How to use this calculator
- Enter Drain length in ft.
- Enter Trench width in in.
- Enter Trench depth in in.
- Press Calculate. The highlighted row is your order quantity; the note beneath it translates the number into real-world terms.
Formula & constants
Trench spec: 12 in wide × 18–24 in deep, wrapped in non-woven fabric, 4 in perforated pipe at the base, clean ¾ in gravel around it. Gravel = trench volume minus pipe volume, ~105 lb/ft³. Every default in the fields above is editable — when your supplier’s spec sheet lists a different density, coverage rate or price, enter that number instead. The waste and compaction allowances follow common trade practice; see our methodology page for the derivation of each formula and the standards it references.
Coverage per ton at 105 lb/ft³
| Depth | 1 ton covers | Tons per 1,000 sq ft |
|---|---|---|
| 2 in | 114 sq ft | 8.8 tons |
| 3 in | 76 sq ft | 13.2 tons |
| 4 in | 57 sq ft | 17.5 tons |
| 6 in | 38 sq ft | 26.3 tons |
Where this fits in the project
Hardscape follows the same layer logic as paving: excavate, place compacted base and bedding sand, then set surface units. Bulk organics like mulch and topsoil are ordered by the cubic yard.
How this connects to your project
No quantity stands alone — each material in the chain depends on the ones around it. These are the planning steps that pair with the french drain calculator:
- Gravel Calculator — French drains are gravel-filled trenches.
- Trench Calculator — French drains are installed in excavated trenches.
- Excavation Calculator — Excavate a trench for French drain installation.
- Gutter Calculator — Downspouts may discharge into French drains.
- Retaining Wall Calculator — French drains relieve water pressure behind retaining walls.
- Rip Rap Calculator — Rip rap is surface drainage; French drain is subsurface.
Frequently asked questions
What slope does a french drain need?
1% minimum — 1 in of fall per 8 ft of run — continuously toward the outlet. No flat spots.
Why clean gravel, not crusher run?
Fines in crusher run clog the voids the drain depends on. Use washed ¾ in stone only.
Last verified: — prices reflect current national averages. See our methodology.
Estimates are for planning. Material densities, coverage rates and prices vary by region and supplier — confirm final quantities before ordering, and check local code where structural or electrical work is involved.