French Drain Calculator

Work out exactly how much gravel you need for a French drain. Accounts for drainage pipe displacement so you don't over-order.

Trench Length

Trench Width

Trench Depth

Gravel Type

Include drainage pipe

Pipe Diameter

Gravel Volume

1.70m³

Weight

2.73tonnes

Bulk Bags (850kg)

4bags

Small Bags (20kg)

137bags

Pipe Displacement

0.095m³

Trench Volume

1.80m³

10m trench at 300mm wide × 600mm deep with 110mm pipe = 1.70 m³ of Grey Granite, weighing approximately 2.73 tonnes.

Standard French Drain Dimensions

Recommended trench sizes for common UK drainage scenarios. All dimensions assume clean 20mm drainage aggregate with no fines.

Application	Width	Depth	Pipe Size	Gravel per metre
Light garden drainage	200mm	450mm	80mm	≈ 0.09 m³
Standard garden drain	300mm	600mm	110mm	≈ 0.17 m³
Waterlogged lawn	300mm	750mm	110mm	≈ 0.21 m³
Driveway edge drain	450mm	600mm	110mm	≈ 0.26 m³

Worked Examples

Garden French Drain

10m trench, 300mm wide × 600mm deep, with 110mm pipe. Using 20mm clean stone at 1,600 kg/m³.

Trench volume: 10 × 0.3 × 0.6 = 1.80 m³
Pipe volume: π × 0.055² × 10 = 0.095 m³
Gravel volume: 1.80 − 0.095 = 1.705 m³
Weight: 1.705 × 1.6 = 2.73 tonnes

Short Rubble Drain (No Pipe)

4m trench, 200mm wide × 450mm deep, gravel only. Using 20mm clean stone at 1,600 kg/m³.

Volume: 4 × 0.2 × 0.45 = 0.36 m³
No pipe displacement
Weight: 0.36 × 1.6 = 0.58 tonnes
That's 1 bulk bag (850kg) with some spare

What Is a French Drain?

A French drain is a simple, time-tested method of managing water in your garden. At its core, it is a gravel-filled trench — sometimes with a perforated pipe at the base — that intercepts and redirects surface or subsurface water away from problem areas. The concept has been used in land drainage for centuries, and it remains one of the most effective and affordable solutions for waterlogged lawns, soggy borders, and driveways that puddle after rain.

In the UK, French drains are particularly useful on clay-heavy soils that drain poorly. They work by providing a path of least resistance for water: instead of sitting in the heavy soil, groundwater flows sideways into the gravel-filled trench and drains away to a soakaway, ditch, or storm drain. The key to a functioning French drain is using the right gravel — clean, single-sized stone with no fines — so the spaces between the stones remain open for water to pass through freely.

Choosing the Right Gravel

The single most important decision for a French drain is the aggregate you fill it with. You need “clean” gravel — meaning stone that has been washed and graded to a single size with no dust, sand, or fines. In the UK, 20mm clean stone is the standard specification. The uniform size creates consistent voids between the stones, and those voids are what give the drain its flow capacity.

Avoid using MOT Type 1, builder's ballast, or any aggregate described as “graded” or “as-dug”. These materials contain fine particles that pack into the gaps and drastically reduce water flow. A drain filled with the wrong material may work initially but will fail within a few years as the fines compact and block the water pathways. It is worth paying slightly more for properly washed, single-sized stone — the long-term performance difference is enormous.

Installation Overview

Start by marking out your trench line, ensuring it follows the natural fall of the ground or has a consistent gradient of at least 1:100 towards the outlet point. Dig the trench to your target width and depth, keeping the sides as straight as possible. Line the entire trench with geotextile membrane, leaving enough excess on each side to fold over the top later.

If using a pipe, lay a 50mm bed of gravel along the trench floor, then place the perforated pipe on top (holes facing downward) and surround it with gravel to within 100–150mm of the surface. Fold the geotextile over the top of the gravel, then backfill with topsoil and turf. The membrane prevents silt from entering the drain from above while still allowing water to percolate through from the sides.

