French Drain Calculator
Stone less the pipe, plus the fall that stops it silting.
Work out French Drain. Stone less the pipe, plus the fall that stops it silting. Waste is a separate input rather than a hidden margin.
Zero for a stone-only trench.
Washed ¾ inch angular stone. Ask the quarry.
1% minimum — about an inch of fall every eight feet.
Use washed angular stone rather than rounded pea gravel — angular locks together and leaves void space, and unwashed stone brings the fines that clog the drain. Perforations face down, because water rises into the pipe from the stone beneath it.
Stone needed
5.23 yd³
7.33 tons · 141.3 cu ft
The pipe displaces 8.73 cubic feet of stone you would otherwise have ordered — small on a short run, real on a long one. The fall matters more: 12.0 inches over 100 feet at 1.0%. Below about 1% a drain silts up instead of draining, and no amount of stone fixes that.
How the French Drain Calculator works
Two things get left out of a French drain estimate. The pipe displaces real volume — a four-inch pipe over a hundred feet is nearly nine cubic feet of stone you do not need — and the fall gets forgotten entirely. A drain laid flat silts up rather than draining, so the slope is computed here alongside the materials.
Also known as: french drain gravel calculator · how much gravel for a french drain · drainage trench calculator · french drain slope calculator
The calculation itself
Trench length times width times depth gives the volume, and the pipe is subtracted from it — a pipe running the length of the trench occupies space that would otherwise be stone.
Fabric is sized for the floor plus both walls plus an overlap at the top, because the fabric wraps the stone rather than merely lining the trench.
Fall is length times the slope percentage. It is not a materials figure, but it is the one that decides whether the drain works.
In practice
A hundred-foot trench, twelve inches wide and eighteen deep, is 150 cubic feet. A four-inch pipe over that run is 8.73 cubic feet.
So the stone is 141.27 cubic feet — 5.23 cubic yards, or about 7.3 tons at 1.4 tons a yard. Ordering on the raw trench volume would over-order by roughly six percent.
Fabric comes to 500 square feet: one foot of floor, three feet up the two walls, and a foot of overlap, times the hundred-foot run.
At one percent the fall is twelve inches over the run — a foot from end to end, which is more than most people picture when they dig it.
Why drains fail
Almost always silt, and almost always because of slope or stone choice rather than quantity. Below about one percent, water moves slowly enough for fines to settle out and pack the void space.
Stone choice compounds it. Washed angular stone locks together and holds open voids; rounded pea gravel packs tighter and drains worse; unwashed stone arrives with the fines already in it.
And the perforations face down, which surprises people. Water rises into the pipe from the stone beneath, so holes facing up mean the trench has to flood before the pipe does anything.
Acting on it
Set the slope with a level and a string line before digging, and check it again before backfilling.
Specify washed angular stone, and wrap the stone in fabric rather than lining the trench.
Plan where the water goes. A drain that moves water to a place with nowhere to leave has relocated the problem rather than solved it.
Frequently asked questions
How much gravel do I need for a French drain?
Trench length times width times depth gives the volume, then subtract what the pipe occupies. A 100 foot trench 12 inches wide and 18 inches deep is 150 cubic feet, less about 8.7 for a 4 inch pipe — so 141 cubic feet, or 5.2 cubic yards.
What slope does a French drain need?
At least 1%, which is about an inch of fall every eight feet. Over a 100 foot run that is a foot of drop from end to end. Less than that and silt settles instead of washing through.
What size stone should I use?
Washed angular stone around ¾ inch is usual — it locks together and leaves void space for water. Rounded pea gravel packs tighter and drains less well, and unwashed stone brings the fines that clog the drain.
Do I need filter fabric?
Generally yes, wrapping the stone rather than just lining the trench, because the failure mode is soil migrating into the void space. This sizes the fabric for the floor, both walls and an overlap at the top.
Which way should the pipe holes face?
Down, which surprises people. Water rises into the pipe from the stone below it, and holes facing up mean water has to fill the trench before it can enter.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/french-drain-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="French Drain Calculator"></iframe>The only condition is that the credit line stays visible. It sits inside the frame, so you do not have to do anything to keep it.
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