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.
- 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
Frequently Asked Questions
What kind of gravel for French drains?
Use clean, washed gravel in 3/4 to 1-1/2 inch size. Avoid crushed limestone with fines, as the fine particles can clog the system over time.
Do I need filter fabric?
Yes, wrap the pipe in filter fabric or line the trench with it to prevent soil from migrating into the gravel and clogging the drain.
Ready to run the numbers?
Open Drainage Gravel Calculator