Free Ideal Weight Calculator — Devine · Hamwi · Robinson · Miller + BMI Range

What This Calculator Does

“Ideal body weight” (IBW) is one of the oldest concepts in clinical medicine and one of the most frequently misunderstood. There is no single equation that produces the right number for a human body — there are several, each developed for a slightly different purpose, and each gives a slightly different answer for the same height. This calculator runs four classic formulas in parallel — Devine, Hamwi, Robinson, and Miller — and pairs them with the BMI healthy range for your height. The result is a small constellation of numbers, not a single target.

The reason we show all four is honesty. If a tool returned a single “ideal weight = X kg” figure, it would be hiding 5–10 kg of legitimate disagreement between the formulas. By showing every output and the average, you can see the spread for yourself — and use the BMI band as a sanity check that tells you the actual healthy range your skeleton can support without strain on either side.

The Four Classic Formulas — The Math

All four formulas share the same skeleton: a base weight at 5 ft (60 in), plus a per-inch increment for every inch above that baseline. They differ only in the base and the slope. Here is what the calculator runs:

Notice how Hamwi has the steepest slope (2.7 kg per inch for men) while Miller has the flattest (1.41). This means the formulas divergeas height increases — they all agree closely at 5 ft, but for a 6′4″ man the spread between Hamwi and Miller can exceed 18 kg. That divergence is real; it reflects the fact that nobody has ever found the “true” relationship between height and lean mass.

Devine vs Hamwi vs Robinson vs Miller — How They Differ

Each formula was designed in a different era with different data, and each has a different clinical use case today.

Hamwi (1964) is the oldest of the four. Dr. G. J. Hamwi published it as a rule of thumb for diabetic dietitians who needed to estimate calorie targets quickly at bedside. It was never meant to be precise — it was meant to be memorable and fast. Because of its steep slope, Hamwi tends to over-estimate ideal weight for tall people compared with the others.

Devine (1974)is the most widely cited formula in clinical medicine today — not because it is the most accurate at predicting body composition, but because it is the formula embedded in most drug-dosing protocols. When a pharmacist calculates a dose of gentamicin or vancomycin based on “ideal body weight,” they are almost always using Devine. So if you are looking up your IBW for a medical reason, Devine is the one to watch.

Robinson (1983) and Miller (1983) were both published as attempts to update Devine using newer population data. Both produce lowernumbers than Devine — Miller especially. They reflect a late-20th-century re-analysis suggesting that Devine’s original baseline was a touch too high for the average frame. Robinson sits between Devine and Miller; Miller is the most conservative of the four.

The pattern: at any given height, you will almost always see Hamwi highest, Devine next, Robinson lower, Miller lowest. The average of the four lands in a defensible middle ground — which is why this calculator surfaces it as the headline number.

The BMI Healthy Range Method

The four formulas all return a single number. The BMI healthy range methoddoes something philosophically different — it returns a range. Specifically, the weight range that gives you a Body Mass Index between 18.5 and 24.9, which the World Health Organization defines as the “healthy weight” band.

The math is direct: BMI = weight (kg) / height (m)². Rearrange to solve for weight:

For someone 1.78 m (5′10″) tall, that is roughly 58.6 kg at the low end and 78.9 kg at the top — a 20 kg window. That window is wider than people expect, and it is deliberately wide. Inside it, the body has enough lean tissue to support normal organ function and enough fat reserve to handle illness and stress, without crossing into the cardiovascular risk zone above BMI 25.

Pair this with the four-formula average and you get a useful sanity check. If the formulas say your “ideal” is 72 kg and the BMI range is 58.6–78.9, then 72 sits comfortably inside the healthy band — there is nothing alarming about being a few kilos in either direction.

