Which of the four formulas should I use?

For a single figure, Devine is the one clinicians reach for most, and the other three were built from it. But the more honest reading is that no one of them is right: they disagree by a few kilograms, all four were made for drug dosing, and the real answer is the healthy range, not any single formula inside it.

Why is the healthy range so much wider than the formulas?

Because it is measuring a different thing. The formulas estimate one "ideal" weight for dosing; the WHO healthy range (BMI 18.5 to 25) marks the whole span of weights that carry no added risk from body mass at a given height. Most people can be anywhere in that twenty-kilogram window and be perfectly healthy.

Do these formulas account for muscle or frame size?

No. They take only height and sex. A muscular person of a given height will exceed their "ideal weight" while carrying little fat, and a large-framed person will sit above it at any healthy body composition. This is the main reason the number should be read as a rough reference, not a goal.

Does age change the ideal weight?

Not in these formulas — they were fitted for adults and use only height and sex. People do lose height with age, which shifts the formula result, but the equations themselves have no age term. A calculator that asks for your age and then ignores it is showing you a field that changes nothing.

Health

Ideal Weight Calculator

The four formulas agree closely. This page shows how narrow that agreement is next to the healthy range it hides inside.

The link carries your figures, so it reopens on exactly these numbers.

The single number against the honest range, height by height

The "ideal weight" barely moves off the middle of the healthy band at any height, and the band stays several times wider than the four formulas ever disagree by. The point is a slice of the range, not an alternative to it.

Height	Ideal weight (average of the four)	Healthy weight at BMI 18.5	Healthy weight at BMI 25
160 cm	57.8	47.4	64
170 cm	65.9	53.5	72.2
175 cmThe default above. Average 70.0 kg sits mid-band; the band spans 56.7 to 76.6.	70	56.7	76.6
185 cm	78.2	63.3	85.6
195 cm	86.4	70.3	95.1

The formula

each formula = base at 5 ft + increment × (height in inches − 60)

Devine: 50.0 kg male / 45.5 female, + 2.3 kg per inch

Robinson: 52 / 49, + 1.9 / 1.7 kg per inch

Miller: 56.2 / 53.1, + 1.41 / 1.36 kg per inch

Hamwi: 48.0 / 45.5, + 2.7 / 2.2 kg per inch

healthy range = 18.5 and 25 × (height in metres)²

Worked example

A man 175 cm tall (5 ft 9 in)

175 cm ÷ 2.54 = 68.9 in, so 8.9 in over five feet
Devine: 50 + 2.3 × 8.9 = 70.5 kg; the four run 68.7 to 72.0 kg
a spread of 3.3 kg — the four barely disagree
WHO healthy range here: 56.7 to 76.6 kg — a 19.9 kg window

Where this goes wrong

Reading the single number as a target

At 175 cm the four formulas land between 68.7 and 72.0 kg — a spread of 3.3 kg — and that closeness looks like accuracy. It is not. Robinson and Miller were both built by adjusting Devine, and Devine built his to estimate drug doses, not to prescribe a healthy weight — he warned against the second use himself. None of the four considers muscle, frame, age or where fat is carried. Set against the WHO healthy range at the same height, 56.7 to 76.6 kg — a 19.9 kg window — the tight cluster of "ideal" numbers is revealed for what it is: one slice of a broad healthy window, presented with a precision the underlying science never claimed. Use the range, not the point, and take an actual weight question to someone qualified to answer it.

Questions

Which of the four formulas should I use?: For a single figure, Devine is the one clinicians reach for most, and the other three were built from it. But the more honest reading is that no one of them is right: they disagree by a few kilograms, all four were made for drug dosing, and the real answer is the healthy range, not any single formula inside it.
Why is the healthy range so much wider than the formulas?: Because it is measuring a different thing. The formulas estimate one "ideal" weight for dosing; the WHO healthy range (BMI 18.5 to 25) marks the whole span of weights that carry no added risk from body mass at a given height. Most people can be anywhere in that twenty-kilogram window and be perfectly healthy.
Do these formulas account for muscle or frame size?: No. They take only height and sex. A muscular person of a given height will exceed their "ideal weight" while carrying little fat, and a large-framed person will sit above it at any healthy body composition. This is the main reason the number should be read as a rough reference, not a goal.
Does age change the ideal weight?: Not in these formulas — they were fitted for adults and use only height and sex. People do lose height with age, which shifts the formula result, but the equations themselves have no age term. A calculator that asks for your age and then ignores it is showing you a field that changes nothing.