Drainage Pipe Calculator
Enter the drain length and slope to calculate total pipe, elevation drop, gravel volume, and filter fabric needed for your drainage project.
Estimate only — not professional advice. Always verify results independently before purchasing materials or beginning work. Terms of Use
What is Drainage Pipe?
A drainage pipe calculator determines the materials needed to install a French drain, perimeter drain, or subsurface drainage system. These systems collect and redirect groundwater away from foundations, basements, and low-lying areas to prevent water damage, soil erosion, and standing water. The calculator accounts for pipe length (with extra for fittings), the required elevation drop for gravity flow, the gravel drainage bed, and the geotextile filter fabric that prevents sediment from clogging the system. The minimum slope for drainage pipe is 1/8 inch per foot (0.125 inches per linear foot), which equals approximately 1% grade. This slope ensures water flows reliably by gravity without pooling in the pipe. Steeper slopes (1/4 inch per foot) are preferred when site conditions allow because they provide faster drainage and are more resistant to sediment accumulation. The total elevation drop across the entire run tells you how deep the outlet must be below the inlet. Pipe length includes a 10% addition beyond the straight-line drain run to account for fittings. Each elbow, tee, and adapter adds effective length to the system. Four-inch corrugated perforated pipe is the standard for residential French drains, while 4-inch solid PVC is used for downspout extensions and connections to the outlet. The perforated pipe collects water from the surrounding gravel, while solid sections transport it to the discharge point. The gravel bed surrounds the drainage pipe and serves two functions: it provides a highly permeable zone for groundwater to flow toward the pipe, and it provides structural support to prevent the pipe from crushing under soil weight. The standard trench cross-section is 12 inches wide and 6 inches deep of gravel (minimum) surrounding the pipe. This provides approximately 0.5 cubic feet of gravel per linear foot of drain, divided by 27 to convert to cubic yards. Filter fabric (geotextile) wraps the gravel bed to prevent fine soil particles from migrating into the gravel and eventually clogging the pipe perforations. The fabric is laid in the trench before gravel placement, wrapped up and over the top of the gravel. Approximately 4 linear feet of fabric width is needed per linear foot of drain to line a 12-inch wide trench with adequate overlap at the top.
How to Calculate
- Measure the total drain run length from the collection point to the discharge point in feet
- Determine the slope — use 1/8 inch per foot minimum (0.125), or 1/4 inch if elevation allows
- Verify the total elevation drop is achievable on your site (outlet must be lower than inlet by this amount)
- Order gravel (3/4-inch washed stone) and filter fabric based on calculated quantities
Formula
Total Pipe Length = Drain Length x 1.10 (10% added for fittings) Total Elevation Drop = Drain Length (ft) x Slope (in/ft) Gravel Bed Volume = Drain Length x 1 ft wide x 0.5 ft deep / 27 Filter Fabric = Drain Length x 4 ft (width needed to line and wrap trench) The 10% pipe addition covers the extra length consumed by elbows and connections. The gravel calculation assumes a standard 12-inch wide by 6-inch deep gravel bed. Filter fabric at 4 ft width allows lining a 12-inch trench bottom, up both sides, with overlap at the top.
Example Calculation
A 100 ft drain run, 1/8" per foot slope: Total Pipe Length = 100 x 1.10 = 110 ft Total Elevation Drop = 100 x 0.125 = 12.50 inches Gravel Bed Volume = 100 x 1 x 0.5 / 27 = 1.85 cu yd Filter Fabric = 100 x 4 = 400 sq ft
Frequently Asked Questions
What is the minimum slope for a French drain?
The minimum slope is 1/8 inch per foot (approximately 1% grade). This provides enough gravity to move water through the pipe without pooling. Steeper slopes of 1/4 inch per foot are preferred because they flush sediment more effectively and handle higher water volumes. Never install drainage pipe dead-level or water will stagnate and breed mosquitoes.
Should I use corrugated or solid PVC pipe?
Use perforated corrugated pipe for the collection portion of a French drain where groundwater seeps into the pipe through the perforations. Use solid PVC (schedule 40) for the discharge portion that carries water from the collection area to the outlet. At the transition, use a corrugated-to-PVC adapter. Some professionals prefer rigid perforated PVC throughout for longevity, though it costs 3-4 times more.
What size gravel should I use for a French drain?
Use 3/4-inch washed crushed stone (not pea gravel). Washed stone has no fine particles that could clog the system. The angular shape of crushed stone provides better structural support than rounded gravel. Never use limestone in acidite soils as it can dissolve over time. River rock (1-2 inch) is acceptable but leaves larger voids that may allow fabric intrusion.
How deep should a French drain be?
For foundation drainage, the trench bottom should be at or below the footing level — typically 18-24 inches deep. For yard drainage collecting surface water, 12-18 inches is adequate. For intercepting groundwater on a slope, dig to the depth where you encounter wet soil. Always maintain the minimum slope from inlet to outlet regardless of depth.
Do I need a cleanout for maintenance?
Yes, install cleanout access points every 50-100 feet and at every direction change greater than 45 degrees. A cleanout is simply a vertical pipe rising to the surface with a removable cap. This allows you to flush the system with a garden hose or run a drain snake if the pipe clogs. Without cleanouts, a clogged system must be excavated for repair.
Where should the French drain discharge?
Discharge to a location that slopes away from all structures: a drainage ditch, storm drain (if permitted by local code), dry well, rain garden, or the lowest point of your property. Never discharge onto neighboring property or into a sanitary sewer. The outlet must be lower than the inlet by at least the total elevation drop calculated for your slope. Use a pop-up emitter or daylight the pipe at grade.