The Short Answer for a 50 Foot Run
A French drain 50 feet long, 12 inches wide, and 18 inches deep holds about 2.78 cubic yards of stone if you fill the whole trench. Deduct the space taken up by a 4 inch perforated pipe and you land at roughly 2.6 cubic yards. Add a normal allowance for settling and ragged trench walls and you are at about 3 cubic yards.
Converted to weight at the usual density of crushed drainage stone, that works out to somewhere in the high 3 ton range. Most people ordering that trench end up buying 4 tons.
Two assumptions are baked into that number. First, a 4 inch perforated pipe running the length of the trench. Second, stone filled all the way to the top of the trench with no soil cap. If you plan to bury the drain under a few inches of topsoil and turf, your stone volume drops, and there is a section further down that shows how to adjust for it.
Very few trenches are exactly 12 by 18, so the rest of this article walks through the arithmetic step by step. Once you have seen it run once, you can redo it for whatever section and length you actually dug.
Step 1: Get the Raw Trench Volume
Everything starts with one formula. Length in feet, times width in feet, times depth in feet, divided by 27. That gives you cubic yards, because there are 27 cubic feet in a cubic yard.
Convert Inches to Feet First
The single most common mistake in this calculation is mixing units. Trench width and depth almost always get quoted in inches, while the run length is in feet. If you multiply 50 by 12 by 18 you get a number that means nothing.
Divide the inch measurements by 12 before you multiply anything. 6 inches is 0.5 feet. 12 inches is 1 foot. 18 inches is 1.5 feet. 24 inches is 2 feet. Write those down on the same scrap of paper as your trench length and the rest goes smoothly.
Length Times Width Times Depth Divided by 27
Here is the standard case. A 50 foot trench, 1 foot wide, 1.5 feet deep. Multiply 50 by 1 by 1.5 and you get 75 cubic feet. Divide 75 by 27 and you get 2.78 cubic yards.
Now a bigger section, because people often widen a trench partway through digging and do not realize what it costs them. A 50 foot trench at 1.5 feet wide and 2 feet deep is 150 cubic feet, or 5.56 cubic yards. That is twice the stone of the first example for the same length of run. Doubling the cross section doubles the order, which is worth knowing before you commit to a wide trench you may not need.
One habit makes all of this easier. Calculate cubic feet per linear foot first, which is just width times depth in feet, then multiply by your run length. A 12 by 18 trench is 1.5 cubic feet per foot of run. A 12 by 24 trench is 2.0. Once you have that per foot figure, a trench that ends up 58 feet instead of 50 is a ten second adjustment instead of a fresh calculation.
Step 2: Subtract What the Pipe Takes Up
Most online calculators skip this step entirely. They hand you a number that includes the space occupied by a length of plastic pipe, which means you are buying stone for a volume that will never hold stone.
The fix is straightforward. A pipe's volume is its cross sectional area times its length. The area of a circle is pi times the radius squared, with the radius measured in feet. For a 4 inch perforated pipe, the outside diameter is a little larger than the nominal 4 inches, but working from roughly 4 inches of outside diameter gets you close enough. That cross section comes out to about 0.09 cubic feet per linear foot.
Over a 50 foot run, that is roughly 4.5 cubic feet, or about 0.17 cubic yards. Applied to the 12 by 18 trench, it takes 2.78 cubic yards down to about 2.6.
Be honest about what that means at this scale. On a 50 foot run, the pipe deduction is well under a quarter of a yard. It is not going to change what you order. Where it starts to matter is on long runs, on jobs using 6 inch pipe, or on a property with several branches feeding one outlet. Add those up and the deduction becomes real money and a partial load you did not need.
There is also a judgment call worth naming. If you are rounding up to the nearest half yard, or ordering by the ton where the supplier is loading a bucket at a time anyway, the pipe deduction and the overage allowance in the next section roughly cancel each other out on a short run. That is fine. It is better to know both numbers and see them offset than to pretend the arithmetic is more precise than the trench is.
Step 3: Add a Settling and Compaction Allowance
A trench on paper is a clean rectangle. A trench in the ground is not. Soil sloughs off the walls while you work, roots pull out and leave voids, and the sides bow outward wherever the digging got easier. Real trenches hold more stone than their stated dimensions suggest, and the rougher the ground, the bigger the gap.
Stone settles too. Angular drainage rock dropped loose into a trench sits high at first. Walk on it, let water move through it, and it knits down. A trench filled flush on Saturday can sit visibly low the following weekend. That is normal and it is not a sign you did anything wrong.
