BMI Calculator Guide: Understand Your Body Mass Index

Body mass index (BMI) is a calculated measure of weight relative to height. It is a quick screening measure, not a direct measurement of body fat and not a diagnosis. The CDC overview of BMI recommends considering an individual result with other information such as medical history, health behaviours, physical examination findings, and laboratory results.

The Nest Calculators BMI tool applies fixed adult categories and is intended only for adults age 20 and older. It does not collect enough information to interpret BMI for children or teenagers.

Metric formula

U.S. customary formula

The units differ, but both formulas produce the same BMI apart from ordinary input and display rounding.

For adults age 20 and older, the CDC adult BMI calculator uses these categories:

• Underweight: below 18.5
• Healthy Weight: 18.5 to 24.9
• Overweight: 25.0 to 29.9
• Obesity Class 1: 30.0 to 34.9
• Obesity Class 2: 35.0 to 39.9
• Obesity Class 3 (Severe Obesity): 40.0 or greater

A category is a screening label. It does not determine whether a particular person is healthy, explain why their weight changed, or prescribe a target weight or treatment.

An adult is 170 cm tall and weighs 70 kg:

A BMI of about 24.2 falls within the adult Healthy Weight category. That statement identifies the category only; it does not measure the person's body-fat percentage or provide a diagnosis.

• It does not directly measure body fat.
• It does not distinguish fat mass from muscle and bone.
• It does not show where body fat is distributed.
• It does not incorporate medical history, blood pressure, laboratory results, medicines, pregnancy, or disability.
• It cannot determine an individual's health or treatment needs by itself.

The CDC's BMI frequently asked questions explain why a healthcare professional may consider BMI alongside other clinical and personal information.

The calculator may show the weight range corresponding mathematically to BMI 18.5–24.9 at the entered height. This is not a personalised “ideal weight,” medical recommendation, or goal. It is simply the weight interval produced by rearranging the BMI formula at those two category boundaries.

For ages 2 through 19, BMI is interpreted using age- and sex-specific percentiles rather than these fixed adult categories. Use the official CDC Child and Teen BMI Calculator or consult a qualified healthcare professional. Do not use this site's adult category result for a child or teenager.

• Check that height, weight, and unit selections are correct.
• Treat small differences near a category boundary cautiously because inputs and display values are rounded.
• Do not start, stop, or change medicine, diet, or treatment solely because of this result.
• Discuss an unexpected result or health concern with a qualified healthcare professional.

Use the Adult BMI Calculator for a transparent weight-for-height screening estimate. Read the category and formula-derived weight range as context—not as a body-fat measurement, diagnosis, or personalised health recommendation.