French Drain Calculator

How to use this French drain calculator

What this calculator does. It estimates the excavation volume, gravel fill, perforated-pipe length and purchase quantity, filter-fabric area, vertical pipe drop, and a basic material cost for one straight French drain. It is useful for early budgeting, comparing trench sizes, preparing a shopping list, and checking how a pipe or fabric choice changes material quantities. It does not size the drain for a design storm, confirm soil infiltration capacity, locate utilities, determine a lawful discharge point, or replace a site-specific drainage design.

When to use it. Use the calculator while planning a yard drainage trench, estimating a foundation-side interceptor drain, comparing a piped system with a gravel-only trench, or preparing quantities before requesting supplier quotes. EPA describes infiltration trenches as gravel-filled practices that intercept runoff and allow storage and infiltration; see its overview of green infrastructure practices.

How to calculate. The calculator opens with a complete demonstration: a 10-inch-wide, 12-inch-deep, 20-foot-long trench, a 4.5-inch outside-diameter pipe, 1.04% slope, 10% gravel wastage, and filter fabric with 12 inches of overlap. The example workbook is ready immediately.

1. Choose Unit system. Switching between Imperial and Metric converts the current dimensional and price-basis values rather than only changing their labels.
2. Replace Trench width, Trench depth, and Trench length with positive project dimensions. Results update live.
3. Keep Include perforated drain pipe checked for a piped drain. Enter the actual Pipe outside diameter, Standard pipe length, and Pipe slope. Clear the option for a gravel-only trench.
4. Set Gravel wastage. Keep Include filter fabric checked when a fabric liner is planned, then enter the Fabric overlap.
5. Enter optional prices for Gravel price, Pipe price per piece, and Fabric price. Read the live estimate and material table, then select Download Excel for a current-state workbook. Reset clears the demonstration and all calculated data; Excel stays disabled until a complete valid state is entered again.

Input guide. Unit system controls feet/inches or meters/centimeters and their matching volume/area price bases. Trench width and Trench depth are required positive cross-section dimensions; examples are 10 in and 12 in. Increasing either raises excavation and gravel almost proportionally. Trench length is the required horizontal run; 20 ft is the example and longer runs increase every length-based quantity. Include perforated drain pipe is optional; when checked, the pipe cylinder displaces gravel. Pipe outside diameter must be the actual external diameter, not merely a nominal label; 4.5 in is typical for nominal 4-inch Schedule 40 PVC. Standard pipe length is a positive purchase length such as 10 ft; the calculator rounds pieces up. Pipe slope accepts 0% through 100%; 1.04% approximates 1/8 inch of fall per foot. Higher slope slightly increases true pipe length and directly increases vertical drop. Gravel wastage accepts 0% through 100%; 10% adds contingency after subtracting pipe displacement. Include filter fabric turns fabric area on or off. Fabric overlap is a nonnegative wrap allowance; 12 in adds one foot to the fabric width. The three price fields are optional nonnegative numbers in the displayed units. A common mistake is mixing inches and feet, using nominal instead of outside pipe diameter, or entering a comma as a decimal separator; this calculator uses en-US decimal notation.

Output guide. Total gravel volume is the primary purchase quantity after pipe displacement and wastage; it is an estimate because trench irregularity and supplier rounding vary. Trench volume is the exact rectangular excavation identity for entered dimensions. Pipe length is the sloped line length, Pipe pieces is that length divided by standard section length and rounded up, and Vertical pipe drop is horizontal length multiplied by slope. Filter fabric area uses the trench base plus two sidewalls plus overlap, multiplied by trench length. Estimated materials and the table costs respond only to the three entered prices. Zero pipe or fabric outputs are valid when their options are off; negative outputs are never valid.

Learn more. Filter selection should match soil and project conditions. The USDA NRCS Subsurface Drain conservation practice standard discusses geotextile filter performance and clogging considerations, while the University of Minnesota Extension explains how fabric and gravel help protect drainpipe from sediment in its drainage guidance.