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Calcimator

Catch Basin Spacing Calculator

Calculate maximum catch basin (inlet) spacing based on gutter flow capacity, cross slope, and design rainfall intensity.

Inputs

ft/ft
ft/ft
ft
ft

HEC-22: high-speed arterial ≤12 ft spread; collector 6–8 ft; local streets up to 10–12 ft

in/hr

Results

Maximum Inlet Spacing

2,330 ft

≈ 8 football fields

Inlets per 400-ft Block

1

Gutter Flow Capacity1.87 cfs
Inlet Intercept Capacity1.54 cfs
Inlet Efficiency82%
How to Use This Calculator
  1. Enter roadway longitudinal slope (%), cross slope (%), gutter width (ft), and maximum allowable spread (ft).
  2. Set rainfall intensity (in/hr) for the design storm event.
  3. Review Maximum Inlet Spacing (ft) and Inlets per 400-ft Block.
  4. Use this to plan storm drain layouts during roadway design or retrofit projects.

How the result changes with Cross Slope

Cross SlopeMaximum Inlet SpacingInlets per 400-ft Block
0.021,496 ft1
0.033,762 ft1
0.047,050 ft1
0.0610,007 ft1

What each input means

Roadway Longitudinal Slope
Longitudinal slope of the road in the direction of flow. Typical urban streets range from 0.5% to 5%.
Cross Slope
Transverse slope of the pavement toward the gutter. Standard is 2% (0.02 ft/ft).
Gutter Width
Width of the gutter pan section. Standard concrete gutter is typically 1.5-2 ft.
Maximum Allowable Spread
Maximum width of water spread into the travel lane per HEC-22 (FHWA Drainage Manual). High-speed roads: spread ≤ 1 lane width (12 ft); local streets: spread up to ½ lane (6 ft); parking/low-speed: up to full gutter width.
Rainfall Intensity
Design rainfall intensity from local IDF curves for the selected return period (e.g., 10-year storm).

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Roadway Longitudinal Slope = 0.02, Cross Slope = 0.02, Gutter Width = 2, Maximum Allowable Spread = 8 = 5 input(s) provided
  2. Calculate Maximum Inlet Spacing
    Maximum Inlet Spacing
    2330 = 2330
  3. Calculate Inlets per 400-ft Block
    Inlets per 400-ft Block
    1 = 1
  4. Calculate Gutter Flow Capacity
    Gutter Flow Capacity
    1.867 = 1.867
  5. Calculate Inlet Intercept Capacity
    Inlet Intercept Capacity
    1.54 = 1.54

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