Size gravity stormwater pipes using Manning's equation. Enter your peak flow Q (from Rational Method), pipe slope, and roughness coefficient — get the minimum pipe diameter to run full. EN/ASTM standard diameters recommended.
For a circular pipe running full: A = π D²/4, hydraulic radius R = D/4. Substituting and rearranging gives the minimum diameter D_min in metres. Always round up to the next standard nominal diameter.
| Pipe material | n (typical) | Notes |
|---|---|---|
| Concrete (precast) | 0.013 | Most common for gravity stormwater |
| Concrete (in-situ) | 0.015 | Rougher finish than precast |
| PVC / HDPE (smooth) | 0.010 | Plastic pipes, very smooth |
| Clay / vitrified | 0.013 | Traditional drainage |
| Corrugated metal | 0.024 | Culverts, higher roughness |
| Cast iron | 0.014 | Older installations |
Always specify the next standard DN above your calculated D_min. Common gravity drainage diameters (mm): 150, 225, 300, 375, 450, 525, 600, 750, 900, 1050, 1200.
Given: Q = 48 L/s = 0.048 m³/s, S = 0.010, n = 0.013 (precast concrete).
Step 1: D_min = (0.048 × 0.013 / (0.3117 × √0.010))^(3/8)
= (6.24 × 10⁻⁴ / 0.03117)^(3/8) = (0.02002)^(3/8) = 0.268 m = 268 mm
Step 2: Next standard size above 268 mm → DN 300 (300 mm precast concrete).
Step 3 — Velocity check: V_full = Q / A = 0.048 / (π × 0.3² / 4) = 0.68 m/s. ✓ Above 0.6 m/s self-cleansing minimum.
Rational Method Q = CiA — Get your Q to feed into this calculator.
Earthwork Cut/Fill Volume — Cross-section prismoidal volumes.
Retaining Wall Stability (EN 1997) — Overturning, sliding, bearing checks.
Interactive Civil Demo — Earthwork + stormwater + wall on a pre-seeded site.