Standard practice is to add about 10 percent to the calculated volume. On a rough trench hand dug through rocky or root filled ground, closer to 15 percent is the safer call.
Applied to the running example, 2.6 cubic yards plus 10 percent is roughly 2.9. Call it 3 yards and move on.
Running short is a worse outcome than running long. Leftover drainage stone always has a home. It makes a decent splash area under a downspout, a base layer under a shed pad, or the start of a second drain run on the other side of the yard. Paying for a second delivery because you are a quarter yard shy is almost always a bad trade, and it stalls the job on a day you had set aside for it.
If you are wrapping the trench in filter fabric, that does not change your stone volume in any meaningful way. The fabric is thin. What it changes is how cleanly the trench holds its shape and how well the stone stays separated from the surrounding soil over the years.
Step 4: Convert Cubic Yards to Tons
Plenty of suppliers sell drainage stone by weight rather than by volume, so the last step is a conversion.
Crushed drainage stone generally runs somewhere in the neighborhood of 1.35 to 1.45 tons per cubic yard. Using 1.4 tons per yard is a reasonable working figure for an estimate.
Run it both ways on the 50 foot example. The raw 2.78 cubic yards times 1.4 is about 3.9 tons. The adjusted 3 yards, with the pipe deducted and the overage added, times 1.4 is about 4.1 tons. Either path puts you at 4 tons, which is why that is the number most people end up ordering for this trench.
Density is not a fixed property. It shifts with the rock the quarry happens to be working, with the stone size, and with how wet the pile is. Wet stone weighs more per yard than dry stone, sometimes noticeably so after a week of rain. Treat any tons figure as an estimate and confirm the conversion with whoever you buy from.
One practical note before you start calling around. Ask whether the supplier quotes by the yard or by the ton before you do your math, so you are comparing the same unit across quotes. It is easy to think one yard is cheaper than another when the two are not even measured the same way. If you are ordering drainage stone for the first time, that single question saves a lot of confusion at the counter.
If you would rather not run the arithmetic by hand, a gravel calculator will take your trench dimensions and give you the volume directly.
Quick Reference Table for Common Trench Sizes
These are the four sections that cover most residential French drains. All figures assume a 4 inch perforated pipe deducted at roughly 0.09 cubic feet per linear foot, a 50 foot run, and a 10 percent allowance folded into the tons column.
- 12 inches wide by 18 deep. 1.5 cubic feet per linear foot, about 1.4 after the pipe. Roughly 2.6 cubic yards for 50 feet. About 4 tons with the allowance.
- 12 inches wide by 24 deep. 2.0 cubic feet per linear foot, about 1.9 after the pipe. Roughly 3.5 cubic yards for 50 feet. About 5.4 tons with the allowance.
- 18 inches wide by 18 deep. 2.25 cubic feet per linear foot, about 2.16 after the pipe. Roughly 4.0 cubic yards for 50 feet. About 6.2 tons with the allowance.
- 18 inches wide by 24 deep. 3.0 cubic feet per linear foot, about 2.91 after the pipe. Roughly 5.4 cubic yards for 50 feet. About 8.3 tons with the allowance.
Scaling the table is easy. Halve the yardage for a 25 foot run. Double it for 100 feet. For any length that is not a round number, use the per linear foot column and multiply, then divide by 27.
As for how deep and wide to dig in the first place, a 4 inch perforated pipe commonly sits in a trench around 12 inches wide, which gives you room for stone on both sides of the pipe. Depth is driven by where the water actually is and how much fall you have across the run, not by a fixed rule. A drain that has to pick up water near a foundation and carry it to a lower point in the yard will be deeper at the outlet end than at the start, and that is the whole point.
Adjustments That Change Your Number
Stone to Grade Versus a Soil Cap
A drain filled with stone all the way to the surface uses every bit of the volume you calculated. That is the version most people picture, and it works well in beds, along fence lines, and anywhere the look of exposed stone is acceptable.
If you are burying the drain under soil and turf so the lawn runs over the top of it, subtract the capped depth from your trench depth before you calculate. A 12 inch wide trench dug 18 inches deep with a 4 inch soil cap is really a 14 inch deep stone fill for math purposes. On a 50 foot run that cuts the order noticeably, and it is a mistake worth catching before the truck is loaded rather than after.
Stone Size and Type
Washed, angular crushed stone in the 3/4 inch range, commonly sold as #57, is the standard choice for drainage. The stones are all roughly the same size, so when they stack they leave large open voids between them, and water moves through those voids fast. That void space is doing most of the work in a French drain.
