Loading Calculator...
Please wait a moment
Please wait a moment
Calculate pipe size, gravel, fabric, and fittings for drainage systems
Did you find this calculator helpful?
A drainage calculator helps you determine the correct pipe size, gravel volume, and materials needed for yard drainage systems. Proper drainage prevents water damage, erosion, and standing water issues around your property.
This calculator considers:
Example: 1,000 sq ft area × 4 in/hr = 2,492 GPM flow rate needed
Minimum 1/8" per foot for proper drainage flow
Perforated pipe in gravel-filled trench, wrapped in fabric. Collects and redirects groundwater away from foundations.
Linear drain with grate, collects surface water from driveways, patios. Connects to solid pipe outlet.
Underground chamber filled with gravel, allows water to percolate into soil. Handles large volumes.
Collection boxes with grates at low points, connected by solid pipe to outlet. Debris settles in basin.
| Pipe Size | Capacity (GPM) | Drainage Area | Best Use |
|---|---|---|---|
| 4" Corrugated | 20 GPM | Up to 500 sq ft | Small yards, downspouts |
| 4" Solid PVC | 25 GPM | Up to 650 sq ft | Outlet pipes, connections |
| 6" Corrugated | 50 GPM | Up to 1,300 sq ft | Medium yards, multiple inputs |
| 6" Solid PVC | 60 GPM | Up to 1,550 sq ft | Main trunk lines |
| 8" Corrugated | 100 GPM | Up to 2,600 sq ft | Large areas, commercial |
Minimum 1% slope (1 inch drop per 8 feet). Use a transit level to verify grade. Water flows downhill—plan outlet location first.
Trench should be 12-18 inches wide and deep enough for gravel + pipe + 2 inches cover. Keep sides vertical.
Lay landscape fabric with extra material on sides. This prevents soil from clogging gravel. Overlap seams 6 inches.
Add 2-3 inches of 3/4" washed gravel before pipe. This creates drainage space under the pipe.
Place perforated pipe holes-down. Connect sections firmly. Use solid pipe at outlets and connections.
Typically 12-24 inches deep. For foundation drainage, dig to footing depth. For yard drainage, 12-18 inches is usually sufficient.
Use 3/4 inch to 1 inch washed gravel (crushed stone or river rock). Avoid limestone which can cement over time. Pea gravel works but is smaller.
Holes should face DOWN. Water rises from below and enters through the bottom holes. This prevents sediment from entering the pipe.
Install French drains 3-5 feet from the foundation. This keeps excavation away from footings while still intercepting water before it reaches the foundation.
Most yard drainage doesn't require permits. However, check local codes if connecting to storm drains or if excavation is near utilities or property lines.
With proper installation and fabric, French drains last 10-15 years or more. Without fabric, they clog with sediment in 5-8 years.