Pipe Sizing and Flow Rates

For most residential French drains, a 110mm perforated twinwall pipe is the standard choice. This provides ample capacity for garden drainage, handling around 3.5 litres per second at a 1:100 gradient. Smaller 80mm pipe is adequate for short runs under 5 metres or very light flow. The twinwall construction (corrugated outside, smooth bore inside) combines flexibility with good flow characteristics.

When using a pipe, note that it displaces some gravel — our calculator accounts for this automatically. A 110mm pipe running through a 300mm × 600mm trench displaces roughly 5% of the total trench volume, so the gravel saving is modest but worth calculating accurately to avoid over-ordering, especially on long runs.

When You Need a French Drain

Common signs that your garden would benefit from a French drain include persistent standing water after rain, a waterlogged lawn that stays boggy for days, damp patches at the base of walls, and surface water running off a slope towards your property. French drains are also frequently used alongside retaining walls to relieve hydrostatic pressure, and at the edges of driveways to prevent water pooling on hard surfaces.

If your problem is primarily surface water from a hard surface like a patio or driveway, a channel drain (also called a linear drain or ACO drain) may be more appropriate. French drains excel at managing groundwater and diffuse surface water from lawns and planted areas. For severe waterlogging across a large area, consider a herringbone pattern of multiple French drains feeding into a central collector pipe.

Need Gravel for Other Projects?

Use our main calculator to work out gravel quantities for driveways, paths, patios, and decorative areas.

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Frequently Asked Questions

What size gravel is best for a French drain?

20mm clean stone (also called 20mm drainage aggregate or 20mm gravel) is the standard choice for French drains in the UK. It must be single-sized with no fines — the gaps between the stones are what allow water to flow freely through the drain. Avoid using gravel with dust or mixed sizes, as the fines will clog the drain over time.

How deep should a French drain be?

A typical French drain for garden drainage should be 450–600mm deep. For drains that intercept surface water runoff, 450mm is usually sufficient. For drains intended to lower the water table or handle significant groundwater, dig to at least 600mm. In all cases the trench should slope at a minimum gradient of 1:100 (1cm drop per metre) towards the outlet.

How wide should a French drain trench be?

Standard French drain trenches are 200–300mm wide. A 300mm (12-inch) width is most common and gives good drainage capacity while being easy to dig by hand. Wider trenches (up to 450mm) are used for commercial or heavy-flow applications but require substantially more gravel.

Do I need a pipe in a French drain?

Not always. A simple rubble drain (gravel-only) works well for short runs and light surface water. However, a perforated pipe at the base significantly increases flow capacity and is recommended for drains longer than 5 metres, or wherever you need to move large volumes of water. Use 80mm or 110mm perforated twinwall pipe wrapped in a geotextile filter sock.

What slope does a French drain need?

The minimum recommended gradient is 1:100 — meaning the trench drops 1cm for every 1m of length. For a 10m French drain, that means the outlet end should be at least 100mm lower than the start. A steeper gradient (1:80 or 1:60) improves flow rate and reduces the risk of silting.

How much does French drain gravel cost in the UK?

20mm clean drainage gravel typically costs £35–£55 per tonne for loose/tipped delivery, or £55–£75 per bulk bag (850kg). For a typical 10m garden French drain at 300mm × 600mm, you will need roughly 1.5–1.7 tonnes of gravel, costing between £55 and £95 for loose delivery.

Should I use a geotextile membrane in a French drain?

Yes — wrapping the trench in geotextile fabric before adding gravel is strongly recommended. The membrane lets water through but stops silt and soil particles from clogging the gravel over time. Line the sides and bottom of the trench, fill with gravel, and fold the membrane over the top before backfilling with soil.

How long does a French drain last?

A well-installed French drain with geotextile membrane and clean, single-sized gravel should last 20–30 years or more. Drains without membrane protection tend to silt up within 5–10 years. The most common cause of failure is using the wrong type of gravel — any material with fines will clog the drain much faster.

Related Guides

How to Lay Gravel — full installation guide including sub-base
Sub-Base Calculator — calculate MOT Type 1 for your sub-base layer
Gravel Depth Guide — recommended depths for every project type
Gravel on a Slope — drainage considerations for sloped areas

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