BMI Change Calculator
See how a change in body weight moves your BMI, from starting BMI to new BMI and any category shift.
How it works
BMI = 703 × weight(lb) ÷ height(in)² · BMI change = new BMI − starting BMI
BMI is body weight scaled by the square of height, so any weight change moves it predictably for a fixed height. This tool computes your BMI before and after a weight change and reports the difference and whether you have crossed a category boundary (underweight, normal, overweight, obese). Remember that BMI is a population screening tool: it cannot tell muscle from fat, so a very muscular person can read "overweight" while being lean.
Worked example
At 5 ft 8 in (68 in), dropping from 200 lb to 180 lb takes BMI from 703 × 200 ÷ 68² ≈ 30.4 (obese) to 703 × 180 ÷ 68² ≈ 27.4 (overweight) — a change of about −3.0.
Frequently asked questions
How much weight changes BMI by one point?
For an adult of average height, roughly 6–8 lb shifts BMI by one point. Taller people need a bit more weight per BMI point, shorter people a bit less.
Is a lower BMI always better?
No. Both very low and very high BMI carry health risks. The "normal" band (18.5–24.9) is a screening range, and athletes or older adults may be healthiest slightly outside it.
Why does BMI use height squared?
Squaring height roughly cancels out the natural scaling of body weight with size, so BMI stays comparable across different heights better than a plain weight-to-height ratio.
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This is a general educational estimate, not medical or dietary advice. Weight, body composition and energy needs respond differently for everyone — for a plan tailored to you, talk to a doctor or registered dietitian.