Ideal Body Weight: How It's Calculated and Why It Varies Between Formulas
Type "ideal body weight" into any online tool and you might get three or four slightly different numbers depending on which formula it uses. That's not a bug — it's because "ideal weight" was never meant to be one single perfect number. It's a set of estimates originally designed for very specific clinical purposes, each with its own assumptions. Here's what's actually going on behind these calculations, and how to use them sensibly.
Why "Ideal Weight" Formulas Exist
Most ideal body weight (IBW) formulas were originally developed decades ago for a very practical medical reason: calculating accurate drug dosages. Certain medications, especially anaesthesia and some antibiotics, need to be dosed based on lean body mass rather than total weight (which can be skewed by excess fat or fluid). Over time, these clinical formulas were adopted more broadly as general "healthy weight" reference points, even though that wasn't their original, narrow purpose.
The Four Major Formulas Compared
Each formula uses height as the primary input, with small adjustments for sex, but they were derived from different population studies and use different constants — which is exactly why they don't agree with each other.
| Formula | Men | Women |
|---|---|---|
| Devine (1974) | 50 kg + 2.3 kg per inch over 5 ft | 45.5 kg + 2.3 kg per inch over 5 ft |
| Robinson (1983) | 52 kg + 1.9 kg per inch over 5 ft | 49 kg + 1.7 kg per inch over 5 ft |
| Miller (1983) | 56.2 kg + 1.41 kg per inch over 5 ft | 53.1 kg + 1.36 kg per inch over 5 ft |
| Hamwi (1964) | 48 kg + 2.7 kg per inch over 5 ft | 45.5 kg + 2.2 kg per inch over 5 ft |
Notice that all four use a base weight at 5 feet and then add a fixed amount per inch of extra height — but the base and the per-inch increment differ across formulas because they were calibrated against different reference populations at different times.
Worked Example: A Man at 5'8" (173 cm)
Applying all four formulas to a man who is 8 inches over 5 feet gives noticeably different results:
Robinson: 52 + (1.9 × 8) = 52 + 15.2 = 67.2 kg
Miller: 56.2 + (1.41 × 8) = 56.2 + 11.3 = 67.5 kg
Hamwi: 48 + (2.7 × 8) = 48 + 21.6 = 69.6 kg
The spread here is roughly 67.2 kg to 69.6 kg — about a 2.4 kg difference just from choice of formula, for the exact same height. None of these numbers is "wrong"; they're simply different statistical estimates of a similar concept.
Why Frame Size and Muscle Mass Matter
None of the four classic formulas account for an individual's skeletal frame size or muscle mass — they only use height. This is a real limitation:
- A person with a naturally larger bone structure (broader shoulders, thicker wrists) will carry more healthy weight at the same height than someone with a smaller frame, without this reflecting excess fat.
- A muscular, physically active person — say, someone who does regular strength training or manual labour — can weigh well above their "ideal weight" from these formulas while having low body fat and excellent health markers.
- Conversely, someone at their exact "ideal weight" number could still have high body fat and low muscle mass (sometimes called "skinny fat"), which isn't necessarily healthier.
This is why doctors increasingly pair ideal weight estimates with body fat percentage or waist circumference measurements for a fuller picture.
Ideal Weight vs BMI-Based "Normal Weight" Range
It's easy to confuse ideal body weight with the "normal weight" range from a BMI chart, but they're calculated completely differently:
- Ideal weight formulas (Devine, Robinson, etc.) give a single target number based on height alone.
- BMI-based "normal" range gives a range of acceptable weights for a given height, calculated using the BMI formula (weight ÷ height squared) and the 18.5–24.9 healthy BMI band (or 18.5–22.9 for the stricter South Asian cutoff).
For the same 173 cm man, the BMI-based normal weight range (using the WHO cutoffs) would be roughly 55.4 kg to 74.5 kg — a much wider band than any single ideal-weight formula's output, because BMI intentionally allows for individual variation rather than pinpointing one number.
Why South Asian Body Composition Needs Adjusted Interpretation
Research consistently shows that Indians and other South Asians tend to carry a higher percentage of body fat at the same weight and height compared to Western populations used to derive these classic formulas. This means:
- An Indian adult sitting right at their "ideal weight" per Devine or Robinson formulas may still carry a higher body fat percentage than a Western adult at the same calculated weight.
- Health risks like insulin resistance and type 2 diabetes tend to appear at lower absolute weights and BMI values in South Asians — sometimes called the "thin-fat Indian" phenomenon, where lower muscle mass and higher visceral fat coexist with an otherwise normal-looking weight.
- For these reasons, many Indian clinicians treat the lower end of ideal-weight ranges as more appropriate targets, and pair weight assessment with waist circumference (under 90 cm for men, 80 cm for women) rather than relying on weight alone.
Practical Use Cases: Where Ideal Weight Actually Matters
- Medication dosing: Anaesthesiologists and some prescribing doctors use ideal or adjusted body weight formulas to calculate safer, more accurate drug doses, particularly for patients with obesity.
- Sports and fitness benchmarking: Coaches sometimes use ideal weight as a rough target alongside body composition testing, though elite athletes routinely exceed these numbers due to muscle mass.
- General health screening: For most people without a specific medical need, ideal weight is best treated as a loose reference point — a conversation starter with your doctor rather than a strict target to chase.
Frequently Asked Questions
Q: Which ideal weight formula is most accurate?
A: None is definitively "most accurate" for everyone — they were derived from different studies decades apart, so most calculators (including ours) show an average or range across formulas rather than picking just one.
Q: Should I try to match my exact ideal weight number?
A: Not necessarily — treat it as a general reference rather than a strict goal, especially if you have a muscular build or larger frame size, both of which these formulas don't account for.
Q: Why is my ideal weight different from my healthy BMI weight range?
A: They use entirely different calculation methods — ideal weight formulas give one number from height alone, while BMI gives a wider healthy range that adjusts for the fact that people of the same height can healthily weigh different amounts.
Q: Do ideal weight formulas apply to children?
A: No, these formulas were developed for adults; children and teenagers are assessed using age- and gender-specific growth charts instead.
Q: Is a lower ideal weight always healthier for Indians?
A: Not automatically — being underweight carries its own health risks (weaker immunity, nutrient deficiencies), so the goal should be a healthy body composition overall, not simply the lowest possible number on the scale.
Check Your Ideal Weight Range
Use our free Ideal Weight Calculator to see your estimated ideal weight across multiple formulas based on your height and gender, in one place.