70 m, the normal weight range according to BMI appears to be approximately 53.5-72.0 kg.
I say approximately, because this is not a medical target. It’s just the range the formula says.
Difference to the Normal Range
Let’s make a more realistic example. Height 1.70 m, weight 85 kg.
The BMI comes out roughly like this:
85 \/ (1.70 x 1.70) = 29.4
This value falls into the overweight range in the adult classification. Since the upper limit of the normal range for the same height is about 72 kg, the tool might show a difference of about 13 kg.
This sentence is sometimes misunderstood. The tool is not saying “you must lose 13 kg.” It’s just saying “mathematically, there are 13 kg between you and the upper limit of the normal BMI range.”
The two are not the same.
The Waist Measurement and Target Weight Issue
BMI is calculated from height and weight. Waist circumference doesn’t change this number. But waist circumference tells something else: it gives a small clue about how the weight is distributed in the body.
That’s where waist measurement comes in handy. Two people who weigh the same on the scale may have different waist circumferences. Both the appearance and health risk may not be the same.
Waist-to-Height Ratio
The waist-to-height ratio is found by dividing waist circumference by height.
Waist-to-height ratio = waist circumference \/ height
If waist circumference is 90 cm and height is 170 cm:
90 \/ 170 = 0.53
The NICE guideline uses the waist-to-height ratio as one of the practical measures for assessing central adiposity. The 0.40-0.49 range is the healthy central adiposity range. 0.50-0.59 indicates increased risk of central adiposity. 0.60 and above means even higher risk.
The simple rule here is memorable: waist circumference should not exceed half your height. Of course, it’s not sufficient on its own for everyone. But for a quick check, it’s not bad.
Target BMI Field
The target BMI option works a bit more mathematically. It calculates what weight the selected BMI value corresponds to at your height.
The formula is straightforward:
Target weight = target BMI x height x height
If height is 1.70 m and target BMI is 24.9:
24.9 x 1.70 x 1.70 = 72.0 kg
If your current weight is 85 kg, a difference of about 13 kg will appear on the screen.
This field is particularly used by those who ask, “what weight does the upper limit of the normal range correspond to at my height?” A logical question. As long as the number doesn’t turn into a goal by itself.
A Few Realistic Examples
When examples are always pristine, the text feels artificial. In real life, things are a bit more complicated. Some haven’t measured their height in years. Some step on the scale after a meal. Some pull the tape so tight when measuring their waist that even they don’t believe the result.
The calculation doesn’t know any of this.
170 cm, 70 kg
Let’s start with a classic example. Height 170 cm, weight 70 kg, age group adult.
70 \/ (1.70 x 1.70) = 24.2
This value remains in the normal weight range. Since the upper limit of the normal BMI range for the same height is about 72 kg, 70 kg is within this range.
This example is boring but useful. It’s a good starting point to check the logic of the tool.
170 cm, 85 kg and 90 cm Waist
This time, let’s say the weight is 85 kg. The BMI comes out approximately 29.4. It’s in the overweight range.
If waist circumference is entered as 90 cm:
90 \/ 170 = 0.53
This ratio is in the increased risk range for central adiposity according to the NICE classification. No panic. But it makes you say, “let me look not only at my weight but also at my waist measurement.”
The two results should be considered side by side: BMI gives the ratio of weight to height, while the waist-to-height ratio looks at central adiposity.
Child/Adolescent Example
For 150 cm and 45 kg, the BMI comes out approximately 20.0.
For an adult, this value might seem normal. But if the age group is selected as child/adolescent, the tool does not make that interpretation. For ages 2-19, percentile is required. Age and sex come into play.
This is where parents most often miss the point. Don’t read a child’s result with the adult table.
When Using the Result
BMI is a good starting point. It’s more meaningful than the number on the scale because it takes height into account. But it’s not the final word.
Take the measurement properly. Measure height without shoes. Track your weight, preferably with the same scale and at similar times. When measuring waist circumference, don’t pull the tape tight. Sucking in the stomach is also considered cheating; at least let’s not cheat ourselves.
And one more thing: is it 169 cm or 170 cm? At some boundary values, this difference can affect the result. Likewise, half a kilogram can cause you to cross between two sides of a category line.
Don’t overthink it.
Where to Place the References?
The adult BMI ranges in this text are consistent with the CDC’s adult BMI categories. The WHO’s definitions of overweight and obesity also use the same main thresholds: 25 and above for overweight, 30 and above for obesity. The 0.40, 0.50, and 0.60 limits for the waist-to-height ratio are based on the NICE guideline.
The job of references is not to burden the text but to show where the used thresholds come from. Those who are curious can look. Those who don’t want to look can do their calculation and move on.
One last piece of advice: don’t get upset at the number and blame the scale. First, make sure you entered your height, weight, and waist measurement correctly. Then look at the number. If it still bothers you, especially if you have a child/adolescent, pregnancy, rapid weight change, or intense sports history, it’s better to discuss it with a health professional.