Healthy weight is a range, not a target number
The 'ideal weight' concept is a fossil from the Metropolitan Life Insurance actuarial tables of the 1940s — a single number for each height, calculated from insurance claim data. Modern medicine has moved past that. Healthy weight is a range, and any weight within that range is medically fine. Chasing a specific number wastes time and creates unnecessary disordered eating.
The current medical consensus uses the BMI 18.5–24.9 range as the healthy zone. For a 5'8" adult that's 122–164 lb — a 42 lb spread. Frame size, age, muscle mass and ethnicity all influence where within that band a person naturally lands. Small-framed 20-year-old sedentary females sit at the lower end. Muscular 40-year-old male athletes sit at or above the upper end. Both can be equally healthy.
Historical formulas — Devine (1974), Robinson (1983), Miller (1983), Hamwi (1964) — return a single 'ideal' number, but they disagree by 10–15 lb for the same person. Devine tends conservative and is still used in clinical drug dosing. Robinson is more current. Averaging all four gives a reasonable center of the healthy range. Frame-size adjustment (wrist circumference or elbow breadth) shifts within it: small frames sit under IBW, large frames comfortably above.
Age changes the target. Longevity research consistently shows that older adults (65+) have better outcomes at the upper end of BMI (22–27) — some fat reserves buffer illness and hospitalization. Elderly patients aggressively slimmed to BMI 20 do worse in outcome studies. Younger, sedentary adults benefit from the lower end. Athletes and lifters shouldn't fixate on BMI at all — body composition matters more than the scale.
The practical framework: use the calculator's range as a target zone, not a bullseye. Aim for the middle if you're average build, upper end if muscular or older, lower end if slim-framed. Combine weight with waist circumference (goal: under half your height), resting heart rate (goal: under 70), and yearly bloodwork. Those four metrics together are a real health picture. Any single one — including weight — lies. Together they don't.