Use this free BMI calculator to turn height and weight into a body mass index number, a Chinese adult reference category, and a height-based healthy weight range. Enter height in centimeters and weight in kilograms. The body mass index calculator runs entirely in your browser and never uploads your measurements. These are screening estimates for adults aged 18 and older, not a medical diagnosis.
Important: Yaya Tools applies Chinese adult BMI cutoffs by default. The BMI number itself is universal, but the category label can differ from WHO or CDC charts used in other countries. Always interpret results with qualified health guidance when they matter for care decisions.
What is BMI?
Body mass index (BMI) is a simple ratio that compares body weight to height. It was developed as a population screening tool because weight alone does not account for how tall someone is. A person who weighs 80 kilograms at 160 centimeters carries a different frame load than someone of the same weight at 190 centimeters; BMI puts both measurements on one scale.
The index does not measure body fat directly. It does not read waist size, muscle mass, bone density, hydration, or where fat is stored. For large groups, however, higher average BMI tends to correlate with higher adiposity, which is why public health agencies still use it in BMI chart tables and quick clinic screenings.
On this page, BMI answers three practical questions at once:
- What is my numeric body mass index?
- Which Chinese adult reference category does that number fall into?
- Given my height, what weight range corresponds to a BMI between 18.5 and 23.9?
Those follow-on answers make the tool more useful than a bare formula, but they remain mathematical references, not personalized medical advice.
Who is this calculator designed for?
The tool is intended for adults aged 18 and older. It is not suitable for children, adolescents, pregnant people, or anyone who needs age- and sex-specific growth curves. Athletes and very muscular adults may also get misleading categories because muscle weighs more than fat without implying the same metabolic risk profile.
If you need to confirm that someone meets the adult age threshold, the age calculator can compute exact years, months, and days from a birth date.
How to use the BMI calculator
The workflow is deliberately short so you can check a number during a health log update or before a doctor visit.
- Open the BMI calculator on Yaya Tools.
- Enter height in centimeters (accepted range: 50–300 cm).
- Enter weight in kilograms (accepted range: 10–500 kg).
- Read the BMI value (one decimal place), the Chinese adult category, and the reference healthy weight range for your height.
Results update instantly as either field changes. The default example uses 170 cm and 65 kg; click Reset example to restore those values after experimenting.
Privacy and local processing
Height and weight are calculated only in your current browser tab. Nothing is sent to a server, stored in a Yaya Tools account, or synced across devices by this page. That local-only design suits quick personal checks, though you should still use a trusted device if the numbers are sensitive. Browser autofill, history, and extensions on shared computers can expose typed values.
Reading the category scale
The on-page scale reflects Chinese adult screening cutoffs:
| Category (page label) | BMI range |
|---|---|
| Underweight (偏低) | below 18.5 |
| Normal (正常范围) | 18.5–23.9 |
| Overweight (偏高) | 24.0–27.9 |
| Obese (肥胖) | 28.0 and above |
Boundary rules match the implementation: 18.5 is the first normal value, 24.0 is the first overweight value, and 28.0 is the first obese value.
The BMI formula
Every reputable body mass index calculator uses the same core equation when inputs are metric:
BMI = weight (kg) ÷ height (m)²
Height must be converted from centimeters to meters before squaring. The page divides centimeters by 100 automatically, so you never need to retype meters.
To recover a target weight from a desired BMI, rearrange the formula:
weight (kg) = BMI × height (m)²
The healthy weight band on this page inserts BMI 18.5 and 23.9 into that rearranged formula, then rounds each result to one decimal place. That band is the Chinese adult normal reference, not a prescription that everyone must hit.
Chinese adult cutoffs vs WHO international cutoffs
The BMI number is identical everywhere; categories differ. Compare the standards side by side:
| Classification | Chinese adult (this tool) | WHO / CDC international adult |
|---|---|---|
| Underweight | below 18.5 | below 18.5 |
| Normal / healthy weight | 18.5–23.9 | 18.5–24.9 |
| Overweight | 24.0–27.9 | 25.0–29.9 |
| Obese | 28.0 and above | 30.0 and above |
China's National Health Commission publishes the 18.5 / 24 / 28 boundaries in its adult weight-management guidance. The World Health Organization obesity and overweight fact sheet describes the widely cited global bands where overweight begins at 25 and obesity at 30. The CDC adult BMI categories page aligns with those international cutoffs for U.S. adult screening.
