What Is a Healthy Weight Range — and How Is It Calculated?

Ask five different formulas what a healthy weight is for a 5'10" person and you'll get five different answers. BMI says one thing, the Hamwi formula says another, and Robinson's equation offers a third. Here's what each approach actually measures and which is most useful in practice.

Why "Ideal Weight" Is Harder to Define Than It Sounds

The concept of an ideal body weight has existed in medicine for over a century, originally developed not for individual health guidance but for actuarial tables — insurance companies needed to estimate mortality risk. Over time, those tables were refined into clinical formulas that physicians used as rough targets for drug dosing, anaesthesia, and nutritional planning.

The fundamental problem is that "ideal" weight for health is not the same as "ideal" weight for longevity, and neither is the same as what any particular formula calculates. Body composition, muscle mass, bone density, age, ethnicity, and fitness level all interact in ways that no single number can capture.

The Hamwi Formula

Developed by Dr. B.J. Hamwi in 1964, this is one of the oldest and most widely taught formulas in clinical nutrition:

For a 5'10" man (70 inches), that works out to 48 + (10 × 2.7) = 75 kg. For a 5'6" woman, it's 45.5 + (6 × 2.2) = 58.7 kg.

The Hamwi formula is simple, fast, and requires no calculator, which is why it's still taught. Its weakness is that it produces a single point estimate rather than a range, and it was developed primarily for average-framed adults of European descent. It does not account for frame size, age, or muscle mass.

The Robinson Formula (1983)

A later refinement, the Robinson formula is considered slightly more accurate for many adults:

For the same 5'10" man: 52 + (10 × 1.9) = 71 kg. Notice this is 4 kg lower than Hamwi. For a 5'6" woman: 49 + (6 × 1.7) = 59.2 kg. The values are similar to Hamwi for women, but differ more for men.

The Miller Formula (1983)

Also published in the same year as Robinson, the Miller formula is:

For our 5'10" man: 56.2 + (10 × 1.41) = 70.3 kg. This is the lowest of the three formulas for men. In practice, Miller tends to yield the most conservative estimates.

BMI-Based Weight Ranges

Rather than a single target, BMI produces a range. For a healthy BMI of 18.5–24.9, the corresponding weight range for a given height is:

For a 1.78 m (5'10") person: the healthy range is (18.5 × 3.168) to (24.9 × 3.168) = 58.6 kg to 78.9 kg. This is a span of over 20 kg — far more useful than a single-point ideal weight formula.

The BMI range approach is now the most commonly recommended by health organisations, precisely because it acknowledges that no single weight is universally correct and that individuals vary significantly even at the same height.

Why the Formulas Disagree

All three clinical formulas (Hamwi, Robinson, Miller) were derived from the same actuarial data but using different statistical methods and assumptions. They were calibrated against population averages from mid-20th century datasets that skewed heavily toward white adults of European descent. The fact that they disagree by 4–6 kg for the same person is not a flaw — it reflects genuine uncertainty in what "ideal" weight means physiologically.

None of these formulas distinguishes between muscle mass and fat mass, which is their most significant limitation. A lean athlete at 85 kg might be healthier than a sedentary person at 70 kg, even if the formula flags the athlete as overweight.

What Healthcare Providers Actually Use

In clinical settings, ideal body weight (IBW) formulas are used primarily for two purposes:

• Drug dosing: Many medications are dosed by weight, and using actual body weight can over-dose obese patients. IBW formulas provide a more appropriate baseline.
• Nutritional planning: Calorie and protein targets for hospitalised patients are often based on IBW rather than actual weight to avoid over-feeding.

For general health advice, most practitioners now use BMI as a starting point (despite its flaws) combined with waist circumference, because central obesity is a strong independent predictor of metabolic risk regardless of total weight.

Adjusting for Frame Size

Some formulas attempt to account for frame size by adjusting the ideal weight estimate up or down by 10% for large or small frames respectively. Frame size is typically estimated from wrist circumference relative to height:

• Small frame: Wrist circumference below 6.5 inches (men) or 5.5 inches (women) — subtract 10% from IBW
• Medium frame: Use IBW as calculated
• Large frame: Wrist circumference above 7.5 inches (men) or 6.5 inches (women) — add 10% to IBW

This is a reasonable adjustment, but still crude. Bone density and skeletal muscle mass are not fully captured by wrist circumference.

The Practical Takeaway

If you want a single, simple target, any of the three formulas is good enough as a rough guide — they differ by less than 10%. If you want a more nuanced picture, use the BMI-based range (18.5–24.9), which acknowledges individual variation.

But the most honest answer is this: a healthy weight is one at which your metabolic markers (blood pressure, fasting glucose, cholesterol, HbA1c) are within normal ranges and you have the energy and mobility to do what you want to do. No formula captures that. Use the numbers as a starting point, not a verdict.