French Drain Trench Gravel & Pipe Displacement Calculator
Estimate exact washed drainage stone (pea gravel or #57 rock) for French drain trenches—accounting for 4-inch corrugated or PVC pipe displacement.
Interactive Material & Cost Estimator
HomeYardMath.com — Hardware Store Shopping List
Quick Reference Coverage Chart (Gravel & Stone)
Instant pre-calculated material estimates for common US residential project sizes before compaction buffer:
| Typical US Project | Area | Depth | Volume | Weight | Store Bags |
|---|---|---|---|---|---|
| Short Downspout Drain (25 ft × 12" wide × 12" deep) | 25 linear ft | 12" Trench Depth | 0.83 cu yd net | 1.08 tons (2,160 lbs) | 45 bags (0.5 cu ft) |
| Standard Yard French Drain (50 ft × 12" wide × 18" deep) | 50 linear ft | 18" Trench Depth | 2.57 cu yd net | 3.35 tons (6,690 lbs) | 139 bags (Buy Bulk!) |
| Perimeter Curtain Drain (100 ft × 12" wide × 24" deep) | 100 linear ft | 24" Trench Depth | 7.00 cu yd net | 9.10 tons (18,200 lbs) | Bulk Dump Truck |
| Wide High-Capacity Trench (50 ft × 18" wide × 24" deep) | 50 linear ft | 24" Trench Depth | 5.35 cu yd net | 6.96 tons (13,910 lbs) | Bulk Delivery |
Engineering Formula & Material Density Constant
A 4-inch internal diameter perforated drain pipe (roughly 4.5 inches outer diameter) displaces approximately 0.11 cubic feet of gravel per linear foot of trench (or 5.5 cubic feet per 50-foot run). Washed #57 stone or river drainage rock averages 2,600 lbs (1.30 tons) per cubic yard.
How to Enter Trench Dimensions into the Calculator Above
To calculate a French drain using the rectangle tool above: enter your Total Trench Length in the Length (Feet) box, your Trench Width in Feet in the Width (Feet) box (for example, enter 1 for a 12-inch trench, or 1.5 for an 18-inch trench), and your Gravel Fill Depth in Inches in the Desired Depth box.
If you plan to cap the top 3 inches of your trench with screened topsoil and turf sod, subtract those 3 inches from your total trench depth so you only calculate the actual stone depth.
Best Gravel for French Drains: #57 Crushed Stone vs. Round River Rock
The single most critical rule of French drain construction is using 100% washed stone with zero rock dust. You have three proven aggregate options:
- 1.5-Inch Washed Round River Rock: Creates the largest internal void spaces (up to 42% porosity), allowing groundwater to surge through the trench rapidly without silting up.
- Washed #57 Crushed Stone (3/4"): The most affordable and widely available bulk drainage aggregate across the US.
- 3/8" Washed Pea Gravel: Ideal for narrow 6-to-8-inch surface trench drains, though if used with slotted pipe, ensure the pipe slots are narrower than 3/8 inch or wrap the pipe in a filter sock.
Trench Slope & Geotextile Burrito-Wrap Technique
A French drain relies on gravity. Always excavate your trench with a minimum downward slope of 1% (1.2 inches of drop per 10 linear feet), or 0.5% minimum on flat coastal lots. Line the raw dirt trench with 4 oz to 6 oz non-woven punched geotextile fabric (never use cheap woven weed barrier, which clogs with mud). Pour 2 inches of washed stone on the bottom, lay your perforated pipe holes-down at the 4 o'clock and 8 o'clock positions, fill with washed gravel to within 3 inches of grade, and fold the geotextile fabric over the top like a burrito before laying sod.
Frequently Asked Questions
How much gravel do I need for a 50-foot French drain?
A standard 50-foot French drain measuring 12 inches wide with 18 inches of gravel depth requires approximately 2.78 gross cubic yards (about 2.57 net cubic yards after subtracting a 4-inch pipe), which equals roughly 3.35 to 3.6 tons of washed stone.
How much volume does a 4-inch French drain pipe displace?
Every 100 linear feet of 4-inch corrugated pipe displaces approximately 11 cubic feet (0.41 cubic yards, or about half a ton) of gravel.
Should I put sock-wrapped pipe AND fabric around the trench?
Wrapping the outside of the entire gravel trench in non-woven geotextile fabric ("burrito wrap") is far more effective than putting a sock directly on the pipe, because it keeps dirt out of the entire stone reservoir.
How deep should a residential yard French drain be?
Most yard surface water French drains are dug 12 inches wide and 18 to 24 inches deep. Foundation footing drains must sit adjacent to the bottom of the concrete footer.