French Drain Calculator
Stone, pipe and filter fabric for any trench, plus the fall and the depth at the outlet.
This French drain calculator works out how much stone, pipe and filter fabric a trench needs. It also gives the fall, and how deep the trench ends up at the outlet. The numbers update as you type.
I built it around two questions most quotes skip. How much stone is that, and does the trench get deeper downhill? Both move the order more than you’d expect.
How the French Drain Calculator Works
The calculator multiplies trench length by width and stone depth, subtracts the pipe, adds a settling allowance and converts the result to cubic yards and tons. It then sizes the fabric and the drop. Here’s what each step assumes.
- Stone depth is trench depth minus soil cover. HGTV’s install guide leaves about 5 inches between the top of the gravel and the ground, so that’s the default.
- The pipe is treated as a solid round tube, and its volume comes out of the stone.
- Fabric covers the trench width, both walls and extra on each side. The same HGTV guide leaves roughly 10 inches of excess on each side to fold over the stone.
- Fall defaults to 1%, which is 6 inches of drop over 50 feet, a tip from the same guide. ADS asks for a minimum 1% slope from the basin to the discharge point in its catch basin guide.
With the example values (100 feet, 12 inches wide, 24 inches deep at the high end, 1% fall on level ground), you order about 8.1 cubic yards of stone, roughly 11.4 tons, and 683 square feet of fabric. I checked the math by hand against the volume formulas and tested the calculator on worked examples.
Does the Trench Get Deeper Toward the Outlet?
On level ground it does. A drain that falls 1% over 100 feet drops 12 inches, so a trench that starts 24 inches deep ends 36 inches deep. The ground setting in the calculator handles that.
The difference is big. The same drain on ground that slopes down with it needs 6.1 cubic yards of stone, while the level-ground version needs 8.13. That’s a third more stone and more digging, and a quote built on the shallow figure won’t cover it.
How Much Stone for a 100-Foot French Drain?
A 100-foot drain in a 12-inch by 24-inch trench needs about 6.1 cubic yards of stone when the depth stays constant. That’s roughly 8.5 tons at 1.4 tons per cubic yard. The table shows other sizes, all with 5 inches of soil cover, 4-inch pipe and 10% extra for settling.
| Trench (width x depth) | Stone | Weight at 1.4 tons per cu yd |
|---|---|---|
| 12 in x 18 in | 4.06 cu yd | 5.7 tons |
| 12 in x 24 in | 6.10 cu yd | 8.5 tons |
| 18 in x 24 in | 9.32 cu yd | 13.1 tons |
| 12 in x 36 in | 10.17 cu yd | 14.2 tons |
Halve the stone for a 50-foot drain. A 50-foot, 12 by 24 trench needs about 3.05 cubic yards.
How Wide and How Deep Should the Trench Be?
There isn’t a single standard, and guides disagree with themselves. HGTV’s steps dig a 6-inch-wide trench, yet its tips ask for at least 2 feet of width, as shallow as 2 feet deep for a slab home and up to 6 feet for a basement.
Treat any one number as a starting point. The trench has to be wider than the pipe, deeper than the pipe and its stone, and deep enough for the water problem you’re solving. If the water comes up under a slab, read the interior French drain guide first, because the depth rules change.
Stone, Weight and What Actually Arrives
Stone weight varies, which is why the calculator lets you change it. Hunker puts 3/4-inch coarse gravel at about 2,565 pounds per cubic yard (1.28 tons), and Omni Calculator uses 1.5 tons per cubic yard for crushed stone. The default of 1.4 sits between them.
Order by the supplier’s figure, and look at the load when it lands. One r/landscaping poster showed what a “washed” order can turn into.
“Ordered 1.5” washed stone and got this mixture…”
– r/landscaping, June 2026 (294 upvotes)
Fabric and Pipe
Choose “Socked pipe only” if your pipe already wears a fabric sleeve, and the calculator drops the roll. Choose “Wrap the trench” if you’re lining it, and it adds the strip width and area. The French drain fabric guide covers which type belongs in a drain.
Where the pipe sits matters as much as the totals. A commenter on a thread about a drain that leaked below its outlet put it plainly.
“Water follows the path of least resistance. With the pipe being on top of the gravel the water will settle to the bottom on the trench and follow the slope. The pipe should have been at the very bottom of the trench.”
– r/landscaping, August 2026 (141 upvotes)
French Drain Calculator FAQ
How much gravel do you need for a 100-foot French drain?
About 6.1 cubic yards, or 8.5 tons, for a 12 by 24 inch trench with constant depth, 4-inch pipe and 5 inches of cover. On level ground with 1% fall, it rises to about 8.1 cubic yards.
Can you use 3/4 gravel for a French drain?
Yes, it’s a common choice. Hill Country Groundworks’ calculator lists washed 3/4-inch drain rock first, ahead of #57 stone and 1 1/2-inch washed rock, and HGTV’s tips call for washed gravel.
How much is a 50-foot French drain?
Angi and Bob Vila reported $10 to $50 per linear foot for exterior drains in the cost per foot breakdown, so 50 feet lands between $500 and $2,500. The optional price fields above total materials only.
What fall does a French drain need?
1%, or 6 inches over 50 feet, is the figure HGTV gives. Type your own value in the “Fall along the drain” box and the calculator shows the drop and the outlet depth.
Still choosing between drains? French drain vs sump pump and dry well vs French drain cover where the water goes.
