BMI and BMR look alike because both start from height and weight, but they answer different questions. Body mass index (BMI) is a size ratio — weight divided by height squared — that sorts adults into screening categories. Basal metabolic rate (BMR) is an energy estimate — the calories a body would use at rest, computed from weight, height, age, and sex. The BMR page makes the separation explicit: the BMI calculator addresses a different measurement and should not be confused with resting energy expenditure.
What each calculator does
The BMI calculator takes age (adults 20 and older only), a metric or imperial unit system, height, and weight. It converts height to meters — centimeters divided by 100, or inches times 0.0254 — and weight to kilograms — pounds times 0.453592 — then computes BMI = weight kg ÷ height m². The result is rounded to one decimal place and paired with a CDC-style screening category: underweight below 18.5, normal weight below 25, overweight below 30, and obese at 30 or higher. The page also computes the healthy weight range for that height, 18.5 × h² to 24.9 × h², displayed in kilograms or converted back to pounds. Age and sex do not appear in the formula; age only gates the result to adults.
The BMR calculator takes a unit system, sex, age, height, weight, and an activity level. It converts inches to centimeters (× 2.54) and pounds to kilograms (× 0.453592), then applies the Mifflin-St Jeor equation: base = 10 × weight kg + 6.25 × height cm − 5 × age, adding 5 for males or subtracting 161 for females. The primary result is that BMR rounded to whole calories per day, with the older Harris-Benedict equation shown as a secondary value. The page then multiplies the Mifflin-St Jeor BMR by the chosen activity factor — sedentary 1.2, light 1.375, moderate 1.55, very active 1.725, extra active 1.9 — to estimate TDEE, also rounded to whole calories per day.
Side-by-side
| BMI calculator | BMR calculator | |
|---|---|---|
| Question answered | What size category is this height-weight combination? | How many calories does this body use at rest? |
| Core formula | weight kg ÷ height m² | 10 × weight kg + 6.25 × height cm − 5 × age, then +5 (male) or −161 (female) |
| Inputs | Age (20+ gate), height, weight | Sex, age, height, weight, activity level |
| Main result | BMI to one decimal, category, healthy weight range | BMR in whole calories per day, plus Harris-Benedict and TDEE |
| Age and sex | Not part of the formula | Age and sex appear in the equation |
| Interpretation | CDC screening categories, not a diagnosis | Resting-energy estimate; TDEE adds an activity factor |
When to use which
Use the BMI calculator when the question is body-size classification: whether a height and weight combination falls into the common adult screening bands, or what weight range at a given height corresponds to a healthy BMI. It is a screening measure, and the page is explicit that the categories are not diagnoses and can mislead for muscular people because BMI uses total weight rather than fat mass.
Use the BMR calculator when the question is energy: how many calories the body would use at rest, and how that estimate grows with an activity factor toward daily needs. Weight-management planning belongs here, and the page recommends comparing the estimate with consistent intake records and weight trends rather than treating BMR as a recommended calorie intake. BMR is below most people’s total needs, and the page warns not to confuse it with body-size measures like BMI.
Limits and disclaimer
Both pages are educational only and are not medical advice. The BMI calculator uses fixed adult categories for age 20 and older, is not for children, pregnancy, or diagnosis, and its hard category cut points mean a small measurement difference near 18.5, 25, or 30 can change the displayed label. The BMR equations are estimates: neither measures lean body mass directly, and real energy needs vary with body composition, medications, thyroid status, illness, training load, spontaneous movement, sleep, and tracking accuracy. Do not start a diet, medication, supplement, or exercise program based only on either result; seek individualized guidance from a qualified healthcare professional.