French Drain Gravel Calculator
Estimate gravel volume and bags for a drain trench after subtracting the pipe.
Last reviewed September 19, 2026
Result
Buy 192 bagsEstimate
No extra is added by default. Use Custom for trench variation, settlement and the supplier or installer allowance.
- Gravel volume to order
- 3.5421 yd³ (95.6367 ft³)Estimate
- Net gravel space
- 95.6367 ft³ (3.5421 yd³)
- Full trench volume
- 100 ft³
- Pipe outside volume
- 4.3633 ft³
https://calcgallery.com/landscaping/french-drain-gravel-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Enter trench length, width and the depth that will actually contain gravel.
- Enter the pipe’s outside diameter.
- Enter the volume printed on one gravel bag.
- Add a custom allowance for trench variation and read bags or bulk volume.
How it is calculated
The gravel occupies the rectangular trench except for the pipe cylinder:
Trench volume = length × width × gravel depth
Pipe volume = π × (outside diameter ÷ 2)² × length
Net gravel = max(trench volume − pipe volume, 0)
Bags = ceil(net gravel with allowance ÷ bag volume)Use outside pipe diameter because the pipe wall also displaces gravel.
Drainage limits
This result does not validate a French drain design. Confirm the drainage route, outlet, pipe size, fall, aggregate grading and filter detail for the site and local requirements.
Enter gravel-filled depth. If part of the trench contains topsoil or another layer, do not include that depth as gravel.
Measure trench variation. Excavated width and depth rarely stay exact. Take several measurements or enter a deliberate custom allowance.
Use suitable clean aggregate. Material selection affects drainage and filter performance. Follow the project specification rather than choosing by bag volume alone.
With your numbers
- Full trench = 100 ft³
- Subtract pipe outside volume 4.363 ft³ to leave 95.64 ft³ for gravel
- With 0% extra = 95.64 ft³
- Divide by the entered bag volume and round up to 192 bags
Quick reference
No extra is added by default. Use Custom for trench variation, settlement and the supplier or installer allowance.
| Job | Gravel bags to buy |
|---|---|
| 50 ft trench, 12 × 24 in | 192 |
| 25 ft trench, 12 × 18 in | 71 |
| 100 ft trench, 18 × 24 in | 561 |
| 40 ft trench with 6 in pipe | 145 |
| 15 m trench, 30 × 60 cm | 104 |
| 10 m trench, 40 × 60 cm | 93 |
Common questions
- How much gravel for a 50 foot French drain?
- For a 12 in wide by 24 in deep trench with a 4 in pipe, the net gravel space is about 95.637 ft³ or 3.542 yd³. At 0.5 ft³ per bag that is 192 bags.
- Why subtract the pipe?
- The pipe occupies part of the trench. Subtracting its outside cylinder prevents that space being counted as gravel.
- Does this calculate filter fabric?
- No. Fabric width and overlap depend on the trench wrap detail and drainage design.
- Can this choose pipe size or trench fall?
- No. Pipe capacity, slope, outlet and soil conditions require a drainage design. This page only calculates material volume from dimensions already chosen.