Storm Water Runoff Calculator
Estimate peak stormwater runoff flow from a drainage basin using the Rational Method. Ideal for sizing storm drains, culverts, and detention ponds for small urban and suburban catchments under 200 acres.
About this calculator
The Rational Method formula is Q = C × i × A, where Q is peak runoff flow (cfs), C is the runoff coefficient (a dimensionless fraction from 0 to 1 representing how much rainfall becomes runoff), i is rainfall intensity (in/hr) for a storm duration equal to the time of concentration, and A is the drainage area in acres. The formula as implemented uses Q = 0.278 × A × i × (C_percent/100), where the landUse field provides C as a percentage and 0.278 is a unit conversion factor for metric output. The time of concentration — the time for runoff to travel from the most remote point to the outlet — determines which storm duration and intensity to use from an IDF (Intensity-Duration-Frequency) curve. The Rational Method assumes uniform rainfall, a single runoff coefficient, and steady-state flow, making it most accurate for small, homogeneous catchments.
How to use
Assume a 10-acre residential drainage area with a runoff coefficient C = 65% (mixed residential), rainfall intensity i = 3.5 in/hr (from a 10-year IDF curve for a 20-minute time of concentration). Apply the formula: Q = 0.278 × 10 × 3.5 × (65/100) = 0.278 × 10 × 3.5 × 0.65 = 0.278 × 22.75 ≈ 6.32 cfs (or the equivalent in your unit system). This peak flow rate is then used to size the storm drain pipe or detention facility outlet structure for the design storm event.
Frequently asked questions
What runoff coefficient C value should I use for the Rational Method?
The runoff coefficient C reflects land surface imperviousness and soil type. Typical values are 0.70–0.95 for commercial downtown areas, 0.50–0.70 for suburban residential neighborhoods, 0.25–0.50 for parks and lawns on flat ground, and 0.10–0.30 for forested or agricultural land. For mixed land uses, calculate a weighted average C based on the area of each cover type. Always consult your local stormwater design manual, as jurisdictions may specify approved C values for permitting purposes.
How does time of concentration affect peak runoff calculations in the Rational Method?
The time of concentration (Tc) is the travel time from the hydraulically most distant point in the watershed to the design point. It governs which rainfall intensity is read from the IDF curve — a shorter Tc corresponds to a shorter, more intense storm, producing a higher peak flow. Tc is estimated using methods like the Kirpich equation, NRCS travel time method, or Manning's kinematic equation for overland flow. Underestimating Tc overestimates peak flow and leads to oversized, more expensive drainage infrastructure.
When is the Rational Method not appropriate for stormwater runoff estimation?
The Rational Method is suited for small, relatively uniform catchments — generally under 200 acres in most guidelines. For larger or complex watersheds with multiple land uses, significant storage (ponds, wetlands), or where runoff timing is critical, methods like NRCS TR-55, HEC-HMS, or SWMM are more appropriate. The Rational Method also cannot model runoff volumes or hydrograph shapes, which are needed for detention basin design. It assumes the entire basin contributes simultaneously, which is unrealistic for large or elongated watersheds.