Orifice Discharge Coefficients (Cd) — Reference
For outlet structures, detention risers, and tank drains: Q = Cd·A·√(2gh), with A the orifice area and h the head on the centroid (free) or the head difference (submerged). Cd = Cc·Cv (contraction × velocity).
Discharge Coefficient by Orifice Type
| Orifice / outlet type | Cd |
|---|---|
| Sharp-edged (thin plate) orifice | 0.61–0.62 |
| Standard stormwater orifice outlet (design value) | 0.60 |
| Rounded / bell-mouth entrance | 0.95–0.98 |
| Short tube (L ≈ 2–3 diameters), flowing full | 0.80–0.82 |
| Borda re-entrant tube, running full | ~0.72 |
| Submerged orifice (use head difference) | ~0.61 |
| Pipe/culvert entrance treated as orifice | 0.60–0.62 |
Component Coefficients (Sharp-Edged)
| Coefficient | Value | Meaning |
|---|---|---|
| Contraction Cc | ~0.62 | vena-contracta area / orifice area |
| Velocity Cv | ~0.98 | actual / ideal jet velocity |
| Discharge Cd | ~0.61 | Cc × Cv |
Weir-to-orifice transition. A riser opening behaves as a weir at low head (Q ∝ h1.5)
and as an orifice once submerged (Q ∝ h0.5). Size the outlet by computing both at each stage
and taking the lower discharge. The flatter orifice curve is why small orifices control low-flow release rates.
Free orifice: Q = Cd A √(2gh) · Submerged: Q = Cd A √(2gΔh)
Sources: Brater & King, Handbook of Hydraulics. ASCE/WEF MOP-77. Standard stormwater outlet-structure practice (Cd = 0.6 for orifice plates).
Sizing an outlet? Open the orifice calculator →
· Weir flow instead? Weir coefficients.
Related cheat sheets and tools
Pair Cd with the orifice tool and the weir coefficient card for the weir-controlled stages of the same riser. For sizing the detention volume that the outlet drains, start from NRCS runoff. For a full stage-storage-discharge outlet design and pond routing, see HydroComplete.