How Much Gravel for a French Drain?

French drain gravel volume depends on your trench dimensions and pipe size. The standard residential French drain uses about 1 cubic yard of clean stone per 75–100 linear feet of trench.

The formula

Volume (cu yd) = Trench width (ft) × Trench depth (ft) × Length (ft) ÷ 27

Subtract the pipe volume for a precise result (usually less than 5% of total — safe to ignore).

Standard trench dimensions

Trench type	Width	Depth	Gravel per 100 lin ft
Narrow residential	8”	16”	0.62 cu yd
Standard residential	12”	18”	0.83 cu yd
Wide residential	12”	24”	1.11 cu yd
Heavy-duty	18”	24”	1.67 cu yd

Gravel needed by drain length

Assuming standard 12” × 18” trench:

Drain length	Cubic yards	Tons (1.40 t/cu yd)
25 ft	0.21	0.29
50 ft	0.42	0.58
75 ft	0.62	0.87
100 ft	0.83	1.16
150 ft	1.25	1.75
200 ft	1.67	2.33

Best gravel types for French drains

Stone type	Description	Rating
#57 crushed stone	¾” angular — best drainage, widely available	★★★★★
¾” clean crushed stone	Similar to #57	★★★★★
1” clean stone	Good for large pipes or heavy flow	★★★★
Pea gravel	Round — allows fines in, avoid if possible	★★★
River rock	Decorative surface layer only	★★
Stone dust / crusher run	Clogs quickly — do NOT use	✗

Installation tips

Slope the trench — Minimum ½” drop per 8 feet toward the outlet point
Line with fabric first — Unroll geotextile filter fabric into the trench before adding gravel
Fill gravel around pipe — Cover pipe with 3–4” of gravel on all sides
Fold fabric over top — Wrap the fabric over the gravel, then backfill with soil
Mark the outlet — Install a pop-up emitter or daylight outlet to prevent backflow

Frequently Asked Questions

How much gravel does a French drain need?

A typical 4-inch pipe French drain in a 12-inch wide × 18-inch deep trench needs about 1.1 cubic yards of gravel per 100 linear feet. Use washed clean stone (¾" or #57 stone).

What type of gravel is best for a French drain?

Use washed, angular clean stone — ¾" crushed stone or #57 stone are the most common. Avoid pea gravel (too round, allows fines to migrate) and stone dust (clogs quickly).

How deep should a French drain be?

Residential French drains are typically 18–24 inches deep and 12 inches wide. The pipe sits at the bottom surrounded by gravel. Slope the trench at least ½ inch per 8 feet toward the outlet.

Do I need filter fabric for a French drain?

Yes — line the trench with landscape filter fabric before filling with gravel. This prevents soil from migrating into the gravel and clogging the drain over time. Wrap the fabric around the pipe and gravel.

How long does a French drain last?

A properly installed French drain with filter fabric lasts 30–40 years. Without fabric or with fine-grain gravel, expect 5–15 years before clogging becomes an issue.