BMI calculator: calculate your body mass index
The BMI calculator divides weight in kilograms by height in metres squared: BMI = kg/m². For example, 70 kg at 1.70 m gives 24.2 kg/m². WHO’s general adult healthy-weight interval is printed as 18.5–24.9. BMI describes populations; on its own, it says nothing about an individual’s health or body composition.
Values recalculate as you type. No measurements are sent or stored: the math runs in your browser.
This calculator is general information, not medical advice. BMI and the ideal-weight formulas are statistical estimates: they describe populations, not you. They cannot diagnose anything. If your weight, your health or a result you have seen here worries you, talk it over with a healthcare professional, who can weigh your body composition, your history and your circumstances. Do not change your food, your medication or your activity because of a number on this page.
If weight is a source of distress for you, seeing an "ideal weight" can do harm. The healthy range is wide on purpose, and a number on its own does not measure your health.
How the calculation works
BMI = weight (kg) ÷ [height (m)]². Height (m) = height (cm) ÷ 100.
Age and sex do not enter the formula; those fields identify relevant limitations. Display uses one decimal, while categories use the unrounded value: 18.5 ≤ BMI <25 is the interval, with no gap between 24.9 and 25.
Limits of the estimate
BMI cannot distinguish muscle from fat or show where fat is carried. Short stature, older age, high muscularity, fluid retention and pregnancy can make the result especially misleading.
BMI helps describe populations and support an initial assessment, but cannot determine an individual’s health, body composition or risk.
These bands and formulas do not apply to people under 19, people who are pregnant or breastfeeding, people with a lot of muscle mass, people retaining fluid, or people with amputations or certain medical conditions. In those cases the reference has to come from a healthcare professional.
Sources: WHO, IMSS 2018, SEEDO 2016 and PRONAM 2025.
Adult BMI reference table
| General adult category | BMI (kg/m²) |
|---|---|
| Underweight | <18.5 |
| Healthy weight interval | 18.5–24.9 |
| Overweight | 25.0–29.9 |
| Obesity class 1 | 30.0–34.9 |
| Obesity class 2 | 35.0–39.9 |
| Obesity class 3 | ≥40 |
BMI for women and men
The BMI formula is the same for women and men. The sex variable only identifies the short-stature thresholds in Mexico’s norm; it does not change weight divided by height squared. BMI alone says nothing about an individual’s health. These categories are not used here during pregnancy or breastfeeding.
BMI for children and by age
No adult category is returned below age 19. WHO growth references for ages 5–19 interpret BMI by age and sex. Children under five need growth indicators such as weight for height. A clinician must interpret those references; a fixed adult table cannot replace them.
From age 60, the adult reading remains a reference with an explicit warning. Mexico’s NOM-008 gives overweight at 28–31.9 kg/m² and obesity at ≥32 kg/m². The IMSS downloadable table uses 25 and 30 kg/m² without naming an age group. Spain’s SEGG geriatric consensus gives normal weight 22–26.9, overweight 27–29.9 and obesity ≥30 kg/m². These different references should be discussed with a professional.
Mexico’s short-stature thresholds
For ages 20–59, NOM-008-SSA3-2017 defines short stature as under 160 cm for men or under 150 cm for women. It gives overweight at 23–24.9 kg/m² and obesity at ≥25 kg/m² for that group. The calculator flags this difference when applicable without replacing clinical assessment or applying these labels to muscularity.
Spain: the SEEDO classification
The clinical SEEDO-SEMERGEN 2020 classification splits overweight and adds a fourth obesity grade. The Ministry’s statistical reporting intervals are not clinical categories. Type 2 starts at 35.0; the 35.3 printed in the 2016 SEEDO document is a typographical error.
| General adult category | BMI (kg/m²) |
|---|---|
| Insufficient weight | <18.5 |
| Normal weight | 18.5–24.9 |
| Overweight grade 1 | 25.0–26.9 |
| Overweight grade 2 | 27.0–29.9 |
| Obesity type 1 | 30.0–34.9 |
| Obesity type 2 | 35.0–39.9 |
| Obesity type 3 | 40.0–49.9 |
| Obesity type 4 | ≥50.0 |
Frequently asked questions
How do you calculate BMI?
Body mass index is weight in kilograms divided by height in metres squared. For 70 kg and 1.70 m, the calculation is 70 divided by 2.89, giving 24.2 kg/m² after rounding. The formula is identical for women and men; it does not directly measure body fat or establish an individual’s health.
What is a normal BMI for women and men?
WHO’s general adult table prints 18.5–24.9 kg/m² as the normal-weight interval. The calculation and general interval are the same for women and men. This is a population reference, not an individual assessment; muscularity, age, pregnancy and other circumstances can make the standard table inappropriate for interpreting a particular person’s body.
Can children use the adult BMI table?
Adult categories should not be applied to children or adolescents. WHO uses age-and-sex BMI growth references for ages five to nineteen, and other growth indicators for younger children. This calculator returns no adult category before age nineteen. A healthcare professional should interpret growth using the appropriate reference and the child’s wider circumstances.
Does BMI change with age?
The mathematical operation does not change with age, but interpretation can. For adults sixty and older, Mexico’s NOM-008 uses overweight from 28 and obesity from 32 kg/m². Spain’s SEGG geriatric consensus uses different intervals. This page keeps the general adult reference visible and explains those differences without selecting a personal weight goal.
What BMI thresholds apply to short adults in Mexico?
Mexico’s NOM-008-SSA3-2017 defines short stature at ages twenty to fifty-nine as under 160 cm for men and under 150 cm for women. It sets overweight at 23–24.9 kg/m² and obesity from 25 kg/m² for this group. The general result must be interpreted alongside that distinction and the person’s clinical circumstances.
Does BMI tell me whether I am healthy?
No. BMI relates weight to height without separating muscle from fat, showing fat distribution, or accounting for symptoms, medical history and other measurements. A number does not determine your health or your worth. If a result concerns you or weight causes distress, seek professional support before changing eating, medication or activity.
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