Hamwi formula: the exact math (and why 5'0" matters)

The Hamwi Ideal Body Weight formula is a height-based equation. It starts with a base weight at 5 feet 0 inches and then adds a fixed amount for every inch above that height. If someone is below 5'0", the same idea can be applied by subtracting per inch below 5'0".

• Male: IBW = 106 lb + 6 lb × (inches over 5'0")
• Female: IBW = 100 lb + 5 lb × (inches over 5'0")

Some references pair Hamwi with a simple frame-size adjustment: small frame −10%, large frame +10%, and medium frame 0%. That’s what the frame-size dropdown in this calculator does.

• 1 inch = 2.54 cm
• 1 lb = 0.45359237 kg
• 5'0" = 60 inches

Why does the formula anchor at 5'0"? Historically, it provided a simple way to scale weight with height using “per inch” increments. It’s not “perfect science” — it’s a practical rule-of-thumb that’s easy to compute.

Worked examples (so you can sanity-check your result)

Height = 5'10" = 70 inches. Inches over 5'0" = 70 − 60 = 10. Hamwi IBW = 106 + 6×10 = 166 lb (≈ 75.3 kg).

Height = 5'4" = 64 inches. Inches over 5'0" = 4. Hamwi IBW = 100 + 5×4 = 120 lb (≈ 54.4 kg).

Height = 4'11" = 59 inches. Inches over 5'0" = 59 − 60 = −1. Hamwi IBW = 100 + 5×(−1) = 95 lb (≈ 43.1 kg).

If Example 1 (166 lb) is a large frame, a quick range is +10%: 166 × 1.10 = 182.6 lb. If it’s a small frame, −10%: 166 × 0.90 = 149.4 lb. Your actual healthy range may be wider than this — frame size is just a rough modifier.

What this calculator is doing (step-by-step)

To keep the tool fast and transparent, everything happens directly in your browser:

• Step 1: Convert your height to total inches (from ft/in or cm).
• Step 2: Compute “inches over 5'0"” = inches − 60.
• Step 3: Apply the Hamwi formula for male or female.
• Step 4: Convert the result to kilograms.
• Step 5: If a frame size is selected, adjust the IBW by −10% or +10%.
• Step 6: Show a clean summary you can screenshot or share.

If you want an “apples to apples” comparison, try running your numbers through multiple tools: Ideal Weight Calculator, Devine Formula, and your BMI or Healthy Weight Range. When several methods point to a similar neighborhood, that can be a useful signal.

Frequently Asked Questions

• Is the Hamwi formula accurate for everyone?

  It’s best viewed as a simple estimate. It uses only height and sex, so it can’t capture body composition (muscle vs fat), age, ethnicity, bone density, pregnancy, or athletic training. Use it as a reference point, not a diagnosis.

• What does “ideal body weight (IBW)” mean?

  In many clinical contexts, IBW is a standardized weight estimate used for calculations (for example, some medication dosing approaches). For personal goals, “ideal” is subjective — health is more than a number.

• Why does the formula change by sex?

  The classic Hamwi equation uses different base weights and per-inch increments for males vs females. It’s a simplified model based on typical body size differences — but real bodies vary widely.

• How do I know my frame size?

  Frame size is sometimes estimated from wrist circumference relative to height, or from clinician judgment. If you’re unsure, leave it on “Not sure / Medium”. The frame adjustment here is a quick ±10% tweak — not a medical measurement.

• Should I try to hit exactly this weight?

  Not necessarily. Many people feel and perform best within a range. If you’re changing weight, focus on sustainable habits: protein, movement, sleep, stress, and consistency — then let your body settle into a healthy range over time.

• Can I use Hamwi if I’m under 5 feet?

  Yes — the same math works with a negative “inches over 5'0"” number, which subtracts weight. This calculator supports that automatically.