French Drain Calculator
Gravel yards and pipe feet from trench size.
What it does
French drains bury a sloped gravel trench with perforated pipe to carry soggy yard water somewhere better. Gravel math decides the delivery ticket, while slope decides whether the system works at all.
The tool above takes trench length, width, and depth. It returns gravel volume with a 10% extra order quantity and the pipe run, in cubic yards or cubic meters.
The defaults dig 50 ft at 12 in wide and 24 in deep: 100 cu ft gross, 3.7 cu yd of gravel, 4.07 cu yd ordered. The metric mirror (15.24 m, 30.5 cm, 61 cm) gives 2.84 cu m with 3.12 cu m ordered.
Volume math follows NIST unit factors with 27 cubic feet per cubic yard. Trench practice follows extension drainage guidance: lined gravel trench with pipe at the bottom sloping at least 1% to an outlet.
How to use
- Pick imperial (feet, inches) or metric.
- Enter trench length.
- Enter width and depth.
- Read gravel volume first.
- Order the 10% extra figure.
- Hold 1% slope from start to outlet.
Formula
- L
- Trench length in feet (or meters).
- W, D
- Width and depth, converted to feet (or meters).
- 27
- Cubic feet per cubic yard.
- 1.1
- Compaction and waste allowance.
Gross volume ignores pipe displacement, so the pipe steals a little gravel back and the waste allowance covers it. Foundation drains run to footing depth instead of the 18 to 24 in yard rule used in the defaults.
Worked example
Example: 50 ft, 12 in wide, 24 in deep
Defaults: imperial, 50 ft long, 12 in wide, 24 in deep.
- Section: 1 ft x 2 ft = 2 sq ft.
- Volume: 50 x 2 = 100 cu ft.
- Yards: 100 / 27 = 3.7 cu yd.
- Order: 3.7 x 1.1 = 4.07 cu yd.
- Pipe: one 50 ft run of perforated pipe.
- Tool shows 3.7 cu yd, 4.07 cu yd with extra, 50 ft pipe. Metric mirror gives 2.84 cu m, 3.12 cu m.
Reference table
| Trench | Volume | Note |
|---|---|---|
| 12 x 12 in | 0.37 cu yd | Shallow |
| 12 x 18 in | 0.56 cu yd | Common |
| 12 x 24 in | 0.74 cu yd | Deep |
| 18 x 24 in | 1.11 cu yd | Wide |
Limits
- Gross volume ignores pipe displacement.
- Slope under 1% risks ponding in the line.
- Foundation drains need footing depth.
- Outlet daylight must exist downhill.
Common mistakes
- Ordering exact gross with no waste.
- Flat trenches that hold water.
- Pipe holes facing up instead of down.
- Skipping fabric so soil fills the gravel.
- Draining onto the neighbor lot.
Try it yourself
Quick checks. Answers sit tucked under each question.
30 ft at 12 by 18 in?
1.67 cu yd, order 1.83 cu yd.
Pipe for a 50 ft run?
50 ft of perforated pipe plus fittings.
Minimum slope?
1% fall to the outlet, more where soil is tight.
Questions
How much gravel for a french drain?
Length times width times depth in feet, divided by 27, plus 10%.
How deep should it be?
18 to 24 in suits yards; foundations run to footing.
Pipe holes up or down?
Down, so water rises into the pipe from the gravel bed.
What slope?
At least 1% fall along the whole run to daylight.
Fabric needed?
Yes around gravel in soil; it stops fines from clogging voids.
Where should it outlet?
Daylight downhill, a dry well, or storm drainage where allowed.