How much does a French drain cost per foot?

It is priced per linear foot of trench, with the rate bundling digging, pipe, gravel, fabric and backfill. A 60 ft run at $30/ft is $1,800. Enter your own $/ft in the French drain tool, since the rate varies with soil, depth and access.

How much gravel does a French drain need?

Gravel cubic yards = length × width × depth ÷ 27. A 60 ft trench 1 ft wide and 1.5 ft deep needs 60 × 1 × 1.5 ÷ 27 = 3.33 cubic yards. Order a little extra for compaction, and remember pipe length equals the trench length.

What slope does a French drain need?

About 1% — roughly 1 inch of fall per 8 feet — using 3–4 inch perforated pipe, wrapped in filter fabric to resist clogging. These are stable planning conventions; confirm local requirements before you dig.

Do I need a permit for a French drain?

Often yes, because trenching near a foundation is excavation, can run near utilities, and affects drainage. Have utilities located first, use licensed pros, and confirm permit and code requirements with your local building department.

Where should a French drain discharge?

Somewhere lower and well clear of the foundation: a daylight outlet where the ground slopes away, a sump with a pump where there is no lower ground, or a storm drain where local code allows it. A drain that empties a few feet from where it started just recirculates the same water, so budget the full run to a real outlet.

French drain cost per foot & how it works

A French drain is a gravel-filled trench with a perforated pipe that carries water away from your foundation. It is priced per linear foot, and the gravel is a straightforward cubic-yard calculation.

By Francesco Zinghinì

A French drain is one of the oldest and most reliable ways to move water: a trench filled with washed gravel around a perforated pipe, usually wrapped in filter fabric, laid at a slight slope so water flows to a lower outlet — a sump, a daylight opening, or a storm drain. Exterior French drains carry surface and subsurface water away from the foundation before it can pool against the wall. The cost is a per-linear-foot figure, which the French drain cost tool computes, along with the gravel take-off.

How it works

Water follows the path of least resistance. A French drain gives it one: the gravel is far more permeable than surrounding soil, so water flows into the trench, drops into the perforated pipe, and runs downhill to the outlet. Three conventions make it work: a ~1% slope (about 1 inch of fall per 8 feet) so it drains but does not silt up, 3–4 inch perforated pipe, and filter fabric to keep fines from clogging the stone. These are stable planning conventions, not code — see the drain-tile conventions table and confirm local requirements.

Cost per foot, worked

French drains are priced by the length of trench. A 60 ft run at $30 per foot is 60 × $30 = $1,800. That per-foot rate — which you enter from your own quote — already bundles digging, pipe, gravel, fabric and backfill, which is why it varies with soil, depth, access and whether the crew hits rock or roots. Enter your own $/ft so the estimate matches your site and stays current as prices move.

Figuring the gravel

If you are buying materials separately, the gravel is a volume calculation. Gravel cubic yards = length × width × depth ÷ 27 (there are 27 cubic feet in a cubic yard). For a 60 ft trench that is 1 ft wide and 1.5 ft deep: 60 × 1 × 1.5 ÷ 27 = 3.33 cubic yards. Pipe is simply the trench length — 60 feet. Order a little extra gravel for compaction and uneven trench bottoms. The cubic-yard conversions table shows the /27 math in general.

Interior vs exterior French drains

The same idea works two ways. An exterior French drain (this tool) intercepts water in the yard before it reaches the foundation and is excavation work. An interior version — installed below the slab at the footing and routed to a sump — manages water that is already getting in; that is the interior drain tile tool. Which you need depends on where the water is: outside and pooling, or inside at the floor.

Pair it with surface fixes

A French drain handles subsurface and pooling water, but do not overlook the cheap upstream fixes that reduce how much water ever reaches it: extend gutters and bury the downspouts so roof water discharges well away from the wall, and regrade so the ground slopes away from the house. Often a combination of surface work and a modest drain beats an oversized drain alone.

Where the drain sends the water

A French drain only works if the water it collects has somewhere lower to go, so the outlet is as important as the trench. There are three common terminations. A daylight outlet is ideal where the ground slopes away: the pipe simply runs downhill and empties onto lower ground, using gravity and no pump. Where there is no lower ground, the drain routes to a sump, and a sump pump lifts the water out — reliable but dependent on power. Some systems tie into a storm drain where local code permits it, which it does not always, so confirm before assuming that option.

Whatever the outlet, the point is to move water away and gone, not in a circle. A drain that discharges a few feet from where it started just recirculates the same water back into the trench. That is also why an exterior French drain pairs naturally with buried downspout extensions: both are about carrying water decisively clear of the foundation. When you scope the job, make sure the quote accounts for the run to a real outlet, not just the trench along the wall — the last stretch of pipe to daylight or a sump is easy to underestimate and essential to the whole thing working.

Excavation means permits and pros

Trenching near a foundation is excavation, and it can run near buried utilities and affect how water reaches (or leaves) your foundation, so it is permit-and-inspection work that belongs to licensed professionals. Always have utilities located before digging. The cost and gravel figures here are planning estimates from your own prices — get itemized written quotes and confirm requirements with your local building department.