Ideal Weight Calculator

Four formulas, four answers, one height

There is no single ideal body weight. There are four widely used formulas, all built between 1964 and 1983, and they disagree by a few pounds at every height. Run all four on a 5 foot 9 man and you get 151.9, 152.3, 155.9 and 159.4 pounds. Same height, same sex, a spread of 7.5 pounds.

The reason is that none of them was built to answer the question people ask today. The Hamwi formula came out of a 1964 clinical shortcut for estimating a starting dose. The Devine formula was published in 1974 to work out drug doses for gentamicin, an antibiotic where the dose depends on lean body mass. Robinson and Miller followed in 1983, each fitted to a different dataset.

That history matters because it tells you what the number is for. It is a reference figure for dosing and for setting a target in a clinical setting, not a verdict on your health. A doctor uses it to pick a starting dose. It was never meant to tell a 45 year old whether they should lose 10 pounds.

The four formulas share one structure: a base weight for 5 feet, plus a fixed number of kilos for every inch above 5 feet. What changes is the base and the per inch step, and the per inch step is where the disagreement grows with height.

The arithmetic behind each formula

Every formula takes the same shape. Start with the weight for 5 feet, add the per inch amount multiplied by inches over 5 feet, and you have kilos. Multiply by 2.2046 for pounds.

Read the table by column and the behavior becomes obvious. Hamwi starts low but climbs fastest, so it gives the lowest answer for short people and the highest for tall ones. Miller starts high and climbs slowest, so it does the reverse. Devine sits in the middle, which is part of why it became the default.

The divergence is easy to see at the extremes. At 5 foot 2, a woman gets 110.0 pounds from Hamwi and 123.1 from Miller, a 13 pound gap. At 6 foot, a man gets 161.2 from Miller and 177.3 from Hamwi, a 16 pound gap. In the middle, around 5 foot 8, the four answers sit within 8 pounds of each other.

A 5 foot 9 man, worked through all four

Take 5 foot 9, which is 69 inches, so 9 inches over the 5 foot mark. Every formula below uses that same 9 inch multiplier, and the only differences are the base and the step.

• Devine: 50 + (2.3 x 9) = 70.7 kg = 155.9 lb, BMI 23.0
• Robinson: 52 + (1.9 x 9) = 69.1 kg = 152.3 lb, BMI 22.5
• Miller: 56.2 + (1.41 x 9) = 68.9 kg = 151.9 lb, BMI 22.4
• Hamwi: 48 + (2.7 x 9) = 72.3 kg = 159.4 lb, BMI 23.5

All four land inside the healthy BMI band for that height, which runs from 125.3 to 168.6 pounds. That is the useful takeaway: the four formulas are not competing answers, they are four points inside the same 43 pound window. The window is far wider than the disagreement between the formulas.

Notice how Miller treats short women and tall men differently from Hamwi. A 5 foot 2 woman reading Miller would think she is 13 pounds heavier than the same woman reading Hamwi. That is not a rounding difference, it is two different models of the human body, and neither is validated on a modern population.

Why BMI gives a range and a formula gives one number

BMI is weight in pounds times 703, divided by height in inches squared. Flip it around and you get the weight that corresponds to any BMI at any height, which is how the healthy range is built: 18.5 and 24.9 plugged into the same equation.

The range exists because the relationship between weight and health is not a point. Mortality data curves, it does not spike, and the bottom of the curve is broad. Two people at 5 foot 9, one at 130 pounds and one at 165, are both inside the healthy band and both have normal risk markers on a standard blood panel.

BMI also ignores what the weight is made of. It cannot tell 180 pounds of muscle from 180 pounds of fat, and at the same BMI two people can differ by 10 percentage points of body fat. The NIH's weight and health page says this directly and pairs BMI with waist measurement for that reason.

Where the formulas fail: muscle, bone and age

The formulas assume an average body composition at a given height, and the assumption breaks first for people who carry unusual amounts of muscle. A 5 foot 9 lifter at 200 pounds and 8% body fat holds 184 pounds of lean mass. Devine says his ideal weight is 155.9 pounds, which would mean losing 44 pounds, and at 8% body fat most of that loss would be muscle.

Turn it around. Keep his 184 pounds of lean mass and put him at a typical 15% body fat, and he would weigh 216 pounds. That is 60 pounds above the Devine figure while being leaner than average. The formula is not measuring his health, it is measuring a population average he does not belong to.

