Frequently asked questions.

Body Mass Index is a one-number summary of weight relative to height. It is the most widely used population-level indicator of weight status in the world. The metric formula is weight (kg) / height (m)²; the imperial form is 703 × weight (lb) / height (in)².

BMI = weight in kilograms divided by the square of height in metres. In imperial units the same number falls out of BMI = 703 × weight (lb) / height (in)². The 703 is a unit-conversion constant. Both forms give the same number to one decimal place.

Both metric (kilograms and centimetres) and imperial (pounds and feet+inches). The toggle at the top of the form switches between them, and we carry your values across the swap so you do not have to re-type. There are also mixed-unit forms: kg + ft/in and lb + in.

BMI is accurate as a screen for the general adult population. It can overestimate body fat in heavily-muscled athletes and underestimate it in older adults with low muscle mass. It is not validated for pregnancy. If your reading surprises you, pair it with waist circumference and a clinician's assessment.

The standard WHO formula: weight in kilograms divided by the square of height in metres. Imperial inputs are converted to metric internally; the imperial form, BMI = 703 × weight (lb) / height (in)², gives the same number.

For ages 2–19 the CDC interprets BMI as a percentile against same-age, same-sex peers from the 2000 growth-reference cohort. Underweight is below the 5th percentile, healthy weight is 5th to less than 85th, overweight is 85th to less than 95th, and obesity is 95th and above. Our Child / Teen mode shows the band; a precise percentile requires the CDC LMS lookup.

The WHO Expert Consultation (Lancet, 2004) reviewed evidence that for many Asian populations cardiometabolic risk rises at lower BMIs than for European-descended cohorts. Several Asian countries therefore use BMI ≥ 23 for overweight and ≥ 27.5 for obesity. Our calculator has an Asian-mode checkbox.

No. BMI is not validated in pregnancy because pregnancy alters weight composition and distribution. Obstetric care is guided by the pre-pregnancy BMI and weight-gain targets that depend on the starting category.

Most adults are well served by staying inside the WHO healthy band (18.5–24.9), but the right target is personal. If you are an athlete, an older adult, recovering from illness, or from a population with lower thresholds, talk to a clinician about what target makes sense for you.

We use the Deurenberg formula (1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4), which has a typical standard error of around 4 percentage points in adults. It is suppressed for under-18s because the formula was derived on adults. For a real measurement, DEXA, bio-impedance or skinfold testing is far more accurate.

We use the Mifflin–St Jeor BMR equation multiplied by an activity factor (sedentary 1.2, light 1.375, moderate 1.55, active 1.725, very active 1.9). If you exercise regularly, pick the activity level that best matches your typical week.

No. The calculator runs entirely in your browser. We do not send your age, height, weight or any input to a server. There is no account, login, or analytics on form fields. Optional: 'Save to history' keeps the last 20 entries in localStorage on your device.

By design — the calculator recomputes on every keystroke so you can see the result update live. If you prefer to enter all fields first and then see the answer, just fill them in before glancing at the gauge.

Take the number to a clinician. They will interpret it alongside waist circumference, blood pressure, fasting glucose, lipids and family history, and discuss interventions that fit your situation. Acting on BMI alone is rarely the right move.

BMI Prime is your BMI divided by 25 — the upper edge of the WHO healthy band. A BMI Prime of 1.00 is the threshold for overweight; 0.74 is the middle of the band. It's a simple way to express how far you are from the upper edge of normal.

The Ponderal Index uses height cubed instead of height squared: PI = weight (kg) / height (m)³. Some researchers argue it correlates better with body fatness in very tall or very short adults. Our calculator reports BMI, BMI Prime and Ponderal Index.

The WHO defines a healthy BMI as 18.5 to 24.9 for non-pregnant adults. For children age 2–19, the CDC defines a healthy weight as a BMI between the 5th and 85th percentile against the 2000 CDC growth reference cohort. For Asian populations, a healthy BMI is often 18.5 to 22.9.

