French Drain Calculator that turns trench length, width and depth into gravel by cubic yard and ton, perforated pipe sections, filter fabric, pipe drop and the material cost total.
Enter the trench size and pipe to get the gravel, pipe, fabric and slope for your French drain.
Gravel, Slope and Fabric
Waste covers settling and spillage. Leave a soil cap if grass goes back over the trench.
Material Prices
Enter your local prices to get the cost. Leave any blank to skip it.
Reference Table
| Width | Gravel per Foot | Weight |
|---|---|---|
| 6 in | 0.847 cu ft/ft | 4.45 tons per 100 ft |
| 8 in | 1.17 cu ft/ft | 6.14 tons per 100 ft |
| 10 in | 1.49 cu ft/ft | 7.83 tons per 100 ft |
| 12 in | 1.81 cu ft/ft | 9.53 tons per 100 ft |
| 18 in | 2.78 cu ft/ft | 14.6 tons per 100 ft |
| 24 in | 3.75 cu ft/ft | 19.7 tons per 100 ft |
Step-by-Step Solution
- Average depth = 18 in + 6.25 in drop ÷ 2 = 21.13 in; gravel depth = 21.13 - 0 in cap = 21.13 in
- Gravel envelope = 50 ft × 1 ft × 1.7604 ft = 88.021 cu ft
- Pipe drop = 50 ft × 1/8 in/ft = 6.25 in; pipe length = √(50² + 0.5208²) = 50.0027 ft
- Pipe volume = π × 0.375² ÷ 4 × 50.0027 = 5.5226 cu ft
- Gravel = 88.021 - 5.5226 = 82.498 cu ft; with 10% waste = 90.748 cu ft
- Weight = 90.748 cu ft × 105 lb/ft³ = 9,529 lb = 4.764 tons
- Fabric width = 2 × 12 + 2 × 21.13 + 12 = 78.25 in; length = 50 + 2 × 1 = 52 ft; area = 6.5208 ft × 52 ft = 339 sq ft
- Result: 3.36 cu yd Gravel to Order
How the French Drain Calculator Works
Enter the trench length, width and depth at the high end, then pick the perforated pipe and how the filter fabric goes in. The French drain calculator returns the gravel to order in cubic yards and tons, the soil to dig out, the pipe sections to buy, the fabric area and the drop the pipe needs to reach its outlet.
The options panel holds the details most tools skip. You can set a gravel waste allowance, a soil cap over the stone for grass, the gravel density, the bag size for store-bought stone, the pipe section length and the fabric overlap. A price panel turns the quantities into a material cost from your own local prices.
Gravel-only drains work too. Pick No Pipe and the tool fills the whole trench with stone, then reminds you to slope the trench bottom toward the outlet so water still drains away.
French Drain Gravel Formula
Gravel fills the trench, less the space the pipe takes up and any soil cap on top. The pipe volume uses its outside diameter, since the stone packs around the outside of the pipe.
The trench deepens toward the outlet to give the pipe its fall, so the tool works at the average depth, the high-end depth plus half the drop. A 50 ft trench 12 in wide and 18 in deep at the high end falls 6.25 in at 1/8 in per foot, so it averages 21.1 in deep and holds 88.0 cu ft. A 4 in pipe (4.5 in outside diameter) takes 5.52 cu ft, leaving 82.5 cu ft of gravel, or 90.8 cu ft and 3.36 cubic yards with 10% waste.
At 105 lb per cubic foot, 90.8 cu ft weighs about 9,530 lb, or 4.76 US tons. The same stone in 0.5 cu ft bags comes to 182 bags, which is why the French drain calculator gives both a tonnage for bulk delivery and a bag count for small jobs. The soil to dig out is 3.26 cubic yards.
French Drain Gravel Chart
Used as a French drain gravel calculator, the tool gives the same figures for any size. The chart gives gravel per foot of trench at an average depth, with a 4 in pipe in place and 10% waste added. Multiply by your trench length to get the total, or read the tons per 100 ft at 105 lb per cubic foot.