Bone structure produces a smaller version of the same error. A man with a broad frame and heavy bones can sit 10 to 15 pounds above the formula at the same body fat. The classic correction is plus or minus 10% for frame size, which turns 155.9 pounds into a band of 140.3 to 171.5.

Age is the third blind spot. The formulas have no age term, but lean mass falls roughly 3% to 8% per decade after 30. A 70 year old man at 160 pounds typically carries more fat and less muscle than a 30 year old at 160 pounds, so hitting the formula weight at 70 can mean carrying more fat than the number suggests. For older adults, strength training and protein intake move health markers more than the scale does.

Frame size and the 10% adjustment

Frame size is the one adjustment the formulas leave out that most people can measure at home. The usual method is to wrap your thumb and index finger around the opposite wrist, just above the wrist bone. If the fingers overlap, the frame is small. If they touch, it is medium. If they do not meet, it is large.

A more repeatable method is elbow breadth, measured with a caliper across the bones on either side of the elbow, but the wrist test is close enough for a 10% adjustment. Applied to a 5 foot 9 man at 155.9 pounds, the band runs from 140.3 pounds for a small frame to 171.5 for a large one.

Treat that band as the real answer. It is 31 pounds wide, which is wider than the gap between any two formulas, and it is the honest expression of how much normal variation exists at a single height. Anyone quoting one number to the pound is quoting a dosing figure, not a health target.

Two cautions on the frame adjustment. It is a rule of thumb with no strong evidence behind it, and it can be used to justify a weight that is too high. If a 10% frame adjustment and a 5 point BMI difference disagree, trust the BMI range and the waist measurement over the wrist test.

What to aim for if you are losing weight

Pick a target inside the BMI range, not the formula number. For a 5 foot 9 man that is anywhere from 125.3 to 168.6 pounds, and the middle of the range, around 147 pounds, is a reasonable place to land for most builds. Aiming for the formula figure of 155.9 is also fine and is easier to justify to a clinician.

The first 5% to 10% of body weight is where most of the measurable health gain sits. For a 200 pound person that is 10 to 20 pounds, which improves blood pressure, fasting glucose and triglycerides before anyone reaches a formula weight. Losing 20 pounds at 1.5 pounds a week takes about 13 weeks.

• Rate: 1 to 2 pounds a week, or 0.5% to 1% of body weight
• Protein: about 0.7 g per pound of target body weight to protect muscle
• Strength work: two sessions a week preserves lean mass while the scale drops
• Waist: track it monthly, because it falls even in weeks the scale does not move

Protein matters more than most people expect during a deficit. At 0.7 g per pound of a 155 pound target that is 109 g a day, which is roughly a palm sized portion at three meals plus a protein snack. Without it, a third of the weight lost can come from muscle.

The NHLBI's overweight and obesity pages cover the clinical thresholds, and its risk assessment page explains how BMI and waist measurement are combined. Both are worth reading before setting a target.

What the number cannot tell you

A weight target says nothing about blood pressure, blood sugar, cholesterol, sleep, fitness or how you feel. Two people at the same weight and height can have completely different metabolic profiles, and the scale cannot distinguish them.

Waist circumference fills part of that gap. Fat stored around the organs carries more risk than fat under the skin, and waist is a decent proxy. Over 40 inches for men and over 35 inches for women is the threshold most guidelines use, independent of BMI. A 5 foot 9 man at 170 pounds with a 42 inch waist carries more risk than the same man at 170 pounds with a 34 inch waist.

Body fat percentage fills another part. A rough healthy band is 10% to 20% for men and 18% to 28% for women, and it can be estimated from a tape measure or a scale with bioelectrical impedance. Neither method is precise, but both beat guessing.

Finally, the formulas were built on mid century military and insurance populations that were less diverse than the country is now. Their accuracy varies by ancestry, and for people of South Asian or East Asian descent the health risk rises at a lower BMI, which is why some guidelines use a 23 cutoff instead of 25 for those groups.

This is an estimate for reference, not medical advice. Ideal body weight formulas are dosing tools from the 1970s and 1980s. Talk to a clinician before setting a weight target, especially with a heart, kidney, thyroid or eating disorder history.