Body Mass Index (BMI) Calculator

How This Calculator Works

Body Mass Index (BMI) is a screening measure calculated by dividing weight in kilograms by height in meters squared — a quick way to check whether a weight falls in a typical range for a given height. Enter your height and weight below to get your BMI and which of the four standard weight categories it falls into.

BMI is a fast, widely-used screening tool, not a diagnosis. It doesn’t distinguish muscle from fat, so it can be misleading for very muscular or very lean people. Use it as a starting point for a conversation with a healthcare professional, not a final answer.

The four categories below apply to adults only. BMI itself is calculated the same way at any age, but a child or teen’s healthy BMI changes as they grow, so it’s assessed against age- and gender-specific percentile growth charts instead of these fixed adult thresholds — a BMI that’s perfectly normal for an 8-year-old could flag as “underweight” against the adult scale, which would be a meaningless comparison. If you enter an age under 20, this calculator says so plainly rather than applying the adult categories anyway, and points to the CDC’s own BMI-for-age percentile calculator (published for exactly this purpose) as the correct reference.

The Formula

BMI is weight divided by height squared, using metric units:

$$\vC{\text{BMI}} = \frac{\vA{\text{Weight (kg)}}}{\vB{\text{Height (m)}}^2}$$

If you enter your height and weight in imperial units (inches and pounds), the calculator converts them to metric first, so the result is identical either way — BMI itself is a unit-independent ratio.

The standard adult categories, per the World Health Organization’s classification, are:

• Under 18.5: Underweight
• 18.5 to 24.9: Normal weight
• 25 to 29.9: Overweight
• 30 and above: Obese

Alongside the category, this calculator also shows three additional measures that answer related but distinct questions:

• BMI Prime — your BMI divided by 25 (the upper edge of the normal range). A value of exactly 1.00 sits right on the normal/overweight boundary; 1.20 means 20% above it. It’s the same information as BMI, just rescaled so “1.0” always means the same thing regardless of height.
• Ponderal Index — weight divided by height cubed instead of squared ($\vA{\text{Weight (kg)}} \div \vB{\text{Height (m)}}^3$). Because it uses height cubed rather than squared, it’s less sensitive to how tall or short someone is for a given build, which is why it’s sometimes preferred for people at the extremes of height where BMI’s squared-height assumption fits less well.
• Healthy weight range — the range of weights, at your exact height, that would put your BMI inside the 18.5-24.9 Normal weight band. This answers “how much would I need to weigh” directly, rather than requiring you to work backward from the category.

Worked Example

Someone who is 1.8 m (about 5’11”) tall and weighs 70 kg (about 154 lb):

1. Square the height: $\vB{1.8} \times \vB{1.8} = 3.24$.
2. Divide the weight by that: $\vA{70} \div 3.24 \approx \vC{21.6}$.

A BMI of about 21.6 falls in the Normal weight range (18.5-24.9).

Common Mistakes

• Treating BMI as a diagnosis rather than a screening tool. BMI flags whether weight relative to height falls outside a typical range — it doesn’t diagnose a health condition on its own, and a doctor would look at other factors before drawing any conclusion from it.
• Applying adult categories to a child or teenager. Growth is still happening at that age, so a fixed 18.5-24.9 “Normal” band doesn’t apply — pediatric BMI is read against age-and-gender growth charts instead (see the note above).
• Ignoring muscle mass. A muscular athlete can land in the “Overweight” range on BMI alone despite having low body fat, since BMI can’t distinguish muscle weight from fat weight — the Body Fat Percentage Calculator gives a more direct estimate for someone in that situation.
• Mixing up height units. Entering a height in inches while the field is still set to centimeters (or vice versa) silently produces a nonsensical BMI — double-check the unit toggle matches what you actually typed.