Drainage

French Drain Gravel Calculator

Enter your trench dimensions and pipe size to see how much drainage stone the trench will swallow. The calculator subtracts the space the pipe occupies, adds the waste allowance you choose, and converts the result into cubic yards and tons so you can order by the load.

Your measurements

Results update as you type. Every unit dropdown converts for you.

Drain length

Trench width

Trench depth

Pipe diameter

Outside diameter of the perforated pipe. Enter 0 for a trench with no pipe.

Waste allowance

Extra material for compaction, spillage and uneven trench walls.

Stone density

Supplier weights vary by stone size and moisture. Ask your quarry for the figure they bill on.

Gravel to order

2.30cubic yards

Volume in cubic feet: 62.16 cu ft
Estimated weightAt the stone density you entered: 3.22 tons
Trench volume: 60.00 cu ft
Pipe displacement: 3.49 cu ft
Net stone before waste: 2.09 cu yd
Waste allowance used: 10%

Show the calculationOpen

Trench volume40.00 ft × 1.00 ft × 1.50 ft=60.00 cu ft
Pipe volumeπ × (0.167 ft)² × 40.00 ft=3.49 cu ft
Net gravel volume60.00 − 3.49=56.51 cu ft
With waste allowance56.51 × (1 + 10 ÷ 100)=62.16 cu ft
Convert to cubic yards62.16 ÷ 27=2.30 cu yd
Estimated tons2.30 cu yd × 1.40 tons/yd³=3.22 tons

Reading your result

The headline figure is what to order: net trench volume, minus the pipe, plus your waste allowance, expressed in cubic yards because that is how stone is sold. The tonnage below it is the same quantity weighed at the density you entered, for suppliers who bill by weight.

If the cubic yard figure lands just under a half-yard step, round up rather than down. Coming up short mid-install means a second delivery charge for a fraction of a load, which usually costs more than the extra stone.

The trench volume and pipe displacement rows are shown separately so you can sanity-check the result. If pipe displacement is a large share of trench volume, your trench is narrow relative to the pipe and worth re-measuring.

How this calculator works

A French drain is a rectangular trench with a perforated pipe running along the bottom and stone packed around it. The stone quantity is the trench volume minus whatever the pipe displaces, which is why pipe diameter appears as an input rather than being ignored.

The calculator works in feet internally. Widths and depths are usually measured in inches, so pick inches in the unit dropdown and the conversion happens before any multiplication — you never have to divide by twelve yourself.

The waste allowance is applied after the pipe is subtracted. Stone settles into soft trench walls, some gets lost in handling, and trenches are rarely dug to exact dimensions, so most people order somewhere between 5% and 15% extra.

Tonnage is a conversion, not a second measurement. Cubic yards are multiplied by the density you enter, so the tonnage is only as good as that figure. Quarries publish weights per yard for each product and those weights climb when the stone is wet.

The formula

Trench volume: length × width × depth
Pipe volumeRadius is half the pipe's outside diameter: π × radius² × length
Net gravel volume: trench volume − pipe volume
Order volume: net volume × (1 + waste ÷ 100)
Cubic yards: cubic feet ÷ 27
Estimated tons: cubic yards × stone density

Worked example

A 60 ft French drain, 12 in wide and 18 in deep, with 4 in perforated pipe, a 10% waste allowance and stone billed at 1.4 tons per cubic yard.

01Trench volume60 ft × 1 ft × 1.5 ft=90 cu ft
02Pipe volumeπ × (0.167 ft)² × 60 ft=5.24 cu ft
03Net gravel90 − 5.24=84.76 cu ft
04Add 10% waste84.76 × 1.10=93.24 cu ft
05Cubic yards93.24 ÷ 27=3.45 cu yd
06Estimated tons3.45 × 1.4=4.83 tons

Order about 3.5 cubic yards, roughly 4.8 tons. Most suppliers sell in half-yard steps, so 3.5 yards is a clean order.

How to take the measurements

Measure the trench length along the centerline of the run, following any curves rather than measuring corner to corner.
Take the width at the bottom of the trench. Bucket width is a good proxy on a machine-dug trench; hand-dug trenches tend to flare wider at the top.
Measure depth from the finished trench bottom to where the stone will stop, not to ground level. If the top few inches will be soil or sod, subtract them.
Use the pipe's outside diameter. A pipe sold as 4 inch is usually a little larger on the outside, though the difference rarely changes the order.
Break long runs into sections if the depth changes, run each section through the calculator, and add the results.

Practical considerations

Stone is usually sold by the cubic yard or the ton, and the two are not interchangeable without a density. Get a quote in one unit and convert with this calculator rather than eyeballing it.
Washed stone with few fines drains fastest but shifts more during placement. Material with fines locks up better underfoot and weighs more per yard, which is why the density field matters for tonnage.
Order stone and filter fabric together if you are using fabric. Running out of fabric halfway along a trench tends to mean stone sitting exposed until the next delivery.
On long runs, stage deliveries near the middle of the trench rather than at one end. Wheelbarrow distance is the hidden cost on a hand-built French drain.

Assumptions and limitations

The calculator treats the trench as a straight rectangular box. Curved runs, flared walls or a rounded bottom all change the true volume, usually upward.
Pipe displacement uses the pipe's outside diameter and assumes the pipe runs the full length. Fittings, cleanouts and catch basins are not accounted for.
Stone density varies with product and moisture. The tonnage is only as good as the figure you enter, so use your supplier's own number where you can.
Nothing here addresses where a French drain may legally discharge, how close it may run to a septic system or foundation, or whether a permit is needed. Those are local questions.

Frequently asked questions

Should I subtract the pipe from the gravel volume?

It makes a real difference on small trenches and almost none on wide ones. A 4 inch pipe displaces about 0.09 cubic feet per foot of run, so a 60 ft drain loses roughly 5 cubic feet — around 5% of a narrow trench. The calculator handles it either way; enter 0 for the diameter if there is no pipe.

How much waste should I add?

Ten percent is a common starting point. Go higher when the trench walls are soft, the sides are irregular, or you are hauling in a wheelbarrow over distance. Go lower on a clean machine-dug trench with a short haul.

Why is my tonnage different from the supplier's?

Because density varies. Clean washed stone often runs near 1.4 tons per cubic yard, but stone with fines packs heavier and wet material is heavier still. Ask what weight per yard your supplier bills on and enter that figure.

Does the calculator include fabric or the pipe itself?

No. It calculates stone volume only. Filter fabric, pipe, fittings and any bedding sand are separate line items on your material list.

Can I use this for a trench drain that has no pipe?

Yes. Enter 0 for the pipe diameter and the calculator returns the full trench volume plus your waste allowance.

Why is my supplier's yardage different from mine?

Suppliers often quote loose delivered volume, while this calculates the volume of the hole. Add your waste allowance in the calculator and compare like with like, then ask whether their figure is loose or compacted.