Storm Water Drainage Calculator
Estimate peak stormwater runoff and determine the minimum pipe diameter needed to handle it. Used by civil engineers and drainage designers when sizing pipes for subdivisions, roads, or urban catchments.
About this calculator
This calculator applies the Rational Method to compute peak flow from a catchment, then uses a Manning-based pipe sizing formula to find the required diameter. Peak runoff is Q = C × i × A / 360, where C is the runoff coefficient, i is rainfall intensity (mm/hr), and A is catchment area (ha), giving Q in m³/s. That flow is then substituted into the full-pipe Manning equation rearranged for diameter: D = [(Q × n) / (0.312 × √(S/100))]^0.375, where n is Manning's roughness and S is pipe slope (%). The runoff coefficient C ranges from 0.1 for grassed areas to 0.95 for impervious rooftops. Manning's n is typically 0.011–0.013 for concrete pipes and 0.022–0.025 for corrugated metal.
How to use
Suppose you have a 2 ha catchment with a design rainfall intensity of 80 mm/hr, a runoff coefficient of 0.75, a pipe slope of 1%, and Manning's n of 0.013. Step 1 — compute peak flow: Q = (2 × 10,000 × 80 × 0.75) / 3,600 = 0.333 m³/s. Step 2 — compute required diameter: D = [(0.333 × 0.013) / (0.312 × √(1/100))]^0.375 = [(0.004329) / (0.0312)]^0.375 ≈ 0.139^0.375 ≈ 0.47 m. The calculator outputs this as approximately 470 mm, so you would select the next standard pipe size up — typically 500 mm.
Frequently asked questions
What is a runoff coefficient and how do I choose the right value?
The runoff coefficient (C) represents the fraction of rainfall that becomes surface runoff rather than infiltrating or evaporating. It ranges from about 0.10 for heavily vegetated, permeable land to 0.95 for nearly impermeable surfaces like rooftops and asphalt. Mixed catchments use an area-weighted average of the sub-area coefficients. Choosing too low a value will undersize the pipe and risk flooding, so engineers typically use conservative (higher) values for design.
What Manning's roughness coefficient should I use for stormwater pipes?
Manning's n depends on the pipe material and its condition. Smooth concrete pipes typically use n = 0.011–0.013, PVC pipes use about 0.009–0.011, and corrugated metal pipes range from 0.022 to 0.027. For preliminary design, n = 0.013 is a widely accepted default for concrete stormwater mains. Using a higher n gives a more conservative (larger) diameter estimate, which provides a safety margin against blockages and aging.
Why is pipe slope so important in stormwater drainage design?
Pipe slope controls the velocity of flow under gravity, which in turn determines how much discharge a given pipe diameter can carry. A steeper slope increases flow velocity and capacity, meaning a smaller pipe can handle the same runoff volume. However, very high velocities (above about 3–4 m/s) can cause erosion and joint damage, while very low velocities (below about 0.6 m/s) allow sediment to settle and block the pipe. A minimum slope of 0.5% is commonly specified to maintain self-cleaning velocity.