What slope does a French drain need?

A minimum fall of about 1/8 inch per foot of run (roughly 1%) is the standard trade guideline for gravity drainage, which is why this calculator defaults to that figure. Steeper is fine and drains faster, but the trench needs to end up correspondingly deeper at the outlet end to maintain a consistent slope over a long run.

Should the holes in the perforated pipe face up or down?

For a standard French drain collecting water from the surrounding soil (the most common residential use), the holes generally face down, so water enters the pipe from below once the surrounding gravel is saturated and the pipe fills, while sediment has less direct entry from above. Some installers and manufacturers recommend holes-up for specific drainage scenarios, so check the pipe manufacturer's guidance for your exact situation.

What is "ag pipe" and is it the same as a French drain pipe?

"Ag pipe" (agricultural pipe) is the common Australian trade name for slotted or perforated drainage pipe, often sold pre-wrapped in a fabric "sock" to keep soil out of the slots. It is the standard pipe used to build a French drain in Australia, and this calculator's pipe-size presets apply equally whether you call the finished trench a French drain or an ag drain.

Do I need council approval for a French drain in Australia?

A simple garden drain on your own property generally does not need council approval, but stormwater discharging onto a neighbouring property or into the street can raise a boundary or stormwater-management issue depending on your local council's rules. Check with your local council before directing an outlet anywhere outside your own block.

French Drain Calculator | Gravel, Pipe & Fabric Quantities

How much gravel and pipe do you need for a French drain?

A French drain is a gravel-filled trench with a perforated pipe running through it, built to carry water away from a soggy lawn, a wet basement wall, or a low spot that never quite dries out. Getting the order right means working out three separate quantities from one trench: how much gravel backfill you need once the pipe itself has displaced some of that space, how long a run of perforated pipe to buy given the slope you are digging to, and how much filter fabric to wrap around it. This calculator takes your trench width, depth and length plus your chosen pipe size and slope, and returns all three figures together with a total material cost.

The part most hand calculations get wrong is treating gravel volume as if it simply equals the trench volume. It does not — the pipe running through the trench takes up real space, so the gravel backfill is the trench volume minus the volume of the pipe itself. Skip that subtraction on a long run and you will over-order gravel by a noticeable margin. This tool does the subtraction for you, along with the slightly longer pipe run length a sloped trench actually needs compared to its flat, straight-line measurement.

How to use the French drain calculator

Enter the trench width, depth and length you plan to dig — width and depth are usually set by your digging equipment and the pipe size, while length is the distance the water needs to travel to a safe outlet.
Set the slope as a drop per unit of run (the calculator defaults to 1/8 inch of fall per foot, a standard minimum for gravity drainage) — steeper slopes drain faster but need a deeper trench at the outlet end.
Choose a pipe size and the standard stick length it is sold in, so the calculator can round the pipe run up to a whole number of pieces rather than leaving you short by a few feet.
Pick a gravel backfill type — standard drain rock, pea gravel, or angular crushed stone — each with its own bulk density, and set a waste allowance to cover spillage and an imperfect trench profile.
Toggle filter fabric on if you are wrapping the gravel (recommended on most soils) to get an estimated fabric area alongside the gravel and pipe figures, then read off the total material cost.

The French drain calculation, step by step

Start with the trench volume, a simple box: Trench volume = width x depth x length. Then work out the pipe volume using the pipe's actual outside diameter as a cylinder: Pipe volume = pi x radius^2 x pipe run length (on the shallow slopes a French drain typically uses, this run length is very close to the trench length itself). Subtract the pipe volume from the trench volume to get the gravel backfill volume: Gravel volume = Trench volume - Pipe volume. Add your waste allowance to that gravel figure, then multiply by the gravel's bulk density to get the weight you need to order.

Worked example: a trench 0.3m wide, 0.5m deep and 10m long, using a 4-inch nominal pipe (114.3mm outside diameter, so a radius of 0.05715m) and standard drain rock at 1,680 kg/m³. Trench volume = 0.3 x 0.5 x 10 = 1.5 m³. Pipe volume = pi x 0.05715^2 x 10 ≈ 0.10 m³ (the pipe run length is close enough to the 10m trench length at this slope to use directly). Gravel volume = 1.5 - 0.10 = 1.4 m³. Adding a 10% waste allowance: 1.4 x 1.1 = 1.54 m³. Weight = 1.54 x 1,680 ≈ 2,587 kg, or about 2.6 tonnes of drain rock to order for this trench.

Pro tips and common mistakes

At a typical Australian price of around AUD 55 per tonne, the worked example on this page — roughly 2.6 tonnes of drainage aggregate for a 10m trench after waste — comes to about AUD 143 in aggregate alone, before ag pipe, sock and excavation cost.

Never lay a French drain flat. Water needs a continuous downhill slope to move through the pipe rather than pooling inside it — a minimum fall of around 1/8 inch per foot of run is the standard trade guideline this calculator defaults to, and a level or back-sloped section anywhere along the run will collect silt and stall the whole drain.
The outlet matters as much as the trench. A French drain needs somewhere for the water to actually go — daylighting to open ground well away from the house, or into a dry well or storm drain — piping water to a low spot right next to the foundation just moves the problem a few metres, it does not solve it.
Wrap the gravel in filter fabric on most soils. Fabric keeps fine soil particles from migrating into the gravel and clogging the voids that make the drain work; the main exception is very coarse, stable soil where some installers skip it, but on silty or clay-heavy ground it is what keeps the drain working for decades rather than a few seasons.
Angular crushed stone drains better than smooth pea gravel of the same nominal size, because angular fragments pack with larger connected voids for water to move through, while rounded stone settles more tightly. Either works, but if your local supplier offers both, angular stone is the better default for a drainage-specific trench.