Yard Drainage Calculator
Estimate GPM, pipe size, and catch basins
Area draining into the system (roof + yard)
Local design storm (1.0 in/hr typical)
Pipe Size
—
in
Catch Basins
—
basins
GPM
—
gallons/min
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Standing water ruining your yard? Sizing a drainage system starts with how much water you need to move. Our calculator converts catchment area and rainfall into GPM, pipe size, and basin count.
- Estimate the catchment area (roof plus yard feeding the drain)
- Find your local rainfall intensity (1.0 in/hr is a common US design storm)
- Enter both values into the calculator
- Round pipe size up and add a cleanout at every direction change
Expert Insights
Smart analysis for your Drainage project
Rational Method Smarts
Our GPM uses the rational method conversion: 1 inch/hour over 1 sq ft = 0.0104 GPM. It's the standard engineers use for small drainage areas.
Pipe Sizing Logic
3-inch pipe handles up to ~20 GPM, 4-inch up to ~50 GPM, and 6-inch for higher flows. Always size up if your slope is shallow.
Slope & Flow Reminder
Drain pipes need at least 1/8 inch of drop per foot (1%) to flow by gravity. Less slope means standing water and clogs.
Calculation Formula
GPM = Ceiling(Catchment area × Rainfall intensity × 0.0104 × 10) / 10
Pipe size (in) = < 20 GPM: 3" | < 50 GPM: 4" | else: 6"
Catch basins = Ceiling(Catchment area / 500)
Drainage Pipe Comparison
| Pipe Type | Diameter | Max GPM (1% slope) | Cost per ft | Best Use |
|---|---|---|---|---|
| Corrugated HDPE | 3" | ~20 | $2-4 | Light yard drainage |
| Solid PVC (SDR 35) | 4" | ~50 | $5-9 | Standard, durable |
| Solid PVC (Schedule 40) | 6" | ~150 | $12-18 | High-flow, deep systems |
Authority References
Our calculations are based on guidelines and standards from these authoritative sources:
- American Society of Civil Engineers (ASCE) — Rational Method for Stormwater Runoff. Learn more
- International Plumbing Code (IPC) — Section 1106 — Size of Conductors, Leaders, and Storm Drains. Learn more
- EPA — National Pollutant Discharge Elimination System (NPDES) — Stormwater Design Guidance. Learn more
Frequently Asked Questions
How do I calculate yard drainage in GPM?
Multiply your catchment area in square feet by the rainfall intensity in inches per hour, then by 0.0104. That gives you the runoff in gallons per minute (GPM) your system must handle.
What size pipe do I need for yard drainage?
A 3-inch pipe handles up to about 20 GPM, a 4-inch pipe handles up to 50 GPM, and a 6-inch pipe is needed for higher flows. Slope the pipe at least 1/8 inch per foot for proper flow.
How far apart should catch basins be?
Plan one catch basin per 500 square feet of drainage area. Space them at low points and along the runoff path so water enters the system before it pools.