BMI and ideal body weight are both weight-for-height measures, but they answer different questions. BMI is a screening ratio — weight divided by height squared — that sorts adults into categories and produces a healthy weight range. Ideal body weight (IBW) is a single-point estimate from the 1974 Devine formula, a dosing heuristic that predates modern screening ranges. The two calculators even overlap on purpose: the ideal body weight page shows the BMI healthy weight band next to the Devine number so the legacy heuristic is presented honestly alongside the population screening range.
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 ÷ 100, or inches × 0.0254 — and weight to kilograms — pounds × 0.453592 — then computes BMI = weight kg ÷ height m², rounded to one decimal place. The category is underweight below 18.5, normal weight below 25, overweight below 30, and obese at 30 or higher, and the page also shows the healthy weight range for that height: 18.5 × h² to 24.9 × h².
The ideal body weight calculator takes sex and height in centimeters or feet and inches. It converts height to inches, counts only the inches over 5 feet (60 inches), and applies the Devine formula: men get 50 kg plus 2.3 kg per inch over 5 ft; women get 45.5 kg plus 2.3 kg per inch over 5 ft. Anyone 5 ft or shorter receives the base weight. The result rounds to one decimal place, and the page also shows the healthy weight band at that height (BMI 18.5–24.9) so the two conventions sit side by side.
Side-by-side
| BMI calculator | Ideal body weight calculator | |
|---|---|---|
| Question answered | What category and healthy range does this height-weight combination fall into? | What single Devine weight-for-height estimate does this height produce? |
| Core formula | weight kg ÷ height m² | 50 + 2.3 × (inches over 60) for men; 45.5 + 2.3 × (inches over 60) for women |
| Inputs | Age (20+ gate), height, weight | Sex, height only |
| Main result | BMI to one decimal, category, healthy weight range | Devine IBW, plus the BMI 18.5–24.9 band for the same height |
| Nature of the number | Screening ratio with a range | Single-point dosing heuristic from 1974 |
| Shared readout | Healthy weight range | Healthy weight band |
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 ideal body weight calculator when the question is the Devine convention itself: what the legacy dosing formula says for a given sex and height, and how that point compares with the BMI-based band. The page’s tips include entering height without shoes, comparing the Devine number with the healthy band rather than treating either as a verdict, and confirming with a pharmacist or clinician which weight convention a specific medication protocol requires.
Limits and disclaimer
Both pages are educational only and not medical advice. BMI uses fixed adult categories for age 20 and older, is not for children, pregnancy, or diagnosis, and its hard cut points mean a small measurement difference near 18.5, 25, or 30 can change the displayed label. The Devine formula is a decades-old heuristic that can overestimate at taller heights and underestimate at shorter heights, counts nothing about muscle or frame size, and is not a weight-loss target or diagnosis. Neither page should be the basis for starting a diet, medication, supplement, or exercise program — seek individualized guidance from a qualified healthcare professional.