Class III (or 'morbid') obesity is BMI ≥ 40. The clinical term has fallen out of favour in recent guidelines in favour of 'class III obesity', but the threshold is the same. Class III obesity is associated with substantially elevated cardiometabolic risk.

BMI = weight (kg) / (height (cm) / 100)². For 70 kg / 170 cm: 70 / (1.70 × 1.70) = 70 / 2.89 = 24.22. The kg/cm form is the most direct because BMI is defined in SI units.

BMI = 703 × weight (lb) / height (in)². For 165 lb / 69 in (5 ft 9 in): (703 × 165) / (69 × 69) = 115,995 / 4,761 = 24.36 — the same number as 75 kg / 1.75 m to one decimal place.

Yes. Use the 'kg + feet/inches' unit toggle. The calculator converts ft+in to centimetres internally and applies the standard formula. You can switch units mid-calculation; your values are carried across the swap.

703 is a unit-conversion constant. To go from the metric form (kg/m²) to the imperial form (lb/in²), you divide by 0.001422, which is the same as multiplying by 703. The 703 is not a magic number — it falls out of the kg-to-lb and m²-to-in² conversion.

Aim for ±1 cm in height and ±0.2 kg in weight. A 1 cm error in height changes BMI by about 0.3 units; a 0.2 kg error changes BMI by about 0.07 units. The calculator rounds the BMI display to one decimal place, so small input errors rarely matter for the category.

For tracking over time, yes — weigh yourself at the same time of day, on the same scale, in the same conditions (e.g. first thing in the morning, after the bathroom, before breakfast). Daily fluctuations of 0.5–1 kg are normal and don't reflect actual changes in body mass.

BMI tends to overestimate body fat in heavily-muscled athletes. A 90 kg rugby player at 180 cm has a BMI of 27.8 (overweight) while carrying 12% body fat. Use the calculator's body-fat estimate as a rough check, and pair it with a DEXA scan or bio-impedance measurement for a real read.

No. Bodybuilders carry substantially more muscle than the average adult, so BMI overestimates their body fat. Use the body-fat percentage from a DEXA scan, bio-impedance, or skinfold testing instead. BMI can be useful for tracking weight gain / loss, but not for adiposity.

BMI underestimates adiposity in older adults with sarcopenia (age-related muscle loss). The number can read as 'healthy' while the body-fat percentage is substantially higher. Pair BMI with waist circumference and a body-fat measurement; the trend matters more than the snapshot.

BMI is the recommended screening tool for children age 2–19, but it is interpreted as a percentile against same-age, same-sex peers from the 2000 CDC growth reference cohort — not against the adult cut-offs. Use the Child / Teen mode in the calculator to label against the CDC bands.

BMI uses height squared, so very tall adults can read as 'overweight' while carrying average body fat. The Ponderal Index uses height cubed and is sometimes preferred for very tall and very short adults. Our calculator reports both.

BMI can underestimate adiposity in very short adults because height squared compresses the effect. The Ponderal Index is more linear across the height range. As a practical matter, use BMI in context with waist circumference and a body-fat measurement.

BMI will overestimate your body fat. Use the Deurenberg body-fat estimate as a rough check, then get a DEXA scan or bio-impedance measurement for a real read. The healthy weight range and ideal weight from the calculator are also useful for context, not targets.

BMI reflects current weight, not the weight trajectory. After significant weight loss, a 'healthy' BMI is encouraging but the body composition still needs to be checked — bone density, lean mass and visceral fat all change during weight loss. Pair the BMI with a body-fat measurement and a clinician's review.

Anorexia nervosa is a clinical diagnosis, not a BMI number. Severe thinness (BMI < 16) is a red flag that warrants urgent clinical evaluation; BMI < 15 is associated with substantially elevated mortality. The calculator's underweight category is a screen, not a diagnosis.

BMI 25.0 to 29.9 is overweight in the WHO standard. The Asian-WHO reference uses BMI 23.0 to 27.4 for overweight.

BMI 30 and above is obese in the WHO standard, with sub-classes I (30–34.9), II (35–39.9) and III (≥ 40). The Asian-WHO reference uses BMI 27.5 and above for obese.

