French Drain Calculator
Use this French drain calculator to estimate the gravel, pipe, and fabric needed for a drainage trench. It starts from the trench dimensions, subtracts the pipe volume, then adds waste and cost so you can plan the material order with fewer surprises.
Result
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What This French Drain Calculator Helps You Do
Use this French drain calculator to estimate the gravel, pipe, and fabric needed for a drainage trench. It starts from the trench dimensions, subtracts the pipe volume, then adds waste and cost so you can plan the material order with fewer surprises.
How to Calculate French Drain Calculator
- Measure the trench - Enter the trench length, width, and depth so the calculator can build the excavation volume.
- Add the pipe data - Enter the pipe diameter and pipe length so the calculator can subtract the pipe from the gravel bed.
- Set allowance and pricing - Enter the waste percentage and, if needed, gravel, pipe, and fabric prices.
- Review the order quantity - Read the base gravel amount, the waste-adjusted order volume, and the rough material cost.
French Drain Calculator Formula
| Symbol | Definition | Unit |
|---|---|---|
| Trench volume | Length × width × depth of the trench | ft3 |
| Pipe volume | Cylindrical volume occupied by the pipe | ft3 |
| Waste | Extra percentage added for ordering | % |
Worked Examples
- length: 40
- width: 2
- depth: 1.5
- pipeDiameter: 3
- pipeLength: 40
- waste: 10
- density: 105
- pricePerYd: 45
- pipePrice: 3.5
- fabricArea: 80
- fabricPrice: 0.65
- gravelYd3: 0
Result: Base gravel = 4.37 yd3
A standard residential trench usually needs a moderate gravel bed and a small waste allowance. The estimate is 4.37 yd3.
- length: 30
- width: 1.5
- depth: 1
- pipeDiameter: 4
- pipeLength: 30
- waste: 8
- density: 105
- pricePerYd: 45
- pipePrice: 3.5
- fabricArea: 80
- fabricPrice: 0.65
- gravelYd3: 0
Result: Base gravel = 1.57 yd3
A narrower trench reduces total volume, but the pipe still removes a meaningful amount of space. The estimate is 1.57 yd3.
- length: 50
- width: 2
- depth: 1
- pipeDiameter: 3
- pipeLength: 50
- waste: 12
- density: 105
- pricePerYd: 45
- pipePrice: 3.5
- fabricArea: 80
- fabricPrice: 0.65
- gravelYd3: 0
Result: Estimated cost = 409.09 USD
A longer run increases the cost because the gravel, pipe, and fabric all scale with length. The estimate is 409.09 USD.
- length: 25
- width: 2.5
- depth: 1.2
- pipeDiameter: 6
- pipeLength: 25
- waste: 7
- density: 105
- pricePerYd: 45
- pipePrice: 3.5
- fabricArea: 80
- fabricPrice: 0.65
- gravelYd3: 0
Result: Gravel weight = 3.68 tons
The wider trench and larger pipe still leave a measurable gravel mass for handling and transport. The estimate is 3.68 tons.
How to Interpret Your Results
| Range | Meaning | Action |
|---|---|---|
| Small trench | Lower gravel and fabric demand | Good for short yard drains and localized runoff control. |
| Typical residential trench | Moderate gravel and pipe demand | Round up a little to cover uneven excavation and cut losses. |
| Long drainage run | Higher cost and material count | Use the cost details and confirm pipe, gravel, and fabric pricing before ordering. |
Frequently Asked Questions
Sources
Last reviewed: March 2026