Pea gravel is rounded and smaller. It packs tighter, leaves less open space between stones, and tends to migrate into the surrounding soil over time. It has its uses, but it is not the default for a drain whose job is to move water quickly.
Fines are the thing to watch for. Crusher run and similar graded materials contain stone dust and small particles specifically so they will compact into a solid base. That is exactly wrong inside a drain, because the fines fill the voids and the trench stops draining. Ask for washed stone and say the word washed out loud when you order.
Stone choice does not change the trench volume math at all. A cubic yard of hole holds a cubic yard of material regardless of what that material is. It changes the tons per yard slightly, and it changes how well the drain performs for the next twenty years.
One last detail on measuring. A French drain almost always runs downhill, which means the trench is deeper at one end than the other. Use the average depth along the run in your formula, not the deepest point. Measure in a few places and take the middle. Using the deep end throughout is how people end up with a yard and a half of stone sitting in the driveway after the job is done.
How We Started
We started Mulch Mound because we got tired of the hassle that came with buying landscaping materials. The options were either loading bags into your car at a garden center or calling around to local suppliers, trying to figure out pricing, minimums, and delivery schedules. Neither option felt convenient or transparent.
Three of us – Alec, Mo, and Tyler – decided there had to be a better way. Alec and Tyler got their start back in 2013 running a landscaping business during college, moving mulch and mowing lawns to pay tuition. That experience taught them how frustrating it was to source materials, and years later, that frustration turned into Mulch Mound.
We focus on making it simple to get mulch, stone, and soil delivered directly to your home. Order online, pick your delivery date, and we handle the rest. No loading bags. No calling multiple suppliers. No wondering if you bought enough or paid a fair price.
We work with quality local suppliers in the areas we serve and aim to be straightforward about what we offer and what it costs. Landscaping is hard work. Buying the materials for it shouldn't be.
Frequently asked questions
How much gravel do I need per foot of French drain?
For the most common section, 12 inches wide by 18 inches deep, figure about 1.4 cubic feet of stone per linear foot once the 4 inch pipe is accounted for. That is the number worth memorizing, because trench runs change mid project more often than trench sections do. You start out planning 40 feet, hit a wet spot, and extend to 55.
A useful rule of thumb that falls out of it: roughly 20 linear feet of that section uses about a cubic yard of stone. If someone asks how much you need and you only know the length, that converts in your head on the spot.
How much gravel should go under and over the perforated pipe?
This is a question about practice rather than arithmetic. The pipe should not sit directly on trench bottom soil. Lay a bed of stone first so the pipe rests on drainage material along its whole length, which keeps it from settling into soft spots and losing its slope. Then backfill with stone up over the top of the pipe, not just alongside it, so water entering from above reaches the pipe through stone rather than through dirt.
The exact depths vary with the trench, but the principle holds everywhere. Stone below, stone beside, stone above. If any part of the pipe is in direct contact with native soil, you have a spot where the drain can clog.
How much does a cubic yard of drainage stone weigh?
Generally somewhere between 1.35 and 1.45 tons, with 1.4 being a fair working number for estimating. The spread exists for two main reasons. Moisture is the first. Stone that has been sitting in an uncovered pile through a rainy stretch carries water in and on it, and that water has weight. Order by the ton after a storm and you get slightly less stone than you would on a dry week.
The second reason is the rock itself. Quarries work different stone, and a denser parent rock produces a heavier yard of crushed product. Neither factor is something you control, which is why a tons estimate should be treated as an estimate and confirmed with your supplier.
Can I use pea gravel instead of crushed stone?
You can, but it is usually the wrong pick inside a drain trench. Rounded stone nests together and leaves less open space for water, and the small round pieces work their way into surrounding soil more readily than angular stone does.
Where pea gravel earns its keep is on the surface. It is comfortable underfoot, it looks finished, and it is a good choice for a garden path, a patio base topping, or a decorative border. If you like the look of it over your drain, run washed crushed stone as the working fill and cap the top couple of inches with pea gravel for appearance.
Do I need to order extra for a trench dug by machine versus by hand?
The cut quality matters more than most people expect. A walk behind trencher makes a consistent slot at a set width, so the finished trench is close to its nominal dimensions and a 10 percent allowance is plenty.
Hand digging and excavator work are different. A shovel trench widens wherever the digging was easy and narrows where it was not, and a bucket cuts wider than the pipe needs almost everywhere. Both end up holding more stone than the number on paper. For those, lean toward the higher end of the allowance range, and if the ground is rocky or full of roots, closer to 15 percent is the realistic call.