The Ideal Weight Formulas Were Designed for Drug Dosing — Not Your Health Goals

The ideal weight formulas doctors use weren't designed to tell you what to weigh

Devine, Robinson, Hamwi, Miller — these are the formulas behind most "ideal body weight" calculators. They're widely used in clinical medicine. They're also more than 50 years old, were developed for a narrow purpose that has nothing to do with aesthetics or fitness, and systematically produce different answers from each other.

Understanding where these formulas came from changes how you interpret the number they give you.

The origin: medication dosing, not body ideals

The ideal body weight (IBW) formulas were developed in the 1960s and 1970s for a specific clinical problem: calculating appropriate drug doses for overweight patients. Many medications are dosed by body weight, but in obese patients, using actual body weight can lead to overdosing (since drugs often don't distribute into fat tissue). Clinicians needed an estimate of what a patient would weigh without excess fat — a lean reference weight for dosing purposes.

B.J. Devine (1974) published the most widely used formula, originally for dosing the drug gentamicin. The formula wasn't validated against health outcomes — it was a practical clinical convenience based on height as a proxy for lean body mass.

• Men: IBW (kg) = 50 + 2.3 × (height in inches − 60)
• Women: IBW (kg) = 45.5 + 2.3 × (height in inches − 60)

This formula gives a 175cm man an IBW of approximately 72.5kg — regardless of his frame size, muscle mass, age, or actual body composition.

How the formulas compare

For a 175cm person, the major formulas produce:

They don't agree. Depending on which formula a calculator uses, the "ideal weight" for the same person varies by several kilograms. This disagreement reflects the fact that none of them is derived from rigorous outcome data — they're all variations on the same rough approximation.

What the formulas can't account for

Muscle mass

A lean, muscular person can weigh substantially more than their IBW without carrying excess fat. An 80kg man at 175cm who is 12% body fat is not overweight — he's muscular. Rigid IBW formulas would classify him as overweight.

Frame size

People of the same height have meaningfully different bone structures. Wrist circumference is a commonly used proxy for frame size, with small, medium, and large frames producing roughly 10% variation in expected healthy weight. The Devine formula has no frame adjustment.

Age

Lean body mass declines with age. A 65-year-old and a 25-year-old of the same height have different body compositions even at the same weight. IBW formulas don't adjust for age.

Sex-specific differences in fat distribution

The male and female IBW formulas differ by about 4.5kg at any given height — a rough approximation of average sex differences in lean mass. But this doesn't capture individual variation within each sex.

Set-point theory: why "ideal weight" is also personal

Research on weight regulation suggests each person has a biologically defended weight range — a set point — maintained by hormonal signals (leptin, ghrelin, insulin) that adjust hunger, satiety, and metabolic rate to resist deviation from this range.

The set point isn't fixed permanently — it can shift upward with sustained overeating and with certain hormonal conditions, and can be influenced (slowly, with sustained behaviour change) by lifestyle. But it explains why weight tends to return to a particular range after disruption, and why two people of the same height and apparent lifestyle can have very different stable weights.

This means there isn't one "ideal weight" for a given height — there's a range of healthy weights, and within that range, individual biology determines where a particular person naturally stabilises with a sustainable lifestyle.

A more useful framework

Rather than a single-number target, consider a three-part assessment:

1. BMI range as a broad screen BMI 18.5–24.9 provides a population-level range, knowing it's imprecise for individuals (discussed in the BMI post). For most people, this maps to a reasonable weight range rather than a single number.

2. Body composition over weight A body fat percentage in the healthy range (roughly 8–20% for men, 15–30% for women) at your actual weight is a more meaningful target than reaching a specific number on a scale.

3. Metabolic health markers Waist circumference under half your height, healthy fasting glucose, normal blood pressure, and good lipid profile indicate a healthy weight regardless of where the formulas put you.

When IBW formulas remain useful

Despite their limitations, IBW formulas serve their original purpose well: clinical drug dosing. For medications where lean body weight matters, Devine's formula is still widely used in pharmacokinetics. For obese patients, adjusted body weight (a blend of IBW and actual weight) is used for certain drug calculations.

For those purposes — which require a consistent, reproducible number rather than a perfect health prediction — these formulas are appropriate. As lifestyle or fitness targets, they're less useful than they appear.

How to use the Ideal Weight Calculator on sadiqbd.com

1. Enter your height in feet/inches or centimetres
2. Enter your sex — the formulas use sex as a variable
3. Read the estimates from multiple formulas — the range they produce is informative
4. Use it as a rough reference, not a prescription

The calculator shows you the output of several formulas simultaneously — which is more honest than showing one formula's number as if it were authoritative. The spread between them illustrates the genuine uncertainty.

Frequently Asked Questions

Should I use ideal body weight as my goal weight? It can serve as a rough reference point, but a more meaningful target is a body composition and metabolic health profile that feels sustainable and healthy for your specific body. Many people are healthy above their IBW; others have risk factors below it.

Why do different ideal weight calculators give different numbers? They use different formulas, developed for different purposes at different times. None has been validated against comprehensive health outcome data. The variation reflects the inherent imprecision of reducing "healthy weight" to a height-based formula.

Is there an ideal weight for athletic performance? Sport-specific. A marathon runner optimises for power-to-weight ratio (lean and light). A powerlifter optimises for strength and muscle mass (heavier). A rugby prop optimises for different characteristics again. Ideal weight for performance is entirely different from ideal weight for general health.

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

The formulas behind ideal weight calculators are 50-year-old clinical shortcuts that were never designed to tell you what to weigh. They're useful starting points, not endpoints. Pair the number with body composition assessment and metabolic markers for a more complete picture.

Try the Ideal Weight Calculator free at sadiqbd.com — see the full range of estimates from multiple formulas for any height and sex.