French Drain Gravel Calculator: Stone, Pipe and Cost

French Drain Gravel Calculator

Estimate drainage gravel after subtracting pipe displacement, plus fabric, pipe, bags, tons and cost.

Soil, sod or other cover above the gravel envelope.
Gravel, fabric, pipe and cost

Replace all example product and price values with supplier data.

Gravel to order—
—Net gravel before allowance
—Bulk gravel weight
—Whole bags
—Bag / bulk project cost
—Pipe to purchase
—Minimum unfolded fabric width
—Fabric area with allowance
—Trench excavation
—Top backfill volume

How to calculate French drain gravel

Enter finished trench dimensions and any non-gravel cover at the top. When the pipe option is selected, the calculator subtracts the pipe’s outside cylindrical volume from the gravel envelope. Use outside diameter, not nominal inside diameter.

Formulas

Gravel-envelope depth = trench depth − top backfill. Envelope volume = length × trench width × gravel depth × run count. Pipe displacement = π × outside diameter² ÷ 4 × length × run count. Net gravel = envelope volume − pipe displacement. Order volume = net gravel × (1 + allowance ÷ 100).

Minimum unfolded fabric width = trench width + 2 × gravel depth + top overlap. Fabric area multiplies that width by total run length and the fabric allowance.

Checked example

A 50 ft trench, 12 in wide and 24 in deep, with 6 in of top backfill and one 4 in outside-diameter pipe has 75 ft³ in its gravel envelope. The pipe displaces about 4.363 ft³, leaving 70.637 ft³. With 10% extra, order about 77.700 ft³ = 2.878 yd³, or 156 bags at 0.50 ft³ each.

Important limits

  • This calculator estimates quantities; it does not design drainage capacity, pipe diameter, trench slope, outlet, cleanouts, geotextile specification or aggregate gradation.
  • The pipe is assumed straight, centered and fully inside the gravel envelope. Fittings, catch basins and structures are not displaced automatically.
  • The fabric width represents the trench bottom, two gravel-height sides and the entered top overlap. Installation details may require more material.
  • Loose density varies with stone size, source, moisture and compaction. Supplier weight and coverage data take precedence.
  • Verify underground utilities, excavation safety, permits, discharge rules, delivery minimums, access and local design requirements.

Related calculators

Use the Gravel Calculator for general aggregate layers, the Volume Calculator for other shapes, or the Square Footage Calculator for fabric areas.