French Drain Calculator

What a French drain does

A French drain is a gravel-filled trench with a perforated pipe at the bottom. Water soaks into the gravel, enters the pipe through its holes, and runs downhill to an outlet such as a ditch, a dry well or the street. It is the standard fix for a soggy lawn, water pooled along a foundation, or a slope that dumps runoff into your yard.

Three things decide whether the drain works: the trench size, the slope, and the pipe. Get the trench size right and you buy the correct amount of gravel in one trip. Get the slope right and water moves instead of pooling. Get the pipe orientation right and water enters instead of sitting in the gravel. This page handles the measuring and the material counts, and it follows the same build the City of Norfolk describes in its drainage guidance: a trench at least 12 inches wide, 18 to 24 inches deep, with the pipe sloped 1 inch for every 8 feet of run.

One warning before you dig: call 811 first. Utility lines are buried shallowly, and a 2 foot trench can reach them. The call is free and the mark-out usually takes a few days.

Sizing the trench: length times width times depth

The trench is a simple box, so the digging math is length times width times depth. Keep the units straight: measure length and width in feet, depth in inches, and divide the depth by 12 to convert it to feet.

Worked example, a 40 foot drain with a 1 foot wide, 12 inch deep trench:

• Soil to remove: 40 x 1 x 12 / 12 = 40 cubic feet
• In cubic yards: 40 / 27 = 1.48 cubic yards
• Gravel in bags: 40 / 0.5 = 80 half-cubic-foot bags

A longer example shows why the volume grows fast. A 60 foot trench at 1.5 feet wide and 18 inches deep holds 60 x 1.5 x 18 / 12 = 135 cubic feet, and 135 / 27 = 5 cubic yards. Bagged, that is 135 / 0.5 = 270 bags. The pipe itself displaces some gravel, roughly a fifth of a cubic foot per 10 foot stick of 4 inch pipe, so contractors order about 5 percent less gravel than the raw box volume and the bed still comes out right.

Depth matters more than width for capacity. Doubling depth doubles the volume, and a deeper drain also reaches the wet soil layer, which is the one you want to intercept.

The slope: about 1/8 inch of fall per foot

Slope is the most common failure point. Water in a flat pipe does not move, sediment settles, and the drain silts up from the flat end first. The working minimum is about 1 percent, which is 1/8 inch of drop for every foot of run. Washington State University Extension's drainage guide for Clark County puts the target between 0.5 and 1 percent, and more is fine if you have the elevation to spare.

To get total fall, multiply run by 0.125:

• 50 foot run: 50 x 0.125 = 6.25 inches of drop
• 100 foot run: 100 x 0.125 = 12.5 inches of drop
• 40 foot run: 40 x 0.125 = 5 inches of drop

Check the fall with a string line and a line level, or a laser level if you have one, rather than by eye. Tie the string at the high end, pull it to the outlet, level it, then measure down every few feet as you dig. Digging slightly deeper at the outlet end and back-checking with the level beats trying to fix a flat spot after the gravel is in.

Pipe size and which way the holes face

Two pipe choices matter: diameter and hole direction.

For diameter, 4 inches is the residential standard and handles yard drainage, foundation perimeter lines and downspout ties. Go to 6 inches only for long runs from large areas or where the water table is high. A bigger pipe does not collect more water through the gravel; capacity in the ground comes from the gravel and the slope.

For hole direction, the usual advice for a yard drain is holes down. Set the pipe so perforations sit near the 4 and 8 o'clock positions. Groundwater rises from underneath, enters through the bottom holes, and leaves immediately, so the pipe carries water while mostly empty. If the holes face up, the whole trench has to fill before water finds the pipe, and the yard stays soggy longer. The Department of Energy's building science resource at PNNL shows the standard assembly, 4 inch perforated PVC laid perforations down in a fabric-lined trench. Note that some septic-system rules require holes up to collect rising groundwater, so follow your local health department where it applies.

Choose rigid PVC for long straight runs and anywhere vehicles may cross. Sock-wrapped corrugated pipe is easier to lay around curves and fine for most yards, but it catches more sediment over the years.<

The two diameters compared:

On slope and flow, a 4 inch pipe on a 1 percent grade carries more storm water than most yards ever deliver, which is why upsizing rarely pays. The money usually does more good in extra depth or a steeper grade than in a wider pipe.

Trench sizes and materials at a glance

This table converts the common trench sizes into what you have to haul. All rows assume a 4 inch pipe and a fabric liner.

Reading the table, the jump from a 40 foot single-foot trench to the 60 foot wide-and-deep version is more than triple the gravel. Past about 2 cubic yards, ordering loose gravel by the yard from a landscape supplier beats bags on price and on the waste you carry to the curb.

Slope needs scale with run, so plan the outlet elevation before you commit to a length:

If your yard cannot give the full fall, shorten the run or let the outlet daylights into a dry well with gravel around it, and steepen what run you do have.

What the materials cost

Gravel for a drain is clean, washed stone, usually 3/4 inch to 2 inch. Bagged decorative gravel at a home center runs about 5 to 7 dollars per half-cubic-foot bag, so the 80 bag example costs 80 x 6 = 480 dollars at the midpoint. Loose washed stone from a landscape yard commonly prices around 40 dollars per cubic yard, so the same 1.48 cubic yards runs near 60 dollars plus a delivery fee, which is most of the difference on any job over 2 yards.

The rest of the shopping list is short: a roll of nonwoven landscape fabric wide enough to line the trench and lap over the top, a 100 foot stick or coil of 4 inch perforated pipe, a bag of gravel for the pipe bed, and a cap fitting for the uphill end. Fabric and pipe for the 40 foot example together usually land in the 80 to 120 dollar range.

Labor is the real cost driver. Digging 40 cubic feet of soil by hand is a solid weekend for one person in average soil, and a trencher rental runs a few hours for the same cut. Hiring the dig out is usually the only part worth pricing against your own time.

Common mistakes, and how to avoid them

Most failed French drains fail for one of four reasons.

• No slope. The pipe looks level and the water sits. Fix: string line plus line level, and check the fall every few feet as you dig.
• No outlet. The drain ends somewhere lower than it started, but the water has nowhere to go. Fix: confirm the daylight point or dry well before digging, not after.
• No fabric. Soil washes into the gravel within a season or two and the drain clogs. Fix: line the trench, fold the fabric over the gravel top, lap the seams.
• Holes up on a yard drain. The trench fills before the pipe carries anything. Fix: set perforations down at 4 and 8 o'clock unless local rules say otherwise.

Two smaller ones are worth naming. Do not route roof downspouts into a yard drain meant for groundwater unless the pipe is sized for storm bursts, and do not plant trees over the line, because roots find the gravel voids and clog them within a few years.

This page is a planning estimate for a straightforward yard drain. Sites with structural water problems, steep slopes or permitting questions need a drainage contractor or your local building department before you dig.