Step on a scale and the number tells you what you weigh right now. It doesn’t tell you what you should weigh. For that, doctors, dietitians and fitness trainers have leaned on a handful of “ideal body weight” (IBW) formulas for decades — Hamwi, Devine, Robinson and Miller — each one a simple equation that turns your height into a target weight in kilograms.
These formulas predate BMI as a clinical shortcut, and they’re still used today for things BMI was never built for, like calculating medication doses. But the four formulas don’t agree with each other, sometimes by 4-5 kg for the same height. This guide walks through the exact math behind each one, works through real height-based examples in kilograms, and explains which formula is actually worth paying attention to.
What “Ideal Body Weight” Actually Means (and How It Differs From BMI)
Ideal body weight and BMI answer two different questions. BMI takes your actual height and actual weight and tells you which category (underweight, normal, overweight, obese) you currently fall into. An IBW formula skips your current weight entirely — it takes only your height and sex and calculates a single target number, the weight a population-average person of that height was historically found to live longest or respond best to treatment at.
That’s a meaningfully different tool. BMI is diagnostic — it describes where you are. IBW formulas are prescriptive — they estimate where research says you’d ideally be, based on old actuarial and clinical datasets. We’ve covered how the two concepts relate and where each one falls short in Ideal Weight vs BMI: What’s the Difference and Which Should You Trust? — this guide focuses specifically on the four IBW formulas themselves and the math behind each.
The Four Ideal Body Weight Formulas, Explained
All four formulas share the same structure: start with a base weight assigned to a height of 5 feet (60 inches), then add a fixed amount of weight for every inch above that. The base weight and the per-inch increment are what differ between them, and each formula uses a different value for men and women.
1. Hamwi Formula (1964)
The oldest of the four, created by Dr. G.J. Hamwi as a quick bedside estimate for dietitians. It’s known for running slightly heavier for taller men than the other formulas.
- Men: IBW = 48 kg + 2.7 kg for every inch over 5 feet
- Women: IBW = 45.5 kg + 2.2 kg for every inch over 5 feet
2. Devine Formula (1974)
Devised by Dr. B.J. Devine specifically to calculate drug dosages, not general health targets. It went on to become the most widely used IBW formula in clinical medicine — it’s the formula still cited in pharmacology references for dosing drugs like aminoglycoside antibiotics, where dosing by actual body weight can be inaccurate.
- Men: IBW = 50 kg + 2.3 kg for every inch over 5 feet
- Women: IBW = 45.5 kg + 2.3 kg for every inch over 5 feet
3. Robinson Formula (1983)
Published as a refinement of Devine, built from a larger and more recent dataset of actuarial life-insurance weights. Many clinicians consider it a slightly more statistically grounded update to Devine, with a higher base weight but a smaller per-inch increment.
- Men: IBW = 52 kg + 1.9 kg for every inch over 5 feet
- Women: IBW = 49 kg + 1.7 kg for every inch over 5 feet
4. Miller Formula (1983)
Published the same year as Robinson, using yet another dataset. Miller has the highest base weight of the four but the smallest per-inch increment, which is why it tends to give the highest estimate for shorter people and the lowest for taller people.
- Men: IBW = 56.2 kg + 1.41 kg for every inch over 5 feet
- Women: IBW = 53.1 kg + 1.36 kg for every inch over 5 feet
Worked Example 1: A 170 cm (5’7″) Man
170 cm converts to 66.93 inches — 6.93 inches above the 60-inch (5-foot) baseline every formula starts from. Plugging that into each formula:
| Formula | Calculation | Ideal Body Weight |
|---|---|---|
| Hamwi | 48 + (2.7 × 6.93) | 66.7 kg |
| Devine | 50 + (2.3 × 6.93) | 65.9 kg |
| Robinson | 52 + (1.9 × 6.93) | 65.2 kg |
| Miller | 56.2 + (1.41 × 6.93) | 66.0 kg |
For this height, the four formulas land within a tight 1.5 kg band — 65.2 kg to 66.7 kg. That’s typical for average adult heights: the formulas were all calibrated against similar population data, so they tend to agree closely near the middle of the height range and diverge more at the extremes.
