French Drain Calculator: Free Online Drainage Material Estimator
Use our free French drain calculator to determine exactly how much excavation, gravel, and pipe you need for your drainage installation project. Designed for drainage contractors, waterproofing specialists, and DIY homeowners, this tool calculates trench excavation volume, gravel backfill volume (accounting for pipe displacement), and pipe dimensions. Enter your trench length, width, depth, and pipe diameter to get instant, accurate material estimates.
How to use this calculator.
- Determine your drain route. Plan the path of the French drain from the problem area to the discharge point. The route should have a continuous downhill slope of at least 1% (1/8 inch per foot).
- Enter trench dimensions. Set the trench width (1-2 feet is typical for residential drains) and depth (18-36 inches). Deeper drains handle more water but require more gravel and excavation.
- Select pipe diameter. 4-inch perforated pipe is standard for residential French drains. Use 6-inch pipe for large properties or areas with heavy water flow.
- Review your results. The calculator shows excavated soil volume, gravel volume (trench volume minus pipe volume), and the pipe cross-section volume for reference.
How do you calculate French drain material volumes?
French drain calculations start with the trench excavation volume, then subtract the volume occupied by the pipe to determine how much gravel backfill is needed.
Excavated Volume (cu yd) = Length (ft) × Width (ft) × Depth (ft) ÷ 27
Gravel Volume (cu yd) = Excavated Volume - Pipe Volume (cu yd)
Pipe Volume (cu ft) = π × (Diameter (in) ÷ 24)² × Length (ft)
Where:
- Filter fabric should line the entire trench, wrapping around the gravel and pipe like a burrito, to prevent soil from infiltrating and clogging the drain.
- 6 inches of gravel minimum should surround the pipe on all sides (below, beside, and above).
- #57 washed stone (3/4-1 inch clean gravel, no fines) is the standard French drain aggregate.
Example calculation.
Let's calculate materials for a foundation perimeter French drain.
Scenario: You are installing a French drain along a 65 ft foundation wall with a 1.5 ft wide trench, 3 ft deep, using 4-inch perforated pipe.
- Calculate excavated volume: 65 ft × 1.5 ft × 3 ft ÷ 27 = 10.83 cu yd
- Calculate pipe volume: 3.14159 × (4 ÷ 24)² × 65 ft = 3.14159 × 0.0278 × 65 = 5.67 cu ft
- Pipe volume in cubic yards: 5.67 cu ft ÷ 27 = 0.21 cu yd
- Calculate gravel volume: 10.83 cu yd - 0.21 cu yd = 10.62 cu yd
Result: You need to excavate approximately 11 cubic yards of soil and order about 11 cubic yards(or about 15 tons) of #57 washed gravel. You will need 65 linear feet of 4-inch perforated pipe and approximately 200 square feet of filter fabric.
Tips from the field.
- Use filter fabric -- always. Every French drain must be wrapped in non-woven geotextile filter fabric. Without it, silt and clay particles migrate into the gravel and pipe, clogging the system within 2-5 years. The fabric is cheap insurance against having to dig it all up again.
- Slope is non-negotiable. A French drain that is flat or back-sloped is just an expensive underground pond. Use a laser level or transit to verify at least 1% continuous slope from start to outlet. Mark grade stakes every 10 feet.
- Provide a proper outlet. The pipe must daylight to a swale, dry well, storm drain, or pop-up emitter. Never terminate a French drain underground without an outlet -- the water has nowhere to go and the system fails.
- Call before you dig. Call 811 (or your local utility locate service) at least 2 business days before excavating. French drains often run near buried utilities, especially along foundation walls. Hitting a gas, electric, or fiber line is dangerous and expensive.
Disclaimer: This calculator provides estimates for planning purposes only. Actual material requirements vary based on soil conditions, groundwater levels, trench stability, and local building codes. French drains should not be connected directly to sanitary sewers. Always check local regulations and call 811 before digging. PavementCalculator.com assumes no liability for drainage system performance or property damage.
Cost, price, and density figures on this page reflect approximate U.S. averages as of August 2026 and vary by region, supplier, and material availability.
Frequently asked questions.
How much gravel do I need for a French drain?
For a 50 ft French drain with a 1 ft wide by 3 ft deep trench using 4-inch pipe, you need approximately 5.6 cubic yards of excavated soil and about 5.4 cubic yards of gravel (after accounting for the pipe volume). Use our calculator for your exact dimensions.
What size pipe should I use for a French drain?
For most residential French drains, 4-inch perforated pipe is standard. For large properties or areas with heavy water flow, use 6-inch pipe. The pipe should be surrounded by at least 6 inches of gravel on all sides and wrapped in filter fabric to prevent silt clogging.
How deep should a French drain be?
French drains should be dug 18-36 inches deep, with the pipe sitting at the bottom. The trench should slope at least 1 inch per 10 feet (1% grade) toward the outlet. Deeper drains (3+ feet) handle more water but require more excavation and gravel.
How do I calculate gravel for a French drain?
Calculate trench volume (length × width × depth in feet ÷ 27), then subtract pipe volume. About 70-80% of the trench is filled with gravel. For 100 ft × 1 ft × 2 ft trench: 200 cu ft ÷ 27 = 7.4 cu yd. Gravel needed: ~5.5 cu yd.
How much does a French drain cost?
DIY French drain: $10-$25 per linear foot for materials (pipe, gravel, fabric). Professional installation: $30-$100 per linear foot. A 100-ft residential French drain costs $1,000-$10,000. Exterior basement drains cost more due to deeper excavation.