BMI Calculator

What is a BMI Calculator?

This calculator takes your height and weight, then instantly shows your Body Mass Index (BMI) — a number that indicates whether your weight falls within a healthy range for your height. It also shows the WHO classification (Underweight, Normal, Overweight, or Obese) with a color-coded result.

How to Calculate BMI

Unit System

Use the toggle at the top to pick your unit system:

• cm / kg — Enter height in centimeters, weight in kilograms. Default for most countries.
• ft·in / lb — Enter height in feet and inches, weight in pounds. Default for English (US) users.

The calculator converts imperial values to metric internally before calculating.

What the Results Mean

Result	What it shows
BMI Number	Your calculated Body Mass Index. Color changes based on classification.
Classification	Where you fall on the WHO scale (see table below)
BMI Range Table	All four categories with their BMI ranges. Your category is highlighted.

WHO Classification

Classification	BMI Range	Color
Underweight	Less than 18.5	Gray
Normal	18.5 - 24.9	Blue
Overweight	25.0 - 29.9	Gold
Obese	30.0 or higher	Red

Good Examples

Example 1: Normal Weight

A person who is 170 cm tall and weighs 65 kg.

• Height: 170
• Weight: 65
• BMI: 22.5 — Normal

Example 2: Overweight (Imperial)

A person who is 5’10” and weighs 195 lb.

• Height: 5 ft 10 in
• Weight: 195 lb
• BMI: 28.0 — Overweight

Example 3: Underweight

A person who is 165 cm tall and weighs 45 kg.

• Height: 165
• Weight: 45
• BMI: 16.5 — Underweight

Example 4: At the Border

A person who is 175 cm tall and weighs 76 kg.

• Height: 175
• Weight: 76
• BMI: 24.8 — Normal (just barely — 25.0 would be Overweight)

The BMI Formula

$BMI = \frac{weight\ (kg)}{height\ (m)^2}$

Step by step: For someone 175 cm, 70 kg:

1. Convert cm to meters: 175 cm = 1.75 m
2. Square the height: 1.75 × 1.75 = 3.0625
3. Divide weight by squared height: 70 / 3.0625 = 22.86

Limitations of BMI

BMI is a useful screening tool, but it has well-known blind spots:

Muscle vs fat. Athletes with high muscle mass often register as “Overweight” or “Obese” despite low body fat. A bodybuilder and a sedentary person can have the same BMI but very different health.

Fat distribution matters. Belly fat (visceral) is more dangerous than fat under the skin (subcutaneous). Two people with the same BMI can have different risk profiles.

Age and sex. Women typically have more body fat than men at the same BMI. Older adults tend to have more body fat than younger adults at the same BMI.

Ethnic variations. Asian populations may face elevated health risks at lower BMI values (23+ is sometimes considered overweight in Asian guidelines).

Not for children. Children’s BMI is interpreted using age- and sex-specific percentile charts, not the fixed thresholds above.

For a fuller picture, consider waist circumference, body fat percentage, or consultation with a healthcare professional.

Key Tips

• Use morning weight. Weigh yourself in the morning after using the bathroom, before eating. Evening weight can be 1–2 kg higher due to food and water.
• BMI is a screening tool, not a diagnosis. A high BMI flags the need for further assessment, not automatic health problems. Always consider the full picture.
• Track trends, not single readings. One BMI measurement means little. Monthly tracking over 3–6 months shows whether you’re moving in the right direction.
• Asian populations use stricter thresholds. In many Asian countries, BMI 23+ is considered overweight (vs. 25+ globally). Use the appropriate standard for your ethnicity.

Frequently Asked Questions

What is a healthy BMI?

According to the WHO, 18.5 - 24.9 is “Normal.” But optimal BMI varies by age, sex, ethnicity, and individual factors. A normal BMI doesn’t guarantee good health, and an abnormal BMI doesn’t necessarily mean poor health.

How often should I check my BMI?

Once or twice a year is enough for most adults. If actively managing weight, monthly tracking helps monitor trends. Focus on the direction, not day-to-day fluctuations.

Can BMI be used during pregnancy?

Pre-pregnancy BMI helps healthcare providers set weight gain recommendations. But BMI should not be calculated or interpreted during pregnancy — weight gain is expected and necessary.

What should I do if my BMI is outside the normal range?

A single number doesn’t define your health. If concerned, speak with a healthcare professional who can assess diet, activity, family history, and other factors. Don’t make drastic changes based on BMI alone.