Estimate peak stormwater flow using the rational method (Q = CiA) and find an appropriate catch basin or storm drain inlet size. Estimation only — consult an engineer for design.
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Shop Express Tools →Catch basin and storm drain inlet sizing is one of the first steps in stormwater system design. The rational method (Q = CiA) is the industry standard for small drainage areas — fast, simple, and accepted by most local agencies. Peak flow Q drives inlet size selection: a curb opening that can only handle 3 cfs will be overwhelmed by a 5 cfs storm event, causing ponding and flooding. This tool provides preliminary sizing guidance using typical inlet capacity values. Actual design must account for inlet geometry, head conditions, bypass flow, and local agency standards. Always have a licensed civil engineer stamp the final drainage design.
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Start free trialThe rational method (Q = CiA) estimates peak stormwater runoff from a drainage area. Q is peak flow in cfs, C is the runoff coefficient (based on surface type), i is rainfall intensity in inches per hour, and A is drainage area in acres. It is widely used for small drainage areas under 200 acres.
Use C = 0.95 for impervious surfaces like asphalt or concrete, C = 0.70 for gravel, C = 0.65 for mixed surfaces, and C = 0.35 for grass or lawn. Composite areas use a weighted average C based on the proportional area of each surface type.
Rainfall intensity depends on your location and design storm frequency (typically 10-year or 25-year return period). The default of 4 in/hr is typical for a 10-year storm in many US regions. Check NOAA Atlas 14 or your local hydrology standards for site-specific values.
After calculating peak flow Q using the rational method, select an inlet with a capacity greater than Q. Curb inlets are rated by opening length; grate inlets by grate dimensions. Always add safety margin — an inlet running at full capacity is already undersized for peak conditions.
No. This tool provides estimates for preliminary sizing and educational purposes only. Final stormwater inlet design requires site-specific hydrologic analysis, local drainage standards, and review by a licensed civil or drainage engineer.
Estimate peak stormwater flow using the rational method (Q = CiA) and find a suitable catch basin inlet size.