BMI below 18.5 is underweight in the WHO standard. The calculator splits it further: severe thinness (< 16), moderate thinness (16.0–16.9), mild thinness (17.0–18.4).

Global mean BMI has risen over the last four decades. In 2016 the global mean adult BMI was about 24.2 for men and 24.8 for women (NCD-RisC, 2017). The mean is well within the WHO healthy band but masks substantial variation by region, age and sex.

BMI measures weight relative to height. Body-fat percentage measures adiposity directly. BMI uses only two numbers; body-fat percentage requires a measurement (DEXA, bio-impedance, skinfold) or a population-level estimate. They tell you different things; the best approach is to use both.

Lean body mass (LBM), also called fat-free mass, is everything in your body that isn't fat: muscle, bone, water, organs, skin and connective tissue. The calculator estimates LBM using the Boer formula: 0.407 × weight (kg) + 0.267 × height (cm) − 19.2 for men; 0.252 × weight (kg) + 0.473 × height (cm) − 48.3 for women.

Ideal weight is the weight at which BMI is in the middle of the healthy band. The calculator reports three formulas — Devine (1974), Robinson (1983) and Miller (1983) — plus the healthy weight range (BMI 18.5–24.9). The numbers are similar; treat them as a range, not a single target.

Basal Metabolic Rate is the number of calories your body burns at complete rest. We use the Mifflin–St Jeor equation: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + (5 for males, −161 for females). BMR × activity factor = Total Daily Energy Expenditure (TDEE).

Total Daily Energy Expenditure is the total number of calories you burn in a day. It is your BMR multiplied by an activity factor. Sedentary 1.2, light 1.375, moderate 1.55, active 1.725, very active 1.9. Eat at TDEE to maintain your weight.

Eat below your TDEE. A 500 kcal/day deficit produces about 0.5 kg of fat loss per week. For most adults that means TDEE − 500 = mild deficit. Pair the deficit with resistance training and 1.6–2.2 g/kg of protein per day to preserve muscle. Take it to a clinician if you have a medical condition.

Eat above your TDEE. A 300 kcal/day surplus produces about 0.25 kg of weight gain per week — slow enough to minimise fat gain. Aim for 1.6–2.2 g/kg of protein per day and prioritise resistance training. The calculator's 'Mild surplus' number (+300) is a reasonable starting point.

A calorie deficit is when you eat fewer calories than you burn (TDEE). A 500 kcal/day deficit produces about 0.5 kg of fat loss per week. The calculator shows your deficit target next to your maintenance.

Waist-to-height ratio (WHtR) is your waist circumference divided by your height. The simple rule of thumb is to keep it under 0.5. A WHtR above 0.5 is associated with elevated cardiometabolic risk; above 0.6 is high risk. The calculator computes it from your waist and height.

WHO and NIH guidance: < 102 cm (40 in) for men and < 88 cm (35 in) for women. Above those thresholds, cardiometabolic risk starts climbing. The calculator combines waist with height to give a waist-to-height ratio with a risk label.

Yes. Click 'Share' to copy a URL that pre-fills the form with your values. The URL is the only thing that leaves your browser; we do not store it on a server. The recipient opens the URL and sees your result instantly.

Yes. Click 'Download PDF' to open a one-page print summary in a new tab. Use your browser's print dialog to save it as a PDF or send it to a printer. The summary includes your inputs, BMI, category, body-fat estimate, healthy weight, ideal weight, lean body mass, BMR and TDEE.

Yes. Click 'Download CSV' to save a CSV file with your inputs and result. Open it in a spreadsheet to track BMI over time or to share with a clinician.

Yes. Click 'Save to history' to keep the last 20 entries in your browser's localStorage. The history is local to your device and is never uploaded. Clear it from the 'Clear history' button.

Yes. The calculator runs entirely in your browser. Your age, gender, height, weight, waist and activity level are never sent to a server. The only persistence is localStorage on your device, used to save your last form state and up to 20 history entries.

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