French Drain Calculator
Estimate drain rock and pipe for a french drain trench, subtracting the pipe volume.
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What this tool does
Estimate drain rock and pipe for a french drain trench, subtracting the pipe volume. Washed ¾ in drain rock at ≈2,600 lb/yd³ (1.3 tons). The trench is assumed fully gravel-filled around the pipe (no separate soil backfill layer deducted).
How to use the French Drain Calculator
- Enter or select trench length in ft.
- Enter or select trench width in in.
- Enter or select trench depth in in.
- Enter or select drain pipe diameter in in.
- Read the calculated result; change any measurement to compare alternatives.
Formula
gravel yd³ = (trench volume − pipe volume) / 27; pipe volume = π×(pipeD/24)²×length; tons = yd³ × 1.3 (washed drain rock ≈ 2,600 lb/yd³)
- trenchL
- Trench length (ft, before metric conversion)
- trenchW
- Trench width (in, before metric conversion)
- trenchD
- Trench depth (in, before metric conversion)
- pipeD
- Drain pipe diameter (in, before metric conversion)
Washed ¾ in drain rock at ≈2,600 lb/yd³ (1.3 tons). The trench is assumed fully gravel-filled around the pipe (no separate soil backfill layer deducted).
Worked example
For french drain calculator, the following measurements illustrate the exact method: Trench length: 50; Trench width: 12; Trench depth: 18; Drain pipe diameter: 4.
Inputs
- Trench length50 ft
- Trench width12 in
- Trench depth18 in
- Drain pipe diameter4 in
Result
- Drain gravel2.62 cu yd
- Drain gravel weight3.4 US tons
- Pipe length50 ft
- Trench excavation2.78 cu yd
Results explained
- Drain gravel
- Drain gravel from the formula above, expressed in yd3 before metric conversion. Washed ¾ in drain rock at ≈2,600 lb/yd³ (1.3 tons). The trench is assumed fully gravel-filled around the pipe (no separate soil backfill layer deducted).
- Drain gravel weight
- Drain gravel weight from the formula above, expressed in ton before metric conversion. Washed ¾ in drain rock at ≈2,600 lb/yd³ (1.3 tons). The trench is assumed fully gravel-filled around the pipe (no separate soil backfill layer deducted).
- Pipe length
- Pipe length from the formula above, expressed in ft before metric conversion. Washed ¾ in drain rock at ≈2,600 lb/yd³ (1.3 tons). The trench is assumed fully gravel-filled around the pipe (no separate soil backfill layer deducted).
- Trench excavation
- Trench excavation from the formula above, expressed in yd3 before metric conversion. Washed ¾ in drain rock at ≈2,600 lb/yd³ (1.3 tons). The trench is assumed fully gravel-filled around the pipe (no separate soil backfill layer deducted).
Frequently asked questions
A 12 in wide × 18 in deep trench holds about 75 cu ft; after the 4 in pipe displaces a little, roughly 2.7 yd³ (about 3.5 tons) of drain rock.
Commonly 12 in wide and 18–24 in deep with the pipe on a couple of inches of gravel and at least 1% slope (1 in drop per 8 ft) toward the outlet.
Washed, angular ¾ in drain rock. Pea gravel and stone with fines mixed in clog and drain slower; wrap the trench in non-woven filter fabric, not the pipe alone.
A 4 in pipe displaces only about 0.09 cu ft per foot — under 5% of a typical trench — but the tool subtracts it anyway.
To daylight at a lower point, a dry well, or a storm system where legal — never onto a neighbour's property or into a septic system.