Drainage Gravel Calculator Formula
Understand the math behind the drainage gravel calculator. Each variable explained with a worked example.
Formulas Used
Gravel Needed
gravel_cuyd = (trench_vol_cuft - pipe_vol_cuft) / 27Approximate Weight
gravel_tons = (trench_vol_cuft - pipe_vol_cuft) / 27 * 1.4Total Trench Volume
trench_vol = trench_vol_cuftVariables
| Variable | Description | Default |
|---|---|---|
trench_length | Trench Length(ft) | 50 |
trench_width | Trench Width(in) | 12 |
trench_depth | Trench Depth(in) | 18 |
pipe_diameter | Drain Pipe Diameter(in) | 4 |
trench_vol_cuft | Derived value= trench_length * (trench_width / 12) * (trench_depth / 12) | calculated |
pipe_vol_cuft | Derived value= trench_length * pi * pow(pipe_diameter / 24, 2) | calculated |
How It Works
Drainage Gravel Calculation
Gravel Volume = Trench Volume - Pipe Volume
The perforated pipe displaces some of the trench volume. Subtract its cross-sectional area multiplied by the trench length to get the net gravel volume. Use washed, angular gravel (3/4 to 1-1/2 inch) for good drainage.
Worked Example
50 ft French drain, 12 in wide, 18 in deep, with 4-inch pipe.
trench_length = 50trench_width = 12trench_depth = 18pipe_diameter = 4
- 01Trench volume = 50 x 1 x 1.5 = 75 cu ft
- 02Pipe volume = 50 x pi x (2/12) squared = 2.18 cu ft
- 03Gravel = (75 - 2.18) / 27 = 2.7 cu yd
- 04Weight = 2.7 x 1.4 = 3.8 tons
Ready to run the numbers?
Open Drainage Gravel Calculator