Sizu Home › Calculators › A Fence Is a Sail
Opens on the book's Route B privacy fence in a 25 m/s storm. Change any number and press Calculate.
Enter a wind speed and press Calculate.
The formula
- Wind pressure q = ½ × 1.225 × V² ≈ 0.613 V² pascals (V in m/s; 1 mph = 0.447 m/s)
- Push on the fence = q × G × Cf (G = 0.85, Cf about 1.5)
- Force on one post = push × spacing × height (its share of the fence)
- Moment at the ground = force × 0.55 × height. Gaps: factor 1 − (1 − solid)^1.5 when under 30 % open
The book's worked example, Chapter 7. At 25 m/s, q = 382.81 Pa; × 0.85 × 1.5 = 488.09 Pa. Route B's posts each carry 1.8 × 1.8288 = 3.292 m² of fence: 1,606.7 N (about 164 kg), and 1,616.1 N·m at the ground. At 2.4 m spacing it would be 2,142.3 N: that is why Route B's posts are 1.8 m apart and 900 mm deep.
A teaching sum, not a design: it takes the code's height and terrain factors as 1. Your local design wind speed, height limits and permit rules decide; in hurricane country, follow the local code and a professional. Prices are illustrative US home-center prices from October 2026, shown without a currency sign; put in your own. Everything stays in your browser: no sign-up, nothing stored.