The most popular ideal weight formulas were built for drug dosing, not fitness goals. Here is what your results actually mean, why three formulas give three different answers, and how to use them wisely.
If you search “what should I weigh,” you will get a number. It will feel precise. It will feel like a goal. And it will almost certainly come from a formula that was never designed to answer that question.
The most widely cited ideal body weight formula — the Devine formula — was published in 1974 in a case study about gentamicin dosing. It was a pharmacokinetics tool, created to help clinicians calculate safe drug doses for patients based on height. It was not based on a study of what healthy people actually weigh. It was not validated against health outcomes. And it was never intended to define what any individual “should” weigh.
Yet today, the Devine formula appears on thousands of fitness and health websites as if it were settled science. When we built the Ideal Weight Calculator, we decided to show this context directly — including results from all three major formulas side by side, so you can see how much they disagree.
Three formulas, three different answers
Our calculator shows results from three established ideal body weight equations. All three use the same approach: start with a base weight at 5 feet tall, then add a fixed amount per inch above that. The difference is in the constants:
Devine (1974): Men = 50 kg + 2.3 kg per inch over 5 ft. Women = 45.5 kg + 2.3 kg per inch over 5 ft. Originally published in a case study on gentamicin therapy. Not referenced to any population data set.
Robinson (1983): Men = 52 kg + 1.9 kg per inch over 5 ft. Women = 49 kg + 1.7 kg per inch over 5 ft. Developed as a refinement of Devine for drug dosage calculations. Derived from Metropolitan Life Insurance height-weight tables.
Miller (1983): Men = 56.2 kg + 1.41 kg per inch over 5 ft. Women = 53.1 kg + 1.36 kg per inch over 5 ft. Also designed for clinical use. Produces higher estimates for shorter individuals and lower estimates for taller ones compared to Devine.
For a 5’10” male, these formulas give you 166, 160, and 170 lbs respectively — a 10-pound spread. For a 5’5” female, you get 126, 130, and 138 lbs — a 12-pound spread. The formulas cannot all be right, because they were never measuring the same thing. They were all approximations for clinical convenience.
This is exactly why our calculator displays all three results in a comparison table rather than presenting one number as “the answer.” The spread itself is the most honest thing the calculator can show you.
The real problem: what these formulas ignore
All three formulas take two inputs — height and sex. That is it. They do not account for:
Body composition. A 5’9” male at 175 lbs could be a lean, muscular athlete at 12% body fat or a sedentary individual at 30% body fat. The formulas would give both the same “ideal” number. Muscle weighs more than fat per unit of volume, and carrying more of it is associated with better metabolic health — but the formula penalizes you for it.
Age. Body composition shifts over the lifespan. Older adults naturally carry less muscle mass and more fat mass at the same weight. A single target number does not account for these changes.
Frame size. Someone with a naturally broader skeletal frame will carry more bone and connective tissue mass than someone with a narrow frame at the same height. The formulas treat them identically.
Ethnicity. Body composition and the relationship between weight and health outcomes vary across populations, just as with BMI. A formula derived from mid-20th-century American insurance data does not generalize cleanly to all populations.
Our calculator addresses this directly. Below the formula results, it shows a BMI-based healthy weight range (18.5–24.9) for your height. This range is wider, accounts for natural variation, and is backed by population health data rather than a pharmacokinetics shortcut.
What to actually do with your results
If the formula number feels impossibly low or discouraging, that is a feature of the formula’s limitations — not a reflection of your health. Here is how to use these numbers constructively:
Treat the BMI range as the practical guide. For most adults, a weight within the 18.5–24.9 BMI range is associated with lower health risks. That gives you a window of 30+ pounds rather than a single number, which is more realistic and more forgiving.
Use the formula results as a rough reference point. If all three formulas cluster around 155 lbs and your BMI range is 130–175 lbs, that midpoint gives you a general sense of where “average” sits for your height. But it is a center of a range, not a target.
Pair it with body composition data. Your weight matters less than what it is made of. Use our Body Fat Calculator to estimate your body fat percentage, and our BMI Calculator to see where you fall on the WHO classification scale. Together, these give you a much more useful picture than any single “ideal” number.
Note for shorter individuals. If you are under 5 feet (152 cm), these formulas become unreliable. They were designed for adults at or above that height, and the math produces increasingly implausible results below it. Our calculator flags this with a warning and recommends using the BMI healthy range instead.
FAQ
Which ideal weight formula is most accurate? None of them are “accurate” in the way most people expect. Research comparing all three found that Robinson’s formula tends to produce results closest to a healthy BMI for men, but no formula consistently matched a healthy BMI for women. The BMI healthy range is a more evidence-based reference.
Why is my ideal weight so much lower than what I actually weigh? The formula results often feel unrealistically low, particularly the Devine formula for women. This is partly because the formulas were not designed as fitness targets and partly because they predate modern understanding of healthy body composition. A weight within your BMI healthy range, combined with a body fat percentage in a healthy zone, is a more realistic and clinically meaningful goal.
Does my ideal weight change with age? The formula results do not change with age since they only use height and sex. However, what constitutes a healthy weight does shift — research suggests that slightly higher BMI ranges (22–27 rather than 18.5–24.9) may be associated with lower mortality risk in older adults. As you age, maintaining muscle mass becomes more important than hitting a specific number on the scale.
Should I use my ideal weight for calorie calculations? Some people use ideal body weight as the input for calorie or protein calculators, especially when significantly overweight. This can work as a practical starting point. Our Protein Calculator offers an optional body fat percentage field that shifts the calculation to lean mass — a more precise approach than estimating from ideal weight.