| Trench Width | 12 in Deep | 18 in Deep | 24 in Deep | Tons per 100 ft, 18 in Deep |
|---|---|---|---|---|
| 6 in | 0.43 cu ft/ft | 0.70 cu ft/ft | 0.98 cu ft/ft | 3.7 tons |
| 8 in | 0.61 cu ft/ft | 0.98 cu ft/ft | 1.35 cu ft/ft | 5.1 tons |
| 10 in | 0.80 cu ft/ft | 1.25 cu ft/ft | 1.71 cu ft/ft | 6.6 tons |
| 12 in | 0.98 cu ft/ft | 1.53 cu ft/ft | 2.08 cu ft/ft | 8.0 tons |
| 18 in | 1.53 cu ft/ft | 2.35 cu ft/ft | 3.18 cu ft/ft | 12.4 tons |
| 24 in | 2.08 cu ft/ft | 3.18 cu ft/ft | 4.28 cu ft/ft | 16.7 tons |
A 100 ft French drain 12 in wide that averages 18 in deep needs about 153 cu ft of gravel, or 5.7 cubic yards and 8 tons. Start the same trench at 18 in and slope it at 1/8 in per foot, and it averages 24.3 in deep, which raises the order to 7.8 cubic yards and 11 tons. Doubling the width roughly doubles the stone, so width and depth are the biggest levers on gravel cost.
French Drain Size: Width and Depth
Good French drain design starts with two questions, how much water and how deep. Size a French drain from the water it has to catch and the depth it has to reach. A drain that collects surface runoff in a soggy yard can be shallow. A drain that lowers groundwater beside a foundation has to sit below the level where water collects.
Omni's French drain guide lists 10 in wide and 12 in deep as a typical trench, and the French drain calculator opens at 12 in wide and 18 in deep. Wider trenches add gravel without adding much flow, since the pipe carries most of the water once it reaches the stone.
A drain can be too deep for its outlet. Every foot of extra depth at the inlet end must still fall to daylight, a dry well or a storm drain, so check the outlet depth before digging. Leave a soil cap of a few inches over the gravel where grass will grow back, and set it in the options so the gravel figure drops to match.
Soil type sets how fast water reaches the drain. The Soil Infiltration Rate Calculator turns a percolation test into an infiltration rate, and the Storm Water Runoff Calculator estimates the flow from a roof or yard that the drain will receive.
French Drain Slope and Pipe Drop
A French drain needs a steady fall so water runs to the outlet instead of sitting in the pipe. The minimum Omni recommends for 3 to 6 in drain pipe is 1/8 in per foot, about 1%, which is 1 in of fall for every 8 ft of run. Small pipes of 2½ in and under need 1/4 in per foot.
| Run Length | Drop at 1/16 in/ft | Drop at 1/8 in/ft | Drop at 1/4 in/ft |
|---|---|---|---|
| 25 ft | 1.6 in | 3.1 in | 6.3 in |
| 50 ft | 3.1 in | 6.3 in | 12.5 in |
| 100 ft | 6.3 in | 12.5 in | 25 in |
| 200 ft | 12.5 in | 25 in | 50 in |
A 50 ft drain at 1/8 in per foot falls 6.25 in, so a trench 18 in deep at the high end reaches about 24 in at the outlet. Long drains add up fast, and a 200 ft run at the same slope needs 25 in of fall. The Drainage Fall Calculator and Pipe Slope Calculator work out fall for any gradient.
A French drain on dead level ground drains slowly and holds water in low spots, so it rarely works well. If the site is flat, dig the trench deeper toward the outlet to create the fall rather than sloping the ground above it.
French Drain Pipe and Gravel
A 4 in perforated pipe is the usual choice for a home French drain, and it is the default pipe in the French drain calculator. Rigid PVC is easier to keep on a straight, even slope, while corrugated pipe bends around corners and costs less. The tool lists Schedule 40 sizes from 2 to 12 in and thin-wall ASTM D2729 perforated PVC at 3, 4 and 6 in. For corrugated pipe, enter the maker's outside diameter as a custom size.
Lay the pipe with the holes facing down. Water rises from the bottom of the trench, so downward holes let it in as soon as the stone starts to fill, as Omni's guide explains. A thin bed of gravel under the pipe keeps the holes clear of the soil below.
For French drain stone, use clean, washed gravel with no fines, so water moves freely through the gap between stones. Some guides prefer angular crushed stone to round pea gravel, which can pack more tightly. The gravel density setting lets you match the weight your supplier quotes.
Filter Fabric for a French Drain
Filter fabric keeps soil out of the gravel and pipe, which is the main reason French drains clog. The French drain calculator handles both common layouts. A full wrap, often called a burrito, lines the bottom and sides and folds over the top of the gravel. A liner covers only the bottom and sides.
