French Drain Gravel Calculator.
Estimate the gravel volume for a French drain trench in cubic yards, cubic feet, cubic metres and litres. Optionally subtract the pipe volume and add your own extra-material allowance.
Drain dimensions
Estimate the gravel volume inside a rectangular trench, with optional pipe displacement.
Enter only the depth you intend to fill with gravel.
Subtracts the cylindrical pipe volume from the gravel estimate.
Enter your own allowance for settlement, spill or site variation.
Gravel volume
Enter the drain length, trench width and gravel depth to estimate material volume.
How much gravel do I need for a French drain?
The starting volume is the drain length multiplied by the trench width and the depth that will actually be filled with gravel. If you enter the outside diameter of the pipe, its cylindrical volume is subtracted from the trench volume before the final material quantity is calculated.
The result is shown in both imperial and metric volume units so you can compare it with the way a local supplier sells aggregate.
Use the gravel fill depth, not the full trench depth
If part of the trench is occupied by soil, top dressing or another material, enter only the depth you plan to fill with gravel. Using the entire excavation depth when only part is filled will overstate the material requirement.
The optional extra-material percentage is entirely your choice. Use it only when you want an allowance for settlement, spill, uneven excavation or other site variation.
Why the calculator gives volume instead of tons
Gravel weight per cubic yard or cubic metre varies with stone type, grading, compaction and moisture. Converting every project with one fixed tons-per-yard number would create false precision.
Use the calculated cubic volume with the density or ordering guidance supplied by the aggregate supplier for the exact material you plan to buy. This tool estimates material quantity; it does not design drainage capacity, pipe slope or outlet suitability.
Frequently asked questions
How do I calculate gravel for a French drain?
Multiply the gravel-filled trench length, width and depth to get the trench volume, then subtract the pipe volume when appropriate. This calculator performs those steps and converts the result into common ordering units.
Does the calculator subtract the drain pipe from the gravel volume?
Yes, when you enter the pipe outside diameter. It subtracts the cylindrical pipe volume over the drain length.
Should I enter the full trench depth?
Enter only the depth that will actually contain gravel. If the top of the trench will be filled with another material, do not include that portion as gravel depth.
How many cubic yards of gravel do I need?
The primary imperial result is shown in cubic yards, with cubic feet also provided. Metric results are shown in cubic metres and litres.
Why does the calculator not show tons of gravel?
Aggregate density varies by material and condition. Use the volume result with your supplier's density or conversion for the specific gravel you are ordering.
Can I add a waste or extra-material allowance?
Yes. Enter your own percentage if you want extra material for settlement, uneven excavation or site variation. The field is optional and is not prefilled with an assumed percentage.
Does this calculate French drain slope or drainage capacity?
No. It is a gravel-volume estimator only. Drainage design, slope, pipe sizing and outlet conditions depend on the site and are outside this calculation.