How to Use This Calculator

1. Pick your unit system. The toggle switches every output between kg/cm and lbs/inches; the underlying math runs in kilograms either way.
2. Choose your gender. All four formulas have separate male/female coefficients because lean-mass distribution at the same height differs on average. The calculator does not infer anything about gender identity — it picks the coefficient set that best fits the body type the formulas were validated against.
3. Enter your age. None of the four formulas actually use age in the calculation, but we collect it as a sanity check (the calculator rejects ages outside 13–100) and because it helps frame the result. A 17-year-old still finishing puberty and a 75-year-old in sarcopenia territory should both be reading the IBW number with extra context.
4. Enter your height. This is the only real input — every formula derives its answer from height alone. Enter centimetres (metric) or total inches (imperial). For imperial users: 5′10″ = 70 in, 6′0″ = 72 in.
5. Read the result. The headline is the average of the four formulas. The detail rows show each formula individually, plus the BMI healthy range. Treat the average as a reasonable midpoint and the BMI range as the boundary on either side.

Three Worked Examples

Three real-world heights, with every formula computed by hand so you can see the math behind each row in the result panel.

Example 1 — 5′10″ (178 cm) male, age 30

Height in inches = 70. Inches over 5 ft = 70 − 60 = 10. Plugging into each formula:

• Devine (M): 50.0 + 2.3 × 10 = 73.0 kg
• Hamwi (M): 48.0 + 2.7 × 10 = 75.0 kg
• Robinson (M): 52.0 + 1.9 × 10 = 71.0 kg
• Miller (M): 56.2 + 1.41 × 10 = 70.3 kg
• Average of four = ~72.3 kg (~159 lbs)

BMI healthy range for 1.78 m: 18.5 × 1.78² = 58.6 kg to 24.9 × 1.78² = 78.9 kg. The average IBW (72 kg) sits comfortably in the upper-middle of the healthy band — exactly where most adult men of this height carry weight without metabolic concern. Note the spread: Miller (70.3) and Hamwi (75.0) disagree by nearly 5 kg for the same body.

Example 2 — 5′4″ (163 cm) female, age 28

Height in inches = 64. Inches over 5 ft = 64 − 60 = 4. Female coefficients:

• Devine (F): 45.5 + 2.3 × 4 = 54.7 kg
• Hamwi (F): 45.5 + 2.2 × 4 = 54.3 kg
• Robinson (F): 49.0 + 1.7 × 4 = 55.8 kg
• Miller (F): 53.1 + 1.36 × 4 = 58.5 kg
• Average of four = ~55.8 kg (~123 lbs)

BMI healthy range for 1.63 m: 18.5 × 1.63² = 49.2 kg to 24.9 × 1.63² = 66.2 kg. The average lands near the lower-middle of the band. At this height, the four-formula spread tightens to about 4 kg — formulas tend to agree more closely at shorter statures because the slope differences haven’t had many inches to compound.

Example 3 — 6′2″ (188 cm) male, age 25, athletic build

Height in inches = 74. Inches over 5 ft = 14. Running the male formulas: Devine 82.2, Hamwi 85.8, Robinson 78.6, Miller 75.9 — average about ~81 kg (~178 lbs). BMI healthy range for 1.88 m: roughly 65.4 to 88.0 kg. By those numbers, any weight up to ~88 kg is “healthy.”

Now here’s the catch: a 25-year-old natural bodybuilder of this height could legitimately weigh 95–100 kg with under 12% body fat— well above the BMI ceiling and well above every formula’s “ideal.” The IBW math has no way to know that the extra mass is muscle. By the calculator’s reckoning the athlete is “overweight.” By any meaningful measure of metabolic health he is the opposite. This is the single most important caveat about every single-number weight target — they are blind to body composition.

The Limits of “Ideal Weight” as a Concept

The term “ideal body weight” sounds like it describes the weight a body shouldbe at. It does not. The four formulas were never validated against long-term health outcomes — they were derived from insurance-actuarial tables (Hamwi), clinical-dosing convenience (Devine), or population averages (Robinson, Miller). None of them asked the question “what weight produces the longest life or the best metabolic markers?”

Modern epidemiological data tells a more interesting story. Studies that follow large cohorts for decades repeatedly find that the lowest all-cause mortality often sits at a BMI around 22–25, with the J-shaped curve rising again on both the underweight and obese sides. People in the upper-healthy and lower-overweight bands sometimes outlive those at “ideal” — particularly past age 60, where a small weight reserve protects against acute illness.