For a 12 in wide trench with 18 in of gravel and a 12 in overlap, a full wrap needs a roll 72 in wide. In the 50 ft default the gravel averages 21.1 in deep, so the roll needs 78.3 in, and with 1 ft extra at each end the wrap takes about 339 sq ft. A liner skips the top width, so it needs less fabric but leaves the top of the stone open to silt.
French Drain Cost
Professional installation of an exterior French drain runs about $10 to $65 per linear foot, and an interior drain about $40 to $100, according to LawnStarter's 2026 cost guide. Labor and digging make up most of the cost of a French drain per foot, not the materials.
| Drain Length | Exterior, Installed | Interior, Installed |
|---|---|---|
| 30 ft | $300 to $1,950 | $1,200 to $3,000 |
| 50 ft | $500 to $3,250 | $2,000 to $5,000 |
| 100 ft | $1,000 to $6,500 | $4,000 to $10,000 |
| 200 ft | $2,000 to $13,000 | $8,000 to $20,000 |
Materials alone cost far less. For 100 ft of trench 12 in wide averaging 18 in deep, the same guide's prices of $25 to $53 per ton for gravel, $0.50 to $3 per foot for pipe and $0.45 to $0.80 per sq ft for fabric give about $200 to $425 for 8 tons of gravel, $50 to $300 for pipe and $275 to $490 for 612 sq ft of fabric.
Enter your own quotes in Material Prices and the French drain calculator gives the total and the cost per foot. Gravel can be priced per ton, per cubic yard, per tonne or per cubic meter. Add a labor or install price per foot and the card shows the installed total with materials and labor split out, so you can compare a quote line by line.
French Drain FAQs
These answers cover the French drain questions people search most, with numbers you can check in the French drain calculator.
How do you calculate French drain size?
Start with the length from the wet area to the outlet, then pick a width and starting depth for the trench, such as 12 in wide and 18 in deep. Gravel is width times average depth times length, less the pipe. A 100 ft drain at that size, sloped at 1/8 in per foot, needs about 7.8 cubic yards of gravel with a 4 in pipe and 10% waste.
Check the fall next, which the French drain calculator shows on its second card. At 1/8 in per foot, that 100 ft drain needs 12.5 in of fall, so the outlet end sits about 30.5 in deep and the trench yields 7.5 cubic yards of soil.
Does a French drain have to empty somewhere?
Yes. A French drain moves water, so it needs an outlet lower than its lowest point, such as a slope that daylights, a dry well, a rain garden or a storm drain where local rules allow it. Without an outlet the trench fills and holds water like a long sump.
Plan the outlet before you dig, since its depth sets how deep the far end of the trench must go.
Can I do a French drain without a pipe?
Yes. A gravel-only drain, sometimes called a rock drain, lets water seep through the stone. It carries less water than a piped drain and works best for short runs with light flow. Pick No Pipe in the French drain calculator and it fills the full trench with stone.
The trench bottom still needs a steady fall toward the outlet, since there is no pipe to speed the flow. The tool warns when a gravel-only drain is flatter than 1/8 in per foot.
Do French drains need to be cleaned?
A well-wrapped French drain needs little care, but silt and roots can still build up over time. A cleanout at the high end lets you flush the pipe with a hose or run a drain snake through it.
If a drain is full of dirt, the fabric has usually failed or was left out. Flushing clears the pipe, but stone packed with soil has to be dug out and replaced with clean gravel inside new fabric.
Which is better, a trench drain or a French drain?
They do different jobs. A trench drain is a grated channel set in concrete or paving that catches surface water running across it, such as at the foot of a driveway. A French drain is a buried gravel trench that collects water soaking through the soil.
For water ponding on a lawn or seeping toward a foundation, a French drain usually fits better. For a sheet of water on a hard surface, a trench drain catches it faster. The Gutter Slope Calculator helps when the water starts at the roof.
Which is better, a sump pump or a French drain?
A French drain relies on gravity, so it only works where an outlet sits lower than the drain. A sump pump lifts water that has no gravity outlet, such as water collected under a basement floor.
Interior drains often feed a sump pump, combining both. Where the yard slopes away, an exterior French drain alone avoids the power use and upkeep of a pump.
Why is it called a French drain?
It is named after Henry Flagg French, a Massachusetts lawyer and farmer who described it in his 1859 book Farm Drainage, not after France. Other names include a rock drain, curtain drain, perimeter drain or weeping tile, depending on where and how it is used.
A curtain drain usually runs across a slope to catch water before it reaches a house, while a perimeter or footing drain wraps around the foundation itself.