French Drain Calculator
Calculate french drain accurately for your build. Get material quantities, waste allowances, and project cost breakdowns.
Formula
Gravel (cu yd) = [L x (W/12) x (D/12) - Pipe Volume] / 27
Calculate total trench volume in cubic feet by multiplying length (ft) by width and depth (converted from inches to feet). Subtract the volume occupied by the perforated drain pipe. Divide by 27 to convert to cubic yards, then multiply by gravel density to get tons. Landscape fabric is sized to wrap the full trench perimeter plus overlap.
Worked Examples
Example 1: Backyard Drainage French Drain
Problem: Calculate materials for a 60 ft long French drain with a 12-inch wide and 18-inch deep trench using washed gravel.
Solution: Trench volume = 60 x (12/12) x (18/12) = 90 cu ft = 3.33 cu yd\nPipe volume (4-in) = 3.14 x 2^2 / 144 x 60 = 5.24 cu ft\nGravel = (90 - 5.24) / 27 = 3.14 cu yd = 4.39 tons\nFabric = 62 x 6 = 372 sq ft
Result: 3.14 cu yd gravel (4.39 tons), 60 ft pipe, 372 sq ft fabric
Example 2: Foundation Perimeter Drain
Problem: Calculate materials for a 120 ft perimeter drain, 12-inch wide and 24-inch deep using crushed stone.
Solution: Trench volume = 120 x 1 x 2 = 240 cu ft = 8.89 cu yd\nPipe volume = 0.087 x 120 = 10.47 cu ft\nGravel = (240 - 10.47) / 27 = 8.50 cu yd = 11.48 tons
Result: 8.50 cu yd gravel (11.48 tons), 120 ft pipe
Frequently Asked Questions
How deep should a French drain be?
A standard French drain should be 18 to 24 inches deep for residential yard drainage. For foundation drainage, the trench should extend to the depth of the footing, typically 24 to 36 inches. The minimum recommended depth is 12 inches, but deeper drains perform better in areas with heavy clay soil or high water tables. The drain should slope at least 1 percent, or roughly 1 inch per 8 feet of run, to ensure proper water flow.
What size gravel is best for a French drain?
The best gravel for a French drain is 3/4-inch washed or crushed stone. This size provides excellent water flow between the stones while preventing soil from clogging the drain pipe. Avoid using fine gravel or sand, as they reduce drainage capacity significantly. Many installers use a combination: 3/4-inch stone around the perforated pipe and larger 1 to 2 inch river rock at the top for aesthetics and additional filtration.
Do French drains need landscape fabric?
Yes, landscape fabric (geotextile) is strongly recommended for French drains. The fabric wraps around the gravel and pipe to prevent fine soil particles from migrating into the gravel and clogging the system over time. Without fabric, a French drain in clay or silty soil can fail within 3 to 5 years. Use non-woven geotextile fabric rated for drainage applications, and make sure it fully wraps the gravel bed with at least 6 inches of overlap at the top.
How much slope does a French drain need?
A French drain needs a minimum slope of 1 percent, which equals about 1 inch of drop per 8 feet of length. A slope of 2 percent or 1 inch per 4 feet is preferred for better performance. For a 50-foot drain run, this means the outlet end should be 6 to 12 inches lower than the inlet end. Always verify the slope with a level or laser during installation to ensure consistent water flow toward the outlet point.
Is French Drain Calculator free to use?
Yes, completely free with no sign-up required. All calculators on NovaCalculator are free to use without registration, subscription, or payment.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.