If two calculators show the same BMI but different labels, compare the reference table first before assuming an error.
BMI calculator examples
Worked examples clarify how centimeters, kilograms, and cutoffs interact on this page.
Example 1: Default inputs — 170 cm and 65 kg
Convert height: 170 cm = 1.70 m.
BMI = 65 ÷ (1.70 × 1.70)
= 65 ÷ 2.89
≈ 22.49 → displayed as 22.5
Category: normal (18.5–23.9).
Healthy weight range from BMI 18.5–23.9:
Lower = 18.5 × 1.70² ≈ 53.5 kg
Upper = 23.9 × 1.70² ≈ 69.1 kg
Displayed range: 53.5–69.1 kg.
Example 2: 160 cm and 72 kg — overweight on the Chinese scale
BMI = 72 ÷ (1.60 × 1.60)
= 72 ÷ 2.56
= 28.125 → displayed as 28.1
Category: obese (28.0 and above).
Note the nuance: at BMI 28.1 this person is obese under Chinese cutoffs but would still fall in the WHO overweight band (25–29.9), not WHO obese (30+). The same number can carry a different public-health label. That is why the standard must be stated explicitly.
Healthy reference range at 160 cm:
18.5 × 1.60² ≈ 47.4 kg
23.9 × 1.60² ≈ 61.2 kg
Range: 47.4–61.2 kg (mathematical reference, not a mandate).
Example 3: 180 cm and 58 kg — underweight
BMI = 58 ÷ (1.80 × 1.80)
= 58 ÷ 3.24
≈ 17.90 → displayed as 17.9
Category: underweight (below 18.5).
Healthy reference range at 180 cm:
18.5 × 1.80² ≈ 59.9 kg
23.9 × 1.80² ≈ 77.4 kg
Range: 59.9–77.4 kg.
Example 4: 175 cm and 75 kg — borderline overweight
BMI = 75 ÷ (1.75 × 1.75)
= 75 ÷ 3.0625
≈ 24.49 → displayed as 24.5
Category: overweight (24.0–27.9). A BMI of exactly 24.0 would also land here; values below 24.0 remain normal on this page.
Under WHO/CDC rules the same 24.5 BMI is still normal (below 25). This example shows how a single reading can look fine on one BMI chart and elevated on another.
Example 5: Tracking a 5% weight change
Suppose someone at 170 cm drops from 70 kg to 66.5 kg. Absolute change is 3.5 kg. Percent change:
(3.5 ÷ 70) × 100 = 5%
Use the percentage calculator for quick ratio math while keeping BMI trend notes in a spreadsheet or journal. Pairing BMI with percentage change highlights velocity, not just category.
Common mistakes when using a BMI calculator
Avoid these errors so your screening number stays meaningful.
Using the wrong units. This page expects centimeters and kilograms. Entering 170 as feet or 65 as pounds without converting first produces nonsense values. Convert imperial measurements before typing.
Assuming WHO labels. Many English-language BMI chart posters follow WHO or CDC cutoffs. This tool deliberately uses Chinese adult boundaries. A BMI of 24.5 is overweight here but still normal on a CDC poster.
Treating the healthy weight range as a personal target. The band is algebra from height and fixed BMI limits. It ignores muscle mass, pregnancy, chronic illness, and individual clinical goals.
Single-shot diagnosis. One reading after a large meal, intense training, or dehydration can shift weight slightly. BMI is better for consistent trend tracking under similar conditions (same scale, similar time of day, comparable clothing).
Applying adult cutoffs to children. Pediatric BMI requires age- and sex-specific percentiles. This calculator will output a number for a child, but the category text is not valid for them.
Ignoring body composition. Strength athletes often sit in the overweight band with low body fat. BMI cannot see that distinction; waist circumference and clinical assessment add context.
Expecting server-side history. Because processing is local, the page does not remember past entries. Save results yourself if you need longitudinal charts.
Replacing professional care. Categories flag possible risk for further evaluation; they do not prescribe diets, drugs, or exercise plans.
Beyond BMI: related tools
BMI describes size relative to height, not daily energy needs. After screening, many adults estimate calories with the BMR/TDEE calculator, which applies the Mifflin–St Jeor equation and activity factors. Use BMI for category screening and BMR/TDEE for maintenance-calorie estimates—each answers a different question.
Together with the age calculator and percentage calculator, you can build a simple personal log: age-confirmed adult status, BMI snapshots over time, and percent weight changes between visits.