Advanced BMI Calculator
for Adults, Children
& Pregnancy
Use kilograms or pounds, and centimetres or feet and inches. Calculate adult BMI, review a pre-pregnancy BMI, or get a child/teen BMI number with appropriate guidance.
Calculate your BMI.
Choose the right context, check your measurements, then calculate.
Your result will appear here.
Enter your measurements on the left. The calculator will show the BMI value and the appropriate context.
Body Mass Index
Your saved calculations.
Saving is optional. Entries stay in this browser on this device, up to 20 results. Avoid saving on a shared computer.
What your BMI result means.
BMI relates your weight to your height. For adults, it provides a screening category that should be considered alongside other health information. Body composition and individual circumstances matter too.
1. Choose the right context
Use adult mode from age 20. For children enter date of birth, measurement date and sex reference. For pregnancy use your weight before pregnancy.
2. Check your units
Make sure the scale reading and height units match the selected option. Waist is always entered in centimetres.
3. Review the result
Read the explanation alongside the number. Classification uses the unrounded BMI, even when the displayed value is rounded.
4. Ask about your situation
Bring questions to your healthcare professional if you need interpretation or individual nutrition guidance.
A simple formula.
Example: 70 kg at 1.70 m gives 70 ÷ 2.89 = 24.22, displayed as 24.2.
For pounds and inches, this tool converts measurements to kilograms and metres before applying the same formula.
Adult BMI categories.
| BMI (kg/m²) | Screening category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to below 25 | Healthy weight |
| 25 to below 30 | Overweight |
| 30 to below 35 | Obesity — class 1 |
| 35 to below 40 | Obesity — class 2 |
| 40 or above | Obesity — class 3 |
Adult BMI categories apply irrespective of sex. They do not explain all differences in health risk or body composition.
Read the CDC adult category reference ↗Why the context can change.
NICE uses lower practical BMI thresholds for people with South Asian backgrounds: overweight from 23 to below 27.5, and obesity from 27.5. The main result retains standard CDC adult categories; the optional additional reference is shown separately.
| BMI | South Asian practical context |
|---|---|
| Below 18.5 | Below the general reference range |
| 18.5 to below 23 | Below the lower overweight threshold |
| 23 to below 27.5 | Overweight threshold |
| 27.5 or above | Obesity threshold |
For adults with BMI below 35, NICE describes waist-to-height ratios 0.4–below 0.5 as healthy central adiposity, 0.5–below 0.6 as increased and 0.6 or above as high. Other results need individual interpretation.
What changes, and what stays the same.
Height and weight determine the BMI number. Adult classification does not require a male/female reference. Some prediction equations do; choose general if you do not want those estimates.
Older adults may need extra assessment of strength, mobility and changes in weight. Child growth uses a different reference and is not assessed against the adult table.
Age and pregnancy need different context.
Children & teenagers
Ages 2–19 need BMI-for-age assessment using age and sex. This tool uses CDC age- and sex-specific LMS data to estimate the percentile. A child’s growth should be reviewed over time.
Use the CDC child/teen calculator ↗Before pregnancy
Use pre-pregnancy weight rather than current pregnancy weight. Discuss pregnancy weight changes and personal goals with your maternity team.
Read CDC pregnancy guidance ↗Estimates with visible methods.
Weight reference
The mathematical interval for adult BMI 18.5 to below 25 at your height. It is not a personalised goal.
Waist-based ratios
Waist ÷ height and waist ÷ hip use the same units. Waist-to-hip is shown numerically without a risk label.
Body fat & RFM
Use a known body-fat value if available. Otherwise, RFM estimates a percentage from height, waist and sex reference. An out-of-range estimate is withheld.
Resting & daily energy
Mifflin–St Jeor estimates resting energy. Multiplying by the selected activity factor gives an approximate daily total; both may differ from actual needs.
Fat and lean mass
Weight is divided using the entered or estimated body-fat percentage. This is a calculation, not a measured body-composition test.
Ponderal index
Weight in kilograms divided by height in metres cubed. Displayed for description only, without diagnostic thresholds.
A number cannot tell the whole story.
BMI does not directly measure fat or distinguish fat from muscle. Interpretation needs additional care for pregnancy, growing children and people whose body composition differs from the reference population.
Waist-based ratios, body-fat equations and energy estimates offer extra context with important limits. They are not diagnostic tests.
Use the result as a starting point.
Write down what you would like to understand. A personalised discussion can consider your food preferences, routine and health history alongside these measurements.
For pregnancy or child growth concerns, involve the appropriate healthcare team. A calculator alone cannot choose your treatment or an eating plan.
Frequently asked questions.
Is BMI a diagnosis?
No. It is a height-and-weight screening calculation. It does not measure body fat directly or establish your overall health.
Which ages use the adult categories?
This calculator uses the CDC adult reference for people aged 20 and older. For ages 2–19, it estimates a CDC BMI-for-age percentile using date of birth, measurement date and sex-specific LMS growth data.
Can I enter kilograms with feet and inches?
Yes. Choose the kg + feet/inches unit option. Pounds with feet/inches is also available.
How does pregnancy mode work?
Enter your weight before pregnancy. The result describes pre-pregnancy BMI, not current pregnancy weight status. Ask your maternity team about your individual care plan.
Why does the range not mean an ideal weight?
The displayed range is the mathematical weight interval corresponding to BMI 18.5 to below 25. It is a screening reference rather than a personal target.
Does this calculate child percentiles?
Yes. It uses the CDC 2000 LMS BMI-for-age reference, with interpolated monthly parameters. Percentiles beyond the standard reference tails are shown as below 3rd or at/above 97th; very high BMI requires extended growth-chart assessment.
Is my result sent to WhatsApp?
No. The consultation button opens a generic booking enquiry. Your measurements and BMI are not added to that message.
Are my entries stored?
Results are not saved automatically. Use “Save on this device” to store a calculation in local browser history. This calculator does not send entries to a server. Other services installed on your website have separate privacy practices.
How are child percentiles calculated?
The BMI is compared with interpolated CDC 2000 LMS values for age in months and sex. This standard reference is not the extended high-BMI chart; extremes require professional assessment.
Can I save or print my result?
Yes. Calculate first, then choose Save on this device or Print result. Saved history can be deleted from this page.
Why do energy estimates need sex and age?
The Mifflin–St Jeor resting-energy equation uses weight, height, age and a male/female reference. General selection omits these equations.
What does South Asian context do?
It adds the lower practical NICE adult BMI thresholds alongside the standard result. It does not replace a personal health assessment.
Is the body-fat value measured?
Only a value you enter from a measurement is labelled as entered. RFM is a prediction from height and waist, and can have substantial individual error.
Does pregnancy mode calculate weight gain?
It shows general total pregnancy weight-gain ranges from pre-pregnancy BMI for one baby. It does not estimate weekly gain, support multiple pregnancies or calculate pregnancy calorie targets.
Transparent references.
- CDC adult BMI categories
- CDC growth-chart LMS data and method
- CDC pregnancy weight-gain guidance
- NICE NG246: BMI and central adiposity
- Mifflin–St Jeor resting-energy equation
- Relative fat mass equation
Child age uses elapsed days ÷ 30.4375, with interpolated CDC 2000 monthly LMS values. Tail percentiles are bounded for display. Activity factors are selectable assumptions. This page does not claim clinical validation, affiliation or endorsement by the reference organisations.
Syed Muhammad Ilyas Shah
Registered Associate Nutritionist (ANutr)
Association for Nutrition (AfN), UK · Reg. 49297
89 Kirkgate, Bradford, BD1 1SZ, United Kingdom
Ask about a consultation ↗