Worked Example 2: A 160 cm (5’3″) Woman
160 cm converts to 62.99 inches — 2.99 inches above the 60-inch baseline. Using the women’s coefficients:
| Formula | Calculation | Ideal Body Weight |
|---|---|---|
| Hamwi | 45.5 + (2.2 × 2.99) | 52.1 kg |
| Devine | 45.5 + (2.3 × 2.99) | 52.4 kg |
| Robinson | 49 + (1.7 × 2.99) | 54.1 kg |
| Miller | 53.1 + (1.36 × 2.99) | 57.2 kg |
Here the spread is much wider — 52.1 kg to 57.2 kg, a gap of 5.1 kg. Miller’s high base weight (53.1 kg) pulls its estimate well above the other three at a shorter height, which is exactly the pattern the formula is known for: Miller tends to run highest for shorter people and lowest for taller people, because its base weight is high but its per-inch increment is the smallest of the four.
This is the practical takeaway: the shorter you are, the more the four formulas disagree with each other, and the choice of formula can shift your “ideal” number by several kilograms.
Which Formula Should You Actually Use?
For most everyday purposes, it doesn’t matter much which one you pick — averaging two or three of them gives a reasonable midpoint. But if you want a single default:
- Devine is the closest thing to an industry standard. It’s the formula referenced in pharmacology and clinical dosing guidelines, and it’s the one most doctors mean when they say “ideal body weight” without specifying further.
- Robinson is generally regarded as a modest statistical improvement on Devine, built from a larger dataset, though it hasn’t displaced Devine’s use in clinical dosing formulas.
- Hamwi and Miller are used less often today but are still cited in nutrition and dietetics references, and some online calculators default to Hamwi simply because it was the earliest and easiest to memorize.
None of the four accounts for muscle mass, bone density, or body frame size — a muscular or large-framed person will legitimately weigh more than any of these formulas suggest without being “overweight” in any meaningful sense. That’s the same blind spot BMI has, which is why pairing an IBW estimate with a lean body mass calculation gives a fuller picture than either number alone. If your actual weight sits a few kilograms outside every formula’s range, that alone isn’t a health verdict — it’s a cue to look at body composition, not just the scale.
Limitations Worth Knowing
A few things these formulas were never designed to handle:
- Children and teenagers: all four formulas were built from adult data and don’t apply to anyone still growing.
- Very short or very tall heights: below 5 feet, the “inches over 60” term goes negative, and the formulas can understate weight needs; well above 6 feet, they can overstate them, since the underlying data thinned out at the extremes.
- Athletes and heavily muscled individuals: none of the formulas distinguish muscle from fat, so a fit, muscular person can measure well above their IBW while being at a healthy body-fat percentage.
- Ethnic and regional differences: the original datasets were drawn from Western clinical and insurance populations, not calibrated for Indian body-composition norms — the same caveat that applies to using an unmodified BMI cutoff instead of the lower Asian-specific thresholds.
Frequently Asked Questions
Which ideal body weight formula is the most accurate?
None of the four is objectively “correct” — they’re all population averages from different decades and datasets. Devine is the most widely used in clinical settings today, mainly because it’s the formula built into standard drug-dosing guidelines, not because it has been proven more accurate than the others for general health purposes.
Can I use centimeters instead of feet and inches?
Yes. All four formulas are built around inches above 5 feet (60 inches), so convert your height to inches first (divide centimeters by 2.54), subtract 60, and multiply by the formula’s per-inch coefficient before adding the base weight — exactly as shown in the worked examples above.
What if my actual weight is nowhere near any of these numbers?
A gap of a few kilograms is normal and not a cause for concern on its own — body frame, muscle mass and fitness level all shift where you’d reasonably sit. It’s more useful as a rough anchor than a precise target. Pairing it with your BMI and, if you want a fuller picture, a body-fat or lean-body-mass estimate, tells you far more than any single IBW number can.
Do doctors still use these formulas?
Yes, primarily Devine — it remains the standard for calculating certain medication doses (particularly for drugs where dosing by actual body weight can lead to overdosing in larger patients) and for some nutrition and critical-care calculations, rather than as a general “healthy weight” target for patients.
Why do the formulas disagree more for shorter people?
Because each formula’s base weight (set at exactly 5 feet) was calculated independently from a different dataset, and those base weights vary more than the per-inch increments do. The closer your height is to 5 feet, the more that base-weight difference dominates the result; the taller you are, the more the per-inch increments — which also differ — take over.
Is a higher or lower ideal body weight “better”?
Neither — the formulas aren’t ranked by which gives a healthier answer. They’re simply four independent estimates from different research teams. The right way to use them is as a range (as shown in the worked examples above), not by picking whichever number you’d prefer to see.