Why Ideal Weight Formulas Break Down for Athletes and Anyone With Above-Average Muscle Mass

A 95kg bodybuilder with 8% body fat and a 95kg sedentary man with 30% body fat get the same "ideal weight" from every classic formula — which tells you these formulas were never measuring what people think they're measuring

Ideal body weight formulas (Devine, Robinson, Miller, Hamwi) take height as the primary input and produce a single weight figure, with minor adjustments for sex. They were developed — as covered in a previous article — primarily for drug dosing calculations, where a population-average relationship between height and a "typical" body weight was clinically useful for estimating distribution volumes. None of these formulas have any mechanism to account for muscle mass.

Why athletes break every classic ideal weight formula

The Devine formula, one of the most widely used:

• Men: 50.0 kg + 2.3 kg × (height in inches − 60)
• Women: 45.5 kg + 2.3 kg × (height in inches − 60)

For a man at 180cm (~70.9 inches): 50.0 + 2.3 × (70.9 − 60) = 50.0 + 25.1 = 75.1 kg

This formula has no input for body composition. A 180cm bodybuilder at 95kg with very low body fat and a 180cm sedentary man at 95kg with high body fat both have identical "deviation from ideal weight" by this formula — despite representing completely different health profiles.

Real-world examples where the formulas clearly fail:

• Competitive rugby players, American football linemen, and powerlifters routinely have body weights 30-50%+ above "ideal weight" formula outputs, while having body fat percentages in healthy or athletic ranges
• Bodybuilders in competition condition (very low body fat, 5-12% for men) are often well above "ideal weight" by these formulas due to high muscle mass
• Endurance athletes (marathon runners, cyclists) sometimes fall below "ideal weight" formula outputs while being in excellent health — lower body fat and sometimes lower muscle mass than population averages, but appropriate for their sport and not indicating any deficiency

BMI has the same blind spot — and it's well documented in sports medicine

This is the same fundamental issue covered in the BMI flaws article, but it's worth examining specifically in athletic populations because the magnitude of the discrepancy is so large:

NFL linemen BMI study: research examining American football offensive linemen has found a very high proportion classified as "obese" by standard BMI categories (BMI ≥30), despite body fat percentages often in the 15-25% range — well within ranges not typically considered unhealthy, and reflecting substantial muscle mass rather than primarily fat mass.

Olympic weightlifters and rugby forwards: similar patterns — BMI values well into "overweight" or "obese" categories, with body composition assessment (when performed) showing body fat percentages consistent with athletic populations.

What's actually useful for athletes: body composition, not weight targets

For athletic populations, the relevant questions aren't "what should my weight be" but:

Body composition relative to sport-specific norms: different sports have different optimal body composition profiles — a marathon runner, a sprinter, a rugby prop, and a gymnast have very different optimal body fat percentages and muscle mass profiles, each appropriate to the demands of their sport.

Performance markers: strength-to-weight ratio (relevant for climbing, gymnastics), power output relative to body mass (relevant for sprinting, jumping), aerobic capacity relative to body mass (VO2max per kg, relevant for endurance sports) — these performance-relevant metrics are far more useful than a generic "ideal weight" for an athlete.

Weight class sports: for sports with weight categories (covered in a previous article on combat sports), the relevant target is the weight class limit — not a population-derived "ideal weight" formula, which has no relationship to competitive weight categories.

Lean body mass as an alternative framework

Some approaches calculate a target weight based on desired lean body mass and a target body fat percentage, rather than height alone:

Target weight = Current lean body mass / (1 − target body fat percentage)

For example, someone with 60kg of lean body mass aiming for 15% body fat: Target weight = 60 / (1 − 0.15) = 60 / 0.85 = 70.6kg

This approach at least incorporates body composition — but it requires an accurate measurement of current lean body mass (from one of the body composition methods covered previously) and a realistic target body fat percentage appropriate to the individual and their goals, neither of which a height-only formula can provide.

When height-based formulas remain genuinely useful

It's worth re-emphasising: these formulas weren't designed to be wrong for their original purpose — they remain used in specific clinical calculations (certain drug dosing protocols, some clinical nutrition calculations for populations where body composition data isn't readily available) where a population-average relationship between height and weight serves as a practical estimation tool within a clinical algorithm, often alongside other clinical judgment and adjustments.

The issue is specifically when these formulas are presented to individuals — particularly athletes or anyone with above-average muscle mass — as a personal target to aim for, which is a misapplication of tools designed for a different purpose.

How to use the Ideal Body Weight Calculator on sadiqbd.com

1. Understand the context: these formulas provide a population-average reference point based on height, originally developed for clinical dosing calculations
2. For athletes or anyone with above-average muscle mass: the output number is likely to be substantially below what represents a healthy weight for that individual — body composition assessment is far more relevant
3. Use alongside body composition tools: the Body Fat Calculator and BMR Calculator on this site, combined with sport-specific or goal-specific body composition targets (discussed with a coach or healthcare provider), provide more individually relevant information than a height-only formula

Frequently Asked Questions

Is there a formula that accounts for muscle mass when calculating ideal weight? Not in the classic "ideal body weight" formula tradition — these remain height-only. The lean-body-mass-based approach described above is one alternative framework, but it requires body composition measurement as an input (rather than producing a target from height alone) and is better understood as a goal-setting calculation than a population-reference "ideal weight."

Why do some online calculators show wildly different "ideal weight" results for the same height? Because different formulas (Devine, Robinson, Miller, Hamwi, and various BMI-based approaches) use different coefficients, derived from different reference populations and decades. None is more "correct" than another in an absolute sense — they're all population averages from different historical datasets, which is part of why none should be treated as a precise individual target.

Is the Ideal Body Weight Calculator free? Yes — completely free, no sign-up required.

Try the Ideal Body Weight Calculator free at sadiqbd.com — see reference estimates from multiple classic formulas based on your height.