So what is IBW good for? Three legitimate uses:

1. Drug dosing.Many lipophilic drugs are dosed by lean body mass, and Devine’s IBW is the standard proxy clinical pharmacists use.
2. Ventilator settings. Tidal-volume settings in ICUs are calculated per kg of IBW, not actual weight, because lung size scales with height not adiposity.
3. A loose orientation point. If your weight is 30 kg above the average IBW for your height, that is a meaningful signal. If it is 3 kg above, it is statistical noise.

Common Mistakes

• Treating the average as a target. The average of four formulas is a midpoint, not a goal. Anything inside the BMI 18.5–24.9 band for your height is defensibly healthy — sometimes a kilo or two outside it is too. Aiming for an exact IBW number is over-fitting to a coincidence.
• Ignoring body composition. A 90 kg lifter at 12% body fat and a 90 kg desk-worker at 32% body fat have the same number on the scale and radically different health profiles. IBW cannot tell them apart. Pair this with a body fat calculator for the full picture.
• Forgetting the formulas were built on small populations. Hamwi, Devine, Robinson and Miller were all developed from cohorts that under-represented certain ethnicities and body types. The numbers are most accurate for adults of European ancestry, simply because that is who was in the validation studies.
• Using IBW for children or adolescents.All four formulas assume an adult skeleton. Pediatric weight assessment uses growth-curve percentiles, not formulas. The calculator’s minimum age of 13 is a guard, not an endorsement of teen use.
• Rounding to a single digit and calling it precise.The formulas themselves carry several kilos of inherent uncertainty — they were fitted to noisy population data. Reporting your IBW as “72.3 kg” implies a precision that isn’t there. “Around 72” is the most you can honestly say.
• Comparing your IBW to last year’s.The formulas only depend on height. If you grew taller, your IBW changes; otherwise it does not. People occasionally re-run the calculator hoping for a different number after a year of training. The number won’t move — but their BMI and body fat will.

When This Calculator Decides For You

The IBW result usually clarifies a real question. The four most common ones:

1. Am I in a healthy weight zone? If your current weight is inside the BMI 18.5–24.9 range for your height, the answer is yes. The exact distance from the formula average is mostly noise.
2. How much should I lose / gain?The honest answer is “to the nearest edge of the healthy range.” If you are above 24.9 BMI, target the upper edge — not the formula average. The healthy range is the goal; the formula average is one possible point inside it.
3. Is my goal weight realistic? Compare your goal to the BMI 18.5 floor. Goals at or below 18.5 are clinically underweight and rarely sustainable; goals around 22–24 are biologically cheap to maintain for most adults.
4. Should I trust an online program’s “target weight”? If a program assigns you a number outside the BMI healthy range you computed here, treat that as a red flag. Your skeleton dictates the band; no marketing copy gets to override it.

Body Composition Matters More Than Single-Number Targets

The deepest limitation of every IBW formula is that they treat the body as if mass were homogeneous. It is not. Two kilograms of muscle and two kilograms of visceral fat have radically different metabolic consequences, and the scale cannot tell them apart. A well-designed health-tracking habit looks at three numbers together, not one:

• Weight band (this calculator) — establishes the rough envelope your height supports.
• Body fat percentage — distinguishes lean mass from fat mass. Use the body fat calculator for an at-home tape-measure estimate that is good to within ±3–4% of a DEXA scan.
• Energy balance — calories in vs out. Use the calorie / TDEE calculator to find your maintenance number, then adjust by 250–500 kcal for slow change in either direction.

Add hydration on top of that and you have most of the inputs to a working metabolic dashboard — the water intake calculator sets the baseline fluid target by weight, activity, and climate. And if you want to put your current weight in context, the BMI calculator renders the same WHO healthy band used here, with your actual weight plotted on it.

The point is not to chase a single perfect number. It is to keep all the relevant numbers roughly inside their healthy ranges, and to recognise that the body is a system. IBW is one useful piece of that system — not the verdict on it.