Orifice Flow Calculator
Tank drains, filter effluent ports, and flow-restricting orifices all follow the same equation. Enter the orifice diameter and head to get the discharge — useful for drain-down times and restriction sizing.
Orifice Discharge
| Item | Value |
|---|
How the calculation works
Orifice equation (from Bernoulli, free discharge):
Q = Cd · A · √(2gH)
A = πD² / 4, g = 32.2 ft/s², H = head on the orifice centerline
gpm = cfs × 448.831; MGD = gpm × 1,440 / 106
Q = Cd · A · √(2gH)
A = πD² / 4, g = 32.2 ft/s², H = head on the orifice centerline
gpm = cfs × 448.831; MGD = gpm × 1,440 / 106
| Orifice type | Typical Cd |
|---|---|
| Sharp-edged | 0.61 |
| Rounded entrance | 0.95–0.98 |
| Short tube (Borda) | 0.82 |
| Submerged discharge | ~0.61 (use differential head) |
Notes & limitations
The orifice equation is the standard free-discharge relation derived from Bernoulli’s equation — see any fluid mechanics text (e.g., Munson, Young & Okiishi). Cd = 0.61 is the classic sharp-edged value for Reynolds numbers above ~10,000.
- For submerged orifices, use the differential head across the orifice, not the upstream depth.
- Cd drops at low Reynolds numbers (small orifices, viscous fluids) — verify against published Cd-vs-Re curves for critical metering.
- Partially open valves are not orifices — use valve Cv/K data instead.
- Vena-contracta and approach-velocity effects are folded into Cd; keep at least 10 diameters of straight approach for the textbook value.
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