Orifice & Throttle Sizing Calculator
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Orifice & Throttle Sizing Calculator
Size the orifice or throttle that limits a SuDS or attenuation outflow to its consented discharge rate, or find the discharge from an orifice at a given head. Based on the standard orifice equation, The SuDS Manual (CIRIA C753) Eq 28.1.
Orifice diameter for a target flow
Enter the discharge you must limit to and the head of water above the orifice. The tool returns the orifice diameter and checks it against blockage limits.
Discharge from a known orifice
Enter an orifice diameter and the head — the tool gives the discharge, and how it changes as the water level rises and falls.
Equation & notes
The orifice equation, discharge coefficients and the practical limits on small orifices.
Orifice equation (C753 Eq 28.1). Q = Cd × A × √(2 g H), where Q = discharge (m³/s), Cd = discharge coefficient, A = orifice area (πd²/4), g = 9.81 m/s², H = head above the orifice centre (m). Discharge rises with the square root of head, so it increases as the storage fills.
| Outlet type | Cd |
|---|---|
| Sharp-edged orifice plate | ~0.60 |
| Rounded edge / short tube | ~0.80 |
| Bellmouth / smooth entry | ~0.97 |
Blockage & minimum size. Small orifices block easily. C753 allows diameters down to ~15 mm only where the opening is well protected (e.g. downstream of pervious surfaces or a cleanable filter). Unprotected orifices are usually kept to at least ~50 mm; where the required flow is too low for a safe orifice, use a vortex / Hydrobrake-type flow control instead, or combine areas so a larger control can be used.
Head. Use the effective head above the centre of the orifice at the design condition (C753 Figure 2.16). Keep the head low and the orifice as large as possible to reduce blockage risk. Provide access to clear the control, and screen upstream where practical.
This orifice flow calculator sizes the orifice or throttle that limits a SuDS or attenuation outflow to its consented discharge rate, or works out the discharge from a known orifice at a given head. It uses the standard orifice equation (The SuDS Manual, CIRIA C753, Eq 28.1) and checks the result against blockage limits. Restricting runoff to an agreed rate is a requirement of the national SuDS standards.
How to size an orifice for a target flow
Enter the discharge you must limit to and the head of water above the orifice at the design condition, and the calculator returns the required orifice diameter. It then flags whether that diameter is small enough to be a blockage risk — the point at which a vortex flow control usually becomes the better option.
Discharge from a known orifice
If you already have an orifice diameter, the calculator gives the discharge it passes and how that changes as the water level rises and falls. This is useful for checking an existing outfall or confirming a manufacturer's figure against first principles.
Discharge coefficient — getting it right
The orifice equation depends on the discharge coefficient (Cd). A sharp-edged plate is about 0.6; a rounded or short-tube outlet is higher, around 0.8; a bellmouth higher still. The calculator lets you pick the right coefficient for your outlet so the diameter is not over- or under-sized.
Further reading: the vortex flow control selection helper for low flows · 2025 national standards for SuDS · gov.uk SuDS technical standards.
Frequently asked questions
Rearrange the orifice equation Q = Cd x A x sqrt(2gH) for area, then convert to a diameter. The calculator does this and checks the diameter against blockage limits.
Very small orifices block easily. Where the required diameter falls below roughly the size at which debris bridges the opening, a vortex flow control is usually specified instead.
They do the same job — restricting flow to a consented rate — but a throttle pipe uses a length of small-diameter pipe rather than a plate. The sizing principle is the same.
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