How much gravel do I need for a French drain?

Multiply trench length by trench width by gravel depth to get trench volume, subtract the volume taken up by the drain pipe if you want a closer estimate, then add a waste allowance. This calculator converts that result to cubic yards, tons, and bag counts.

What size gravel is best for a French drain?

Many French drains use clean, washed drainage stone around 3/4 inch to 1-1/2 inches. Avoid fine material that can compact and slow water movement. Local soil, pipe, and code requirements can change the best choice.

How much slope should a French drain have?

A common planning target is about 1% slope, equal to 1 foot of drop for every 100 feet of run. Some projects use less or more depending on the outlet and site conditions.

Drainage project calculator

French Drain Gravel Calculator

Estimate the drainage rock, filter fabric, slope drop, bags, tons, and material cost for a French drain before you start digging.

Gravel needed1.31 cu ydAbout 1.84 tons with 10% waste.

Trench dimensions

Drain length (feet)Measure the full trench run to the outlet. Trench width (inches) Gravel depth (inches)Use the rock-filled depth, not just pipe diameter.

Pipe outside diameter (inches)Set to 0 for a rock-only drain. Waste allowance (%) Target slope (%)

Rock, bags, and fabric

Stone density (tons / cu yd)Washed drainage rock often estimates near 1.3–1.5 tons per cubic yard. Bag size (cu ft) Price per ton ($)

Fabric overlap (inches) Fabric roll width (feet) Pipe price per foot ($)

How to use this French drain calculator

Enter the trench length, width, and planned gravel depth. The calculator estimates the rock-filled trench volume, subtracts the drain pipe volume, adds your waste allowance, and converts the result into cubic yards, tons, and bag count.

The fabric estimate assumes enough landscape fabric to line the bottom, both trench walls, and overlap across the top. For real projects, round up for curves, uneven digging, and extra fabric at the outlet.

Planning tips

Use clean drainage stone so water can move freely through the trench.
Keep fine soil out of the gravel with non-woven filter fabric where appropriate.
Confirm the drain has a safe outlet before buying material.
Round up stone orders; delivery and compaction can make exact math feel stingy in the dirt goblin zone.

Frequently asked questions

How deep should gravel be around a French drain pipe?

Many designs surround the pipe with several inches of clean stone above, below, and beside the pipe. The exact depth depends on water volume, soil, trench depth, and local requirements.

Should the pipe volume be subtracted?

Subtracting pipe volume gives a closer material estimate. It is still smart to keep a waste allowance because trenches are rarely perfectly rectangular.

Can I use bagged gravel instead of bulk stone?

Small drains can use bagged gravel, but larger runs usually become cheaper and easier with bulk stone delivery. Compare the bag count against local bulk pricing.