French Drain Calculator

A French drain is a gravel-filled trench, usually with a perforated pipe near its floor, that gives surface water and high groundwater somewhere to go. This page is the shopping list for that trench: how much gravel the excavation needs once the pipe has taken its share, how many whole pipe pieces the run requires, how much filter fabric a chosen wrap uses, and what those materials cost at prices you supply.

The quantities and costs are read-only: they are answers, not inputs. You describe the trench you have or the one you are about to dig, and everything downstream follows. That direction is deliberate — the page is built to price a drain you have measured, not to shrink or stretch a drain until it meets a budget.

It is worth deciding early whether the trench gets a perforated pipe, because that single choice changes the gravel figure. A pipeless trench is filled with gravel from floor to top; a piped trench subtracts the space the pipe occupies. The same trench can therefore need noticeably different amounts of stone depending on one checkbox.

Work through the boxes in the order they appear. Each one keeps its own unit, so a single calculation can mix feet for the run, inches for the width and a cubic yard price without any conversion on your side.

A complete example, end to end

Take a trench 10 inches wide, 12 inches deep and 20 feet long, holding a 4″ Schedule 40 pipe. Wrap the whole trench in fabric with a 1-inch overlap, keep the 10% wastage, leave the density at 1,680 kg/m³ and enter 40 per cubic metre of gravel, 12.50 per pipe piece and 5 per square metre of fabric. Set the width, depth and overlap boxes to inches, the length to feet, the volumes to cubic feet, the fabric width to inches and the fabric area to square feet.

A 4″ Schedule 40 pipe falls in the 0.125 inch-per-foot slope band, so the run drops 2.5 inches and the pipe itself measures 20.0011 ft (6.09633 m) along the slope. Its outside diameter displaces 2.20905 ft³ of the excavation, which leaves 14.4576 ft³ for gravel. The 10% allowance takes that to 15.9034 ft³, and at 1,680 kg/m³ the order weighs 1,667.93 lb, or 756.56 kg.

With those prices the three costs come out at 18.01 for the gravel (charged on the wastage-inclusive 15.9034 ft³, not the neat 14.4576 ft³), 37.50 for three pipe pieces and 34.84 for the 75 ft² of fabric. Untick the pipe and the same trench needs the full 16.6667 ft³ of gravel, 18.3333 ft³ with wastage, weighing 1,922.78 lb (872.159 kg).

Read the numbers in that order: the total gravel volume is what you order, the whole-pipe count is what you collect, and the fabric area is what a roll has to cover. A supplier who sells by weight needs the weight, not the volume, which is why both appear.

Every quantity comes from the trench you described and the pipe you selected. This section shows the arithmetic so you can sanity-check a result, or reproduce it by hand when a supplier asks how you arrived at a figure.

Excavation

$w$ is the trench width, $d$ its depth and $L$ its length. The boxes may mix units — 10 inches, 12 inches and 20 feet in one calculation is fine, because each box is converted before the three are multiplied.

Recommended slope and its drop

Small pipes need a steeper fall to carry the same water, so the page applies a minimum slope in inches per foot that depends on the pipe size and on the schedule you chose. The two schedules do not switch bands at exactly the same diameter: SDR35 reaches the shallowest band at its 8-inch pipe, while Schedule 40 keeps the middle band through its 8-inch size.

Dividing inches per foot by 12 turns the slope into a plain fraction of the run, so the drop comes out in the same unit as the length. On the 20 ft example that is 20 × 0.125 / 12 = 0.2083 ft, or 2.5 inches. Treat the recommended grade as a starting point: shallow grades are easier to dig but less forgiving of settlement, and local drainage practice or the outlet arrangement may call for more.

Pipe length, gravel and weight

The pipe follows the slope, so it is the hypotenuse of the run and its drop. The extra is tiny at drainage grades — a fifth of a foot on a twenty-foot run — but it explains why the pipe length printed is never shorter than the trench.

$D_{\mathrm{o}}$ is the outside diameter of the pipe, not its nominal trade size, and the formula treats the pipe as a smooth cylinder. That cylinder is the volume of gravel the pipe takes the place of.

$u$ is 1 when the trench holds a perforated pipe and 0 when it does not, so a gravel-only trench keeps the whole excavation. Then the wastage allowance is added and the weight follows from the bulk density:

$\phi$ is the wastage written as a fraction, so 10% enters as 0.10, and $\rho$ is the bulk density of the loose gravel. The default, 1,680 kg/m³, is a common figure for washed gravel and reads as roughly 105 lb/ft³; your supplier’s bulk density is the number to use if they quote one.

Whole pipes, and why the count rounds up

$L_{\mathrm{piece}}$ is the length of one piece on the shelf. 6.09633 m of sloped pipe over 3 m pieces is 2.03 pieces, which becomes 3 whole pieces: offcuts cannot be stretched end to end, so the page never rounds down. The standard choices are labelled in feet and metres, and the metric figure in the label is the length the page uses — the “10 ft (3 m)” option is treated as exactly 3 m.

Costs, and the two gravel prices

A gravel supplier may quote by volume or by weight. Entering either price fills in the other through the density, because a cubic metre of gravel at 1,680 kg/m³ weighs 1.68 tonnes:

On the worked example the volume price of 40 per cubic metre reads as 0.0238095 per kilogram, and the cost is charged on the wastage-inclusive volume. Filter fabric is priced the same way, by area, once you have chosen a wrap.

Filter fabric keeps fine soil from washing into the gravel and blinding the pipe. The page offers three ways to place it, and they use very different amounts of material for the same trench. Each wrap has its own width formula.

$o$ is the overlap you enter, and it is added once. The one-inch (2.54 cm) minimum reflects the practical point that butted edges can open up as the backfill settles.

Here is what the three wraps cost in material on the example trench — 10 in wide, 12 in deep, 20 ft long, 4″ pipe, 1 in of overlap, fabric at 5 per square metre:

Fabric is sold by area on a roll of some fixed width. A 75 ft² requirement can come from a 3 ft roll cut into 25 ft lengths, or a 5 ft roll cut into 15 ft lengths; the cutting plan is a separate question from the total area, so the page quotes the area and the width you would need, and leaves roll choice to you.

One more habit worth forming: order the pipe before the stone. A size change moves the displaced volume, the slope band and the fabric sleeve together, so a quick recalculation after the pipe arrives is cheaper than discovering the mismatch on site.