Catch Basin Sizing Calculator

Unit system

Switching units updates default coefficients.

Drainage area

m²

Runoff coefficient (C)

Typical: roofs 0.90, pavement 0.85, lawns 0.15–0.35.

Rainfall intensity

mm/hr

Safety factor

Accounts for uncertainty and localized flow concentration.

Clogging reduction

Fractional capacity loss, 0.00–0.90.

Inlet capture method

Use combined for grates with an opening below.

Ponding depth above inlet

m

Weir length (effective)

m

Weir coefficient (Cw)

Adjust to match your inlet guidance.

Orifice discharge coefficient (Cd)

Common range: 0.60–0.70 for sharp-edged openings.

Orifice shape

Provide diameter or width & height below.

Orifice diameter

mm

Available curb length (optional)

m

Optional storage check (rectangular internal dimensions)

Basin internal length

m

Basin internal width

m

Sump depth

m

This estimates storage volume only; it does not replace a full routing analysis.

Example Data Table

Scenario	Area	C	Intensity	Ponding	Weir Length	Orifice
Commercial parking	9,500 m²	0.85	75 mm/hr	0.10 m	0.80 m	150 mm Ø
Residential block	4,200 m²	0.55	60 mm/hr	0.08 m	0.60 m	120 mm Ø
Industrial yard	1.8 acres	0.80	3.5 in/hr	0.35 ft	3.0 ft	6 in Ø

Use local standards to select rainfall intensity, coefficients, and inlet geometry.

Formula Used

Peak runoff (Rational Method)

Q = C · i · A / 3,600,000 (Metric: Q in m³/s, i in mm/hr, A in m²)
Q = 1.008 · C · i · A (Imperial: Q in cfs, i in in/hr, A in acres)

Use an intensity consistent with your chosen storm return period and time of concentration.

Inlet capture (simplified)

Q_weir = Cw · L · h^3/2
Q_orifice = Cd · A · √(2gh)
Q_effective = Q · (1 − clogging)

These equations approximate capture under ponded conditions. Calibrate Cw and Cd to your standard details.

How to Use This Calculator

Select your unit system and enter the drainage area.
Choose a runoff coefficient that matches surface cover.
Enter the design rainfall intensity from local IDF data.
Provide ponding depth and inlet geometry for capture capacity.
Set safety and clogging factors to reflect field conditions.
Click Calculate, then export the summary to CSV or PDF.

For final design, check gutter spread, bypass flow, and inlet spacing against applicable roadway drainage guidance.

Runoff Inputs and Design Storm

Catch basin sizing starts with the Rational Method peak flow. Use drainage area, runoff coefficient, and rainfall intensity from local IDF curves for the selected return period. For mixed surfaces, compute a weighted C. Higher intensities or larger areas increase peak runoff and can require more inlets or larger openings.

Inlet Capture Under Ponding

This calculator estimates capture with ponded flow using a weir component and an orifice component. The weir term grows with the 3/2 power of water depth, so small increases in allowable ponding can raise capacity noticeably. The orifice term depends on opening area and the square root of depth.

Clogging and Safety Factors

Field performance is reduced by debris, sediment, and partial blockage. Apply a clogging reduction to represent typical grate impairment, then apply a safety factor to cover uncertainty in intensity selection, surface changes, and flow concentration. Conservative projects often use 10–25% clogging and 1.10–1.25 safety factors.

Select coefficients that match your standard details. If bypass flow is allowed, compute remaining discharge to the next inlet. Use site grading to confirm that ponding depth is achievable without hazards.

Spacing and Storage Checks

After a required basin count is computed, spacing can be estimated by dividing available curb length by the number of basins. This supports preliminary layout, but final spacing must satisfy gutter spread, bypass flow, and roadway constraints. A simple storage volume check helps confirm sump capacity for sediment management.

Worked Example Data

Example (metric): A = 9,500 m², C = 0.85, i = 75 mm/hr gives Q ≈ 0.168 m³/s. With h = 0.10 m, L = 0.80 m, Cw = 1.70, and a 150 mm circular opening (Cd = 0.62), combined capacity is about 0.066 m³/s; with 15% clogging and SF 1.10, the layout suggests 4 basins.

FAQs

1) What design storm should I use for rainfall intensity?

Use the return period and duration required by your local drainage standard, then read intensity from IDF data for a duration near the time of concentration of the drainage area.

2) How do I choose the runoff coefficient C?

Select C based on surface type and slope. For mixed areas, compute a weighted average using each surface area fraction. Use local guidance when available.

3) Why does ponding depth affect inlet capacity so much?

Weir capture scales with h^3/2 and orifice capture scales with √h, so even small allowable increases in depth can raise capture noticeably.

4) What values are reasonable for clogging reduction?

For preliminary design, 0.10–0.25 is common depending on debris load, maintenance, and grate type. Increase the value where leaf litter or sediment is expected.

5) Should I use weir, orifice, or combined capture?

Use combined when a grate provides both overflow edge capture and an opening below. Use weir-only for shallow edge capture and orifice-only when flow enters primarily through a defined opening.

6) Does the “recommended basins” result replace spacing checks?

No. It is a capacity-based estimate. Final layout should confirm gutter spread, allowable bypass flow, inlet throat effects, and roadway safety constraints per the applicable roadway drainage manual.

7) What should I export in the CSV/PDF for design documentation?

Export the input set, selected method, effective capacity after clogging, safety factor, and the resulting basin count. Include the example scenario and note any coefficients calibrated to standard details.