Drainage Inlet Capacity Calculator
Estimate how much stormwater a grated inlet (pit or gully grate) can capture, from the open perimeter of the grate and the depth of water ponding over it. The tool checks both the shallow weir-flow and the deeper orifice-flow limits and reports the governing design capacity.
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Minimum 0
Minimum 0
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the effective open perimeter or length L of the grate opening in metres, and the ponding depth H over the grate in metres.
- Keep the weir coefficient Cw at 1.66 (SI) unless a local standard specifies otherwise.
- Optionally enter the clear open area A of the grate (and orifice coefficient Co, default 0.6) to also get the deep-submergence orifice capacity — the tool then reports the lesser of the two as the governing design capacity.
How it works
At shallow ponding the grate perimeter behaves as a weir: Q = Cw · L · H^(3/2) with Cw ≈ 1.66. As water deepens the openings drown and the grate acts as a submerged orifice: Q = Co · A · √(2gH) with Co ≈ 0.6 and g = 9.81 m/s². The two curves cross; below the crossover weir flow limits capacity, above it orifice flow does, so the real inlet follows whichever passes less water. When you supply the open area the tool computes both and takes the minimum as the governing capacity.
Worked example
Worked example. A grate with an effective open length of L = 1.2 m ponds to H = 0.06 m. Weir control gives Q = 1.66 × 1.2 × 0.06^1.5 = 0.02928 m³/s = 29.28 L/s. With a clear open area of 0.09 m² the orifice check gives 0.6 × 0.09 × √(2·9.81·0.06) = 58.6 L/s, so at this shallow depth the weir value of 29.28 L/s governs.
Common mistakes
- Using the full grate footprint for the open area or perimeter — only the clear (unblocked) opening carries flow, and bars reduce it.
- Ignoring blockage: leaves and debris commonly clog part of the grate, so codes apply a blockage factor (often 50%) to these ideal capacities.
- Forgetting that a grate on a continuous grade only intercepts part of the gutter flow — full capture assumes a sag (low-point) inlet where water ponds.
Frequently asked questions
Which capacity should I design to?
The governing (lesser) of the weir and orifice values at the design ponding depth, then reduced further by a blockage factor from your local drainage code. This tool gives the ideal hydraulic limits; real inlets capture less.
What is the open perimeter L?
It is the effective length of grate edge over which water spills in as a weir. For a grate in a sag it is often taken as the perimeter of the opening (with one side deducted where it sits against a kerb); for on-grade grates a shorter effective length is used. Follow the convention in your design standard.
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- Manning Open Channel Flow Calculator
- Broad-Crested Weir Flow Calculator
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- Trapezoidal Channel Flow Calculator
- Manning Pipe Flow Calculator
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Cite this tool
Zerdly. (2026). Drainage Inlet Capacity Calculator. Retrieved from https://www.zerdly.com/tools/drainage-inlet-capacity-calculator/
Tip: Enter any known values to calculate the remaining results.
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