French Drain Calculator

A French drain calculator returns the gravel volume, perforated pipe length, and filter fabric area for a drainage trench. It multiplies length, width, and depth for the trench volume, subtracts the pipe's own volume, and converts the rest to cubic yards. Filter fabric area is the trench's cross-section perimeter times its length.

Last reviewed September 2026Method & sources Report an error

Not to scale.

Gravel Needed

2.75 cu yd

Shopping list

Gravel2.75 cu yd
Filter Fabric250 sq ft
Perforated Pipe50 ft

Breakdown
Gravel Needed	2.75 cu yd
Filter Fabric	250.0 sq ft
Perforated Pipe	50 ft

How to Calculate French Drain

A French drain trench's raw volume (`length × width × depth`, converted to consistent units) isn't all gravel. The perforated pipe running through it displaces its own cylindrical volume, so that's subtracted out first. The remaining gravel volume is converted to cubic yards and rounded up to the nearest common quarter-yard increment. Filter fabric wraps around the trench's cross-section (bottom, both sides, and typically folded over the top before backfilling with topsoil) to keep soil from clogging the gravel over time. Its area is based on the trench's perimeter cross-section times its length.

Example: 50 ft trench, 12 in. wide, 18 in. deep, with 4 in. perforated pipe

A 50 ft trench, 12 in. wide, 18 in. deep, with 4 in. perforated pipe: 1. Trench volume: 50 × (12 ÷ 12) × (18 ÷ 12) = 50 × 1 × 1.5 = 75 cu ft 2. Pipe volume: π × (4 ÷ 12 ÷ 2)² × 50 ≈ π × 0.0278 × 50 ≈ 4.36 cu ft 3. Gravel volume: 75 − 4.36 ≈ 70.64 cu ft ÷ 27 ≈ 2.62 cu yd, rounded up to 2.75 cu yd 4. Fabric area: 50 × (2 × 1 + 2 × 1.5) = 50 × 5 = 250 sq ft. That trench needs about 2.75 cu yd of gravel, 250 sq ft of filter fabric, and 50 ft of perforated pipe.

French Drain: Compare Scenarios

Scenario	Gravel Needed	Filter Fabric	Perforated Pipe
Small (20 ft trench, 12 in. wide, 12 in. deep)	0.75 cu yd	80.0 sq ft	20 ft
Medium (50 ft trench, 12 in. wide, 18 in. deep)	2.75 cu yd	250.0 sq ft	50 ft
Large (100 ft trench, 18 in. wide, 24 in. deep)	11.00 cu yd	700.0 sq ft	100 ft

What Is French Drain?

A French drain calculator converts a drainage trench's dimensions into the gravel, perforated pipe, and filter fabric needed to build it. It is a common DIY solution for redirecting water away from a foundation, low spot, or chronically wet area in a yard.

French Drain: Pros and Cons

Pros

Subtracts the pipe's own volume from the gravel figure, rather than treating the trench as if it were solid gravel
Rounds gravel to the quarter-yard increments bulk material is commonly ordered in
Works for any trench length, width, and depth combination

Cons

Doesn't determine the trench's slope (French drains need a consistent downhill grade, commonly at least 1% or about 1/8 in. per foot, to actually drain). See the Elevation Grade calculator to check a planned slope
Doesn't size the drain's outlet or daylight discharge point. Where the collected water actually exits needs its own planning separate from the trench itself
Assumes a simple, single-depth trench. A trench that changes depth or width along its run needs each section calculated separately

French Drain: What a Pro Checks

Grade the trench downhill toward its outlet at a consistent slope, commonly at least 1% (about 1/8 in. of fall per foot). A French drain moves water by gravity alone, so a flat or reverse-sloped run won't drain. Check your planned slope with the Elevation Grade calculator.
Wrap the gravel in filter fabric rather than skipping it. It keeps surrounding soil from migrating into the gravel and clogging the drain's ability to move water over time.

Frequently Asked Questions

How much gravel do I need for a French drain?

Find the trench's total volume (length × width × depth), subtract the pipe's own volume, then convert to cubic yards. A 50 ft trench, 12 in. wide and 18 in. deep with 4 in. pipe, needs about 2.75 cu yd of gravel.

What size pipe should I use for a French drain?

4 in. perforated pipe is common for a typical residential drainage trench. 6 in. is used for a higher water volume or a longer run where more capacity is needed.

Do I need filter fabric for a French drain?

Yes, it's strongly recommended. Filter fabric wrapped around the gravel keeps surrounding soil from migrating into the gravel and clogging the drain's ability to move water over time.

How deep should a French drain trench be?

Commonly 12-24 in. deep, though it depends on what you're draining (surface water vs. a foundation footing drain) and your local frost depth if the drain needs to stay functional through freezing conditions.

Does a French drain need to slope?

Yes. Gravity does the work, so the trench needs a consistent downhill grade toward its outlet, commonly at least 1% (about 1/8 in. of fall per foot of length). Check your planned slope with the Elevation Grade calculator.

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