Search for your “ideal weight” and you’ll likely get several different answers depending on which calculator or formula you use – not because any of them are wrong exactly, but because “ideal weight” has been defined multiple different ways over the decades, each with different assumptions built in.
Why Multiple Formulas Exist
Several ideal body weight formulas have been developed since the mid-20th century, originally for varied purposes including medication dosing calculations and insurance industry actuarial tables, rather than as a definitive personal health target. Each uses slightly different assumptions and reference populations, which is why they don’t always agree.
Common Ideal Weight Formulas
| Formula | General Approach |
|---|---|
| Devine formula | Originally developed for medication dosing; widely used baseline in clinical settings |
| Robinson formula | A later refinement of the Devine formula |
| Hamwi formula | An older, simpler formula, still occasionally referenced |
| BMI-based range | Uses the healthy BMI range (18.5-24.9) applied to your height, giving a weight range rather than a single number |
Each of these can produce a somewhat different “ideal” figure for the same height, sometimes by several kilograms, which is exactly why relying on any single formula as a precise personal target is less useful than understanding the broader picture they’re all approximating.
Why a Range Makes More Sense Than a Single Number
Rather than fixating on one specific “ideal” figure, using the healthy BMI range (18.5–24.9) applied to your height – via our BMI Calculator – gives a more sensible weight range to consider, since it acknowledges that a healthy weight isn’t one fixed number but a reasonable span, influenced further by individual factors like muscle mass, frame size and overall body composition.
What These Formulas Don’t Account For
Like BMI itself, none of the common ideal weight formulas distinguish between muscle and fat mass, meaning a muscular individual can appear to be well above their “ideal weight” by these calculations despite being lean and healthy – the same fundamental limitation covered in our guide on BMI accuracy for athletes. They also don’t account for frame size, bone density, or individual variation in healthy body composition.
A More Useful Way to Think About “Ideal” Weight
Rather than searching for a single precise number, it’s generally more useful to think in terms of a healthy range for your height, combined with other indicators of health – energy levels, fitness, blood pressure, and how you feel day to day – rather than treating any calculator’s output as a rigid target to hit exactly. Two people at the same height can have quite different, equally healthy “ideal” weights depending on their individual build and muscle mass.
Using Ideal Weight Figures Sensibly
These calculators can still be genuinely useful as a rough starting reference point, particularly for people with no strong sense of what a reasonable target range might look like. The key is treating the output as a broad guide rather than a precise number to hit exactly – similar to how BMI itself, covered in our guide on what BMI can and can’t tell you, works best as a starting conversation point rather than a definitive verdict.
Frequently Asked Questions
Which ideal weight formula is most accurate?
None is definitively “most accurate,” since they were developed for different original purposes and use different assumptions; a healthy BMI-based range is generally a more practically useful reference than any single ideal weight formula.
Should I aim for the exact number an ideal weight calculator gives me?
Treating it as a rough guide within a healthy range, rather than an exact target, is generally more sensible, particularly given these formulas don’t account for individual muscle mass or frame size.
Why do different ideal weight calculators give me different answers?
Because they’re based on different original formulas and reference populations, developed for different purposes historically, which is precisely why no single number should be treated as definitively correct over the others.
Is a doctor’s assessment more reliable than an online calculator?
A GP can consider your ideal weight range alongside your broader health picture, family history and individual body composition, offering more personalised context than any calculator can provide in isolation.