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French Drain Gravel Calculator
How much gravel for a French drain, once the pipe and topsoil are taken off.
Part of the 🏠 Homeowners and DIYers toolkit, with more tools made for the job.
Often 15 to 30 cm (6 to 12 inches). Wetter, clay soil usually needs wider.
The stone does not fill this part. Leave empty for 10 cm.
The pipe takes up space the stone does not. Leave empty for no pipe.
For uneven trench walls and settling. Leave empty for 10%.
Type the trench size, and the gravel you need appears here.
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Enter the length, width and depth of the trench, how much topsoil you will leave on top and the size of the pipe. This shows how much gravel to order.
A French drain is a trench filled with gravel, usually with a perforated pipe in it, that carries water away from a wall or a wet patch. The gravel only fills part of the trench, because the pipe takes up some room and the top layer is usually soil or turf. Getting the volume right matters because gravel is heavy and expensive to move twice. Trenches are often 15 to 30 centimetres (6 to 12 inches) wide, and wetter, clay soils usually need wider ones.
How it’s worked out
- Gravel depth is the trench depth minus the topsoil you leave on top.
- Trench volume is the length times the width times the gravel depth.
- The pipe volume is subtracted. It is the area of the pipe's circle times its length. If you give no pipe, none is taken off.
- An allowance is added for uneven walls and settling. The tool uses 10 percent unless you change it.
A 12 m trench 25 cm wide and 45 cm deep, leaving 10 cm for topsoil, has a stone zone of 1.05 cubic metres. With a 110 mm pipe it needs about 0.9 cubic metres, and with 10 percent extra about 1 cubic metre.
Common mistakes
- Measuring the trench once. Widths and depths vary along a trench, so measure in several places and average.
- Filling to the top. The top layer is usually soil or turf, so the stone stops a little below the surface.
- Forgetting the fall. A drain needs a gentle slope to move water, and somewhere to go, so check the levels.
- Ordering by volume when stone is sold by weight. Ask the supplier what a cubic metre or yard weighs.
Questions
How wide should the trench be?
Often about 15 to 30 centimetres (6 to 12 inches). Clay soils drain slowly and usually need wider trenches than sandy ones. Follow advice for your soil.
Should I use fabric?
Many guides line the trench with geotextile fabric to stop soil clogging the gravel. It is a separate material that wraps the trench, so allow for it.
Do I need a pipe?
It depends on the job and the amount of water. Many French drains have a perforated pipe, some use gravel only. Leave the pipe box empty for none.
Where does the water go?
Somewhere that is allowed and suitable, such as a soakaway or a suitable outlet. Where you can discharge water, and whether it affects neighbours, is covered by local rules, so check yours.
Where the method comes from
Estimates only
These are estimates and general information, not building, safety, legal or professional advice. Materials, coverage, waste and rules vary by product, project and place, so check the maker's instructions and your local rules, and use a qualified person for gas, electrical and structural work.
Rain stopped play. Officially research.