Pressure calculator
How much pressure is on each nail of a sadhu board?
Enter your weight, the board's nail spacing, and your posture. The math is grounded in plantar biomechanics and the Shergold–Fleck puncture model — see the methodology page for sources.
70 kg
Nail spacing
Posture
467g
per nail
≈ a medium apple
150
Nails in contact
4,576 kPa
Pressure per tip
654%
of skin-puncture threshold
Numbers are geometric estimates of nail contact under static load. They are not a safety claim. See contraindications.
Common questions
About the numbers.
- How much pressure does each nail on a sadhu board exert?
- For a 70 kg adult standing on a 10 mm-spacing board, each nail carries about 4.6 N of force — roughly 470 grams of equivalent weight, distributed across ~150 nails in contact with both feet. The tip-level pressure is about 4.6 MPa, which sounds extreme until you realize it's static, brief, and elastically buffered by the skin.
- Why doesn't a sadhu board pierce skin if it's above the 0.7 MPa puncture threshold?
- The 0.7 MPa figure is for an indefinitely sustained, single sharp indenter. A sadhu board is the opposite: dozens of nails sharing load simultaneously, skin deforming elastically around blunt copper tips (real contact patch grows above 1 mm² under load), and brief exposure (~30 s for a beginner). Practitioners also develop a slight callus that raises the threshold further.
- Does wider spacing (15 mm) really push harder than narrow (8 mm)?
- Yes — and the calculator shows it. Your weight is constant; fewer nails per unit area means each one carries more. An 8 mm board fits ~1.8× more nails in the same footprint as a 15 mm board, so the per-nail force is ~45% lower. That's why beginners start at 8 or 10 mm and advanced practitioners move to 12 or 15.
- How is this different from a fakir's bed of nails?
- Lying spreads the load across ~1500 cm² of body surface; standing concentrates it onto ~130 cm² of foot. Same weight, an order of magnitude fewer contact nails, ~10× the per-nail force. That's why fakir physics demonstrations are a textbook example of pressure-equals-force-over-area and a sadhu-board practice is a discipline.
- Where do the constants come from?
- Plantar contact area follows Cavanagh, Rodgers and Iiboshi (1987), cross-checked against more recent pedobarography literature. The 0.7 MPa puncture threshold is from Shergold and Fleck (2004), "Mechanisms of deep penetration of soft solids," Proc. R. Soc. A 460:3037–3058. The full methodology page lists every source.