A person's ideal body weight is calculated from a single line of arithmetic: a base weight set for someone exactly 5 feet (152 cm) tall, plus a fixed number of kilograms for every inch above that, with a separate version of that line for each sex. The four formulas most calculators use — Devine (1974), Robinson (1983), Miller (1983), and Hamwi (1964) — all follow this pattern and take only two inputs: height and sex. Age does not appear in any of them, because each researcher fitted a straight line to adult weight data and found that height, broken down by sex, explained most of the variation. A companion number that better reflects age-related changes in body composition is the healthy BMI range, but even that only takes height and weight, not your birth year. So when you search for an ideal weight based on height and age, the honest answer is that no widely cited formula plugs in your age — you get an estimate built from height and sex, with the BMI range as the most useful reference point alongside it.
Why age doesn't appear in the classic ideal-weight formulas
Every formula you'll meet in a clinical or fitness setting was designed for adults, and the original authors made an early, deliberate choice: keep the inputs simple. Dr. G. J. Hamwi published his formula in 1964 to help estimate nutrition requirements in hospitals, so it had to be fast at the bedside. Dr. B. J. Devine published his in 1974 to set drug doses — gentamicin and theophylline among them — and a calculation that takes only height and sex saves a clinician from re-checking a patient's birth date. Robinson (1983) and Miller (1983) reworked Devine's numbers against fresh data sets; both kept the same two-input structure.
What the original papers also found is that, across adults of the same sex, average weight rises with height in a near-straight line. Age adds noise on top of that line — lean mass tends to fall and fat mass to rise between 30 and 70, which is exactly the trend most readers notice — but the slope of weight against height stays steady enough across adulthood that adding age didn't improve the fit enough to justify the extra step. In short: height gives the target, sex sets the baseline, and age is treated as a second-order effect that the formulas deliberately leave out.
The four formulas side by side
Each formula has the same skeleton — a base weight at 5 feet (152 cm), plus a slope for every inch above — but the two numbers differ by author and year. The values below are the male versions of each formula; the female versions use lower base weights and slightly smaller slopes, so every "ideal" you read for a woman sits below the corresponding male value.
| Formula | Year | Male base (5 ft) | Male slope (per inch) |
|---|---|---|---|
| Hamwi | 1964 | 48.0 kg | 2.7 kg |
| Devine | 1974 | 50.0 kg | 2.3 kg |
| Robinson | 1983 | 52.0 kg | 1.9 kg |
| Miller | 1983 | 56.2 kg | 1.41 kg |
Two patterns stand out. Hamwi sets the lowest base weight for men but the steepest slope, so its result overtakes the others once you go past roughly 5'7" — which is why it ends up giving the highest figure for tall adults. Miller sets the highest base weight but the gentlest slope, so it falls behind for tall adults and ends up at the bottom of the range at the top end. Devine sits in the middle and is the one most often quoted in clinical dosing guides, which is why it has become the default in medicine. Because the four slopes differ, a tall person sees a wider spread between the formulas than a short person does.
How to calculate ideal weight from height
The fastest way is to let the Ideal Weight Calculator apply all four formulas in one pass, but the manual steps are worth knowing so you can sanity-check the result.
- Decide whether to use the male or female version of the formula. All four formulas in the table above have a parallel column for the opposite sex; pick the one that matches you.
- Convert your height to inches above 5 feet. Divide your height in centimetres by 2.54 to get inches, then subtract 60. If you are shorter than 5 feet, set "inches over 5 ft" to 0 — the formula collapses to its base weight and stops being meaningful, which is why the calculator points you at the BMI range instead.
- Multiply the inches above 5 feet by the slope from the table. For Devine on a male that is 2.3 kg per inch; for Hamwi on a male it is 2.7 kg per inch; for Robinson 1.9 kg per inch; for Miller 1.41 kg per inch.
- Add that result to the base weight for your sex. The sum is the formula's ideal weight in kilograms.
- Repeat for the other three formulas if you want the full picture. The average of all four is usually a more honest single number than any one of them, because it dampens the spread between the high and low formulas.
- Read the result alongside the healthy BMI range shown below the four numbers, which the calculator updates instantly in both kilograms and pounds.
Worked example using Devine for a man who is 175 cm tall. Step 1: 175 ÷ 2.54 = 68.9 inches. Step 2: 68.9 − 60 = 8.9 inches over 5 ft. Step 3: 8.9 × 2.3 = 20.47 kg. Step 4: 50 + 20.47 = 70.47 kg, which rounds to about 70.5 kg or 155 lb. Run the same person through the other three formulas and you'll see roughly 72.0 kg (Hamwi), 68.9 kg (Robinson), and 68.7 kg (Miller) — four numbers in a tight band, with Devine in the middle.
The healthy BMI range as a second opinion
The single most useful number next to the formulas is a band, not a point. Body mass index is weight divided by height squared, and the WHO treats 18.5 to 25 as the normal range for adults. Multiplying each end of that band by your height in metres squared turns it into a real weight range. At 175 cm (1.75 m), the lower end is 18.5 × 1.75² ≈ 56.7 kg and the upper end is 25 × 1.75² ≈ 76.6 kg — so a 175 cm man's healthy weight sits roughly between 57 and 77 kg. The four formulas land somewhere inside that band; the BMI range is what tells you the band itself is sensible.
Two reasons to lean on the BMI range. First, it is grounded in population health data rather than a mid-20th-century rule of thumb. Second, it is a range, so it survives the fact that no two adults of the same height weigh exactly the same thing for healthy reasons. If you want a single quick read on where you sit, the BMI Calculator gives you the same band and category in one step.
Where these formulas stop being useful
All four formulas share the same blind spots, and they matter as soon as your body deviates from the average adult they were fitted to. The formulas cannot see muscle, bone density or frame size, so a lean, muscular athlete can sit well above their "ideal" weight and still be perfectly healthy, while a small-framed person with very little muscle can sit below it and also be healthy. The formulas also assume a binary male or female input and were calibrated for adults of 5 feet and taller; below that height they collapse to their baseline and become meaningless, which is why the calculator flags short heights and points you at the BMI range instead. The same limitation applies to children, for whom none of the four formulas apply at all.
Age matters here, even though the formulas don't take it. From your 30s onward, lean mass tends to fall and fat mass tends to rise at the same weight, so a number that was healthy at 25 can sit on the upper edge of the BMI band at 55. None of the formulas adjust for this drift — the BMI band is the only part of the picture that nudges the right direction with age, and even it doesn't take age as an input. Treat the result as a rough, informational estimate, not a personal target or a diagnosis; for guidance on a healthy weight for you, a doctor or a registered dietitian is the right next step.