Calculator Inputs
Use this tool for ponds, rain barrels, raised beds, and tanks where overflow must safely pass incoming water.
Formula Used
This calculator uses the orifice flow relation for free discharge:
Q = Cd × A × √(2gh)
- Q = flow rate (m³/s)
- Cd = discharge coefficient (dimensionless)
- A = opening area (m²)
- g = gravity (9.80665 m/s²)
- h = water head above the opening (m)
For sizing, the calculator applies a safety factor and divides the flow across the selected outlets.
How to Use This Calculator
- Select Size opening to design a new overflow outlet.
- Enter your inflow rate, expected head, and a safety factor.
- Choose an outlet shape and the number of outlets.
- Submit to see recommended dimensions and total capacity.
- Download a CSV or PDF to share with your installer.
Example Data Table
| Scenario | Inflow | Head | Outlets | Cd | Safety | Suggested opening (each) |
|---|---|---|---|---|---|---|
| Rain barrel overflow | 0.6 L/s | 12 cm | 1 | 0.62 | 1.25 | ≈ 24 mm diameter |
| Pond spill outlet | 2.0 L/s | 20 cm | 2 | 0.62 | 1.30 | ≈ 29 mm diameter |
| Raised bed slot | 1.2 L/s | 10 cm | 1 | 0.62 | 1.25 | ≈ 70×25 mm slot |
Examples are typical design cases. Always verify with site conditions and keep overflow paths clear.
Overflow Outlet Planning Article
Why Overflow Outlets Matter
Overflow outlets protect planters, pond edges, and rain barrels from sudden inflow. A properly sized outlet prevents soil washout, root drowning, and liner overtopping. During a heavy downpour, a small opening can become the bottleneck even if the storage volume is large.
How Head Drives Capacity
The driving force is water head, the vertical distance from the water surface to the opening centerline. Capacity rises with the square root of head, so doubling head increases flow by about 41%. Shallow heads under 3 cm are sensitive: a 1 cm change can noticeably shift discharge.
Circular vs Slot Openings
Circular outlets match pipes and bulkhead fittings and are easy to screen. Slots are useful when water levels are shallow or when you want a wide, gentle drawdown. For the same area, a slot may pass debris differently, but it can clog if the height is too small.
Sizing with a Safety Margin
This calculator applies a discharge coefficient and a user safety factor. For sharp-edged openings, Cd around 0.60–0.65 is common. A safety factor of 1.2–1.5 is typical for gardens because leaves and algae reduce effective area. Splitting flow across two outlets halves the per-outlet requirement and reduces splash velocity.
Practical Installation Checks
Keep the outlet invert above sediment zones, and place a removable screen that you can clean quickly. Add a short stilling section or baffle inside a tank to calm turbulence near the opening. After installation, test with a hose at the expected inflow and verify free discharge at the outlet face. Route discharge away from beds with an elbow or splash pad. If the outlet empties to soil, armor the area with gravel to prevent erosion. Inspect screens in leaf season and after storms. In freezing climates, avoid standing water that can crack fittings.
FAQs
1) What does the calculator assume about the outlet?
It assumes free discharge to air with a clean, sharp-edged opening. Submerged outlets, long pipes, or tight bends add losses and will reduce real capacity. Use a higher safety factor or model pipe losses if needed.
2) How do I measure water head correctly?
Measure vertically from the expected maximum water surface to the center of the opening. For slots, use the mid-height of the clear opening. If the water level varies, size for the smallest head you expect during peak inflow.
3) Which discharge coefficient should I pick?
For a sharp-edged hole or bulkhead opening, 0.60–0.65 is typical. Smooth, rounded entrances can be higher, while screens and rough edges can be lower. If unsure, choose 0.62 and add extra safety margin.
4) What safety factor is reasonable in gardens?
Many garden systems use 1.2–1.5 to cover leaves, algae, partial blockage, and uncertain rainfall intensity. If you cannot inspect frequently, increase the factor. If the outlet is screened and accessible, a lower factor may be acceptable.
5) Should I use one outlet or several?
Multiple outlets reduce the required size per opening and lower exit velocity, which helps with noise and splash. They also provide redundancy if one screen clogs. Space outlets so water can reach each one easily.
6) How is a rectangular slot sized here?
The tool calculates the required area per outlet, then divides by your chosen slot height to find slot width. Use the clear, unobstructed height. Very shallow slots can clog, so consider a taller slot or more outlets.
7) Why is capacity sensitive at small heads?
Flow increases with √h, so small heads provide limited driving force. A minor drop in water level can noticeably reduce discharge. If your design head is under a few centimeters, add outlets, increase opening area, or create more head.