The body frame size calculator uses your height and wrist circumference to classify your skeletal frame as small, medium, or large — then applies the Hamwi formula to give you a personalised ideal body weight range adjusted for that frame. It is the same wrist-circumference method used in clinical nutrition and dietetics, and it runs entirely in your browser with no data uploaded anywhere.
Why frame size matters
Standard height–weight tables and even BMI assume that everyone of the same height has the same bone mass. In practice, bone density and skeletal breadth vary considerably between individuals. A person with a large skeletal frame naturally carries more bone mass — and therefore more weight — without any excess body fat. Using a single-number ideal weight for people of very different frame sizes leads to either unrealistically low targets for large-framed individuals or unnecessarily high targets for small-framed ones.
Body frame size gives you a physiologically honest reference point. Once you know whether your frame is small, medium, or large, you can interpret your weight in context rather than comparing yourself to a population average that may not represent your skeletal structure.
The formula
The method is based on the r-value — the ratio of height to wrist circumference, both measured in the same unit (centimetres):
r = height (cm) ÷ wrist circumference (cm)
Because the wrist has no meaningful amount of muscle or fat (only skin, tendons, and bone), its circumference closely reflects skeletal size rather than body composition. A taller person with the same wrist size will have a higher r-value and therefore a relatively finer skeleton for their height — small frame. A shorter person with a larger wrist will have a lower r-value — large frame.
Classification thresholds
| Frame | Men (r-value) | Women (r-value) |
|---|---|---|
| Small | r > 10.4 | r > 11.0 |
| Medium | 9.6 – 10.4 | 10.1 – 11.0 |
| Large | r < 9.6 | r < 10.1 |
These thresholds are taken from the clinical kinesiology and dietetics literature (Grant, 1982; Metropolitan Life Insurance tables cross-validated with wrist-circumference data).
Ideal body weight — Hamwi formula
Once the frame is known, ideal body weight (IBW) is calculated using the Hamwi formula:
- Men: 48 kg for the first 152.4 cm + 2.7 kg per additional cm (proportioned from the 2.7 kg/inch original)
- Women: 45.5 kg for the first 152.4 cm + 2.2 kg per additional cm
This gives a medium-frame baseline. The frame adjustment is:
- Small frame: IBW − 10 %
- Large frame: IBW + 10 %
The result is a range rather than a single number, reflecting the natural biological variation within each frame category.
Worked example
A woman, 168 cm tall, wrist circumference 15.8 cm:
- r = 168 ÷ 15.8 = 10.63 → women’s threshold: r > 11.0 = small, 10.1–11.0 = medium → medium frame
- Height above 152.4 cm = 15.6 cm → 15.6 × (2.2 ÷ 2.54) = 13.5 kg extra
- IBW base = 45.5 + 13.5 = 59.0 kg
- Medium frame: no adjustment → IBW = 59.0 kg (range: 53.1 – 64.9 kg)
A man, 180 cm tall, wrist circumference 20.0 cm:
- r = 180 ÷ 20.0 = 9.0 → men’s threshold: r < 9.6 = large → large frame
- Height above 152.4 cm = 27.6 cm → 27.6 × (2.7 ÷ 2.54) = 29.3 kg extra
- IBW base = 48 + 29.3 = 77.3 kg (medium baseline)
- Large frame: +10 % → IBW = 85.1 kg (range 69.6 – 85.1 kg)
Every number shown in those examples is reproduced exactly by the calculator above.
Measurement tips
Measure your wrist at the narrowest point, just distal to the two bony styloid processes (the small knobs you can feel on either side of your wrist). Keep the tape snug but not tight. Measure twice and average if the readings differ. For height, stand barefoot against a wall with your heels together and eyes level with the horizon. Small measurement errors (±1 mm on the wrist) shift the r-value by about 0.03–0.07, which rarely changes the frame classification but can matter near a boundary value.
This tool is for screening and educational purposes only and is not a substitute for clinical assessment or professional nutritional advice.