BMI Calculator – Check Your Body Mass Index & Healthy Range
About the BMI Calculator
Body Mass Index is a single number derived from your height and weight. In metric units the formula is \text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}, and in imperial units it is \text{BMI} = \frac{703 \times \text{weight (lb)}}{\text{height (in)}^2}. This calculator works in either system, returns your BMI to one decimal place, places it on a visual scale, and shows the weight range that would put you in the standard healthy category for your height.
BMI exists because it is cheap and quick. It was devised in the 1830s by Adolphe Quetelet as a statistical description of populations, not as a measure of individual health, and it was adopted for clinical screening much later precisely because it needs nothing more than a scale and a tape measure. That history explains both its usefulness and its limits.
The important limitation is that BMI measures mass, not composition. It cannot distinguish muscle from fat, so a trained athlete with substantial muscle mass can register as overweight while carrying very little body fat. It says nothing about where fat is distributed, and abdominal fat carries different risk than fat elsewhere. Its thresholds were derived largely from European-descent populations, and health bodies in several Asian countries use lower cut-offs because cardiometabolic risk appears at lower BMI values in those groups. It is also not appropriate during pregnancy, and it needs different interpretation entirely for older adults, whose body composition shifts with age.
Most usefully, BMI is a screening indicator that tells you whether a fuller conversation is worth having. It is not a diagnosis and not a verdict on your health. A doctor looking at the same question would also consider waist measurement, blood pressure, blood markers, family history, activity level, and how you actually feel, none of which fits into a two-input formula. If your result surprises or concerns you, that conversation with a healthcare professional is the genuinely informative next step.
The BigToolSite team checked this calculator’s output against the standard WHO formula in both unit systems, verified that the metric and imperial paths produce matching results for equivalent inputs, and confirmed the healthy weight range is derived from your own height rather than a fixed table.
If you want to express the gap between two figures as a percentage rather than an absolute difference, the Percentage Calculator handles that arithmetic directly.
For the other everyday numbers our calculators cover, the File Size Calculator is one of the more frequently used tools in the same collection.
How to Use the BMI Calculator
- Choose your unit system: Metric (kg, cm) or US Units (lbs, ft/in). The input fields change to match.
- Enter your Age in years and select your Gender. These are recorded for context but do not change the BMI formula itself, which uses only height and weight.
- Enter your Height. In metric this is a single figure in centimeters; in US units it is feet and inches in separate boxes.
- Enter your Weight in kilograms or pounds.
- Click Calculate BMI, or just press Enter from any field.
- Read your result: the large number is your BMI, the colored badge names the standard category, and the marker on the gradient bar shows where you fall between 15 and 40.
- The Healthy Weight Range box converts the 18.5 to 24.9 BMI band into actual weights for your height, in whichever units you selected.
- The reference table below lists the full WHO categories, including the three obesity classes, with their BMI ranges.
- Click Reset to clear the form and start again.
Example Usage
Metric input: Height 175cm, Weight 70kg
Calculation: \frac{70}{1.75^2} = 22.9
Result: BMI 22.9, Normal weight, healthy range for that height roughly 56.7kg to 76.3kg
US input: Height 5ft 9in, Weight 165 lbs
Calculation: \frac{703 \times 165}{69^2} = 24.4
Result: BMI 24.4, Normal weight, healthy range roughly 125 lbs to 168 lbs
The healthy range is the same formula rearranged as \text{weight} = \text{BMI} \times \text{height (m)}^2, solved at 18.5 and at 24.9. That figure is often more practical than the BMI number itself, because it is expressed in units you actually recognize. It also shows how wide the band is: for a given height, the normal category typically spans around 20 kilograms or 45 pounds, which is a reminder that there is no single correct weight for a person of a given height.
The gauge caps at 40 on the right-hand side. A BMI above that still displays its exact value in the label, but the marker sits at the far end of the bar rather than extending past it.
Frequently Asked Questions
In metric, divide your weight in kilograms by your height in meters squared. In imperial, divide your weight in pounds by your height in inches squared, then multiply by 703. Both produce the same number for the same person.
The WHO classifies 18.5 to 24.9 as normal weight, below 18.5 as underweight, 25 to 29.9 as overweight, and 30 or above as obese. These are population-level screening thresholds rather than individual targets.
No. It cannot separate muscle from fat, so muscular people often score as overweight despite low body fat. It also does not account for fat distribution, and several Asian health authorities use lower thresholds because risk appears at lower BMI values in those populations.
The categories shown here are for adults aged 20 and over. For children and adolescents, BMI is interpreted against age and sex percentile charts rather than fixed cut-offs, so a pediatric growth chart from a healthcare provider is the correct reference.
The BMI formula itself uses only height and weight. Age and sex matter for interpreting the result, since body composition changes over a lifetime and differs between men and women at the same BMI, but they do not enter the calculation.
No. Weight gain during pregnancy is expected and monitored differently, so BMI categories do not apply. Pregnancy weight guidance comes from a midwife or doctor based on your pre-pregnancy measurements.
No single number replaces it, but waist circumference and waist-to-height ratio capture abdominal fat, which BMI misses entirely. Body fat percentage measured by a clinical method gives a fuller picture. Clinicians typically use several measures together.
Treat it as a prompt to talk with a doctor rather than a conclusion. They can look at the measures BMI cannot see and tell you whether the number means anything for you specifically.
The formula and the standard adult thresholds are identical. Women generally carry a higher proportion of body fat than men at the same BMI, which is one more reason the number is a starting point rather than an assessment.
No. The calculation runs entirely in your browser and nothing is saved or transmitted.
