Calculate sag inlet capacity, standalone weir or orifice flow, rational-method runoff, blockage effects, or inverse ponding depth and clear opening area.

Screening models only. The sag envelope assumes a horizontal opening with a common average-head datum and an assumed common hydraulic derating. It does not model on-grade capture, calibrated transition, tailwater or drainage-system limits.

For a rectangular grate against a curb, exclude the curb-side edge; confirm the active perimeter for your geometry.

Weir: crest. Horizontal grate: opening plane. Standalone vertical orifice: opening centroid under full submergence.

Default 1.66 is an SI assumption; use the coefficient for your geometry. It is not a US-unit coefficient.

Default 0.60 is an assumption; confirm for the opening.

Optional settings

Blank means 0. Scales both hydraulic limits equally. Actual debris changes perimeter and area differently; this is not a measured blocked-area rule.

Inlet Capacity Formula

The equation below describes a fully submerged opening. Use head above the opening centroid for a vertical orifice; for a horizontal grate, use the applicable head over its plane. The calculator also preserves standalone weir flow, rational peak runoff and a simplified horizontal sag-grate envelope. The envelope is not a calibrated transition law or an on-grade interception model.

Q = C × A × SQRT (2 × g × H)
  • Where Q is the inlet capacity (m^3/s)
  • C is a dimensionless orifice discharge coefficient; 0.67 is used in this example, not a universal value.
  • A is the inlet opening area (m^2)
  • g is the acceleration due to gravity
  • H is head in metres relative to the appropriate opening datum, under the equation’s submergence assumptions

For a submerged opening, Q = Co A sqrt(2gh). The weir relation is Qw = Cw L h^(3/2), with Cw entered in SI m½/s even when dimensions use feet or inches. The sag screening envelope is F × min(Qw,Qo), where F is the assumed common hydraulic derating. Real debris reduces opening area and effective perimeter differently; this common factor is only an explicit sensitivity assumption. Confirm coefficients, active perimeter, head datum, transition, tailwater and downstream limits using the FHWA HEC-22 Urban Drainage Design Manual.

What is an Inlet Capacity?

Definition:

An inlet capacity is a measure of the volumetric flow rate of water the inlet to a water drain can handle.

How to Calculate Inlet Capacity?

Example Problem:

The following example outlines the steps and information needed to calculate Inlet Capacity.

First, determine the orifice coefficient. In this example, this is found to be .67.

Next, determine the clear opening area. This is measured to be 2 m^2.

Next, determine the allowable head. This drain system has an allowable head of 3m.

Finally, calculate the inlet capacity using the formula above:

Q = C * A * SQRT (2*g*H)

Q = .67 * 2 * SQRT (2*9.80665*3)

Q ≈ 10.279 m³/s with standard gravity 9.80665 m/s². This standalone-orifice example uses zero derating. Rational-method runoff uses Q = C i A with rainfall converted to m/s and area to m²; choose an applicable design intensity and catchment time of concentration under local guidance.