A 50-foot French-drain stone envelope, 12 inches wide and 18 inches deep, holds about 2.616 cubic yards of gravel after subtracting an approximate 4-inch-diameter pipe. With an example 10% quantity allowance, that is 2.878 cubic yards. This assumes the full 18-inch depth is stone, not soil or sod cover. Actual pipe outside diameter, trench geometry and supplier delivery increments can change the order.
Use the French drain calculator for a straight rectangular stone envelope, collecting-pipe length, full-wrap fabric and slope drop. This guide calculates quantities; it does not choose a safe excavation depth, approve a drain layout or guarantee that the water problem will be solved.
The gravel formula
Stone-envelope volume (cu ft) = length (ft) × width (in) × stone depth (in) / 144.
Approximate pipe volume (cu ft) = length × π × (diameter / 2)² / 144.
Net gravel (cu yd) = (envelope volume – pipe volume) / 27.
Quantity with allowance = net gravel × (1 + allowance percent / 100). These are our geometry formulas. Keep full precision until final display and ordering decisions.
The pipe model subtracts a smooth cylinder. Nominal 4-inch pipe does not guarantee a 4-inch actual outside diameter, especially for corrugated products. Confirm the selected product profile with the installer. This model does not subtract couplers or calculate gravel-free packaged aggregate products.
Stone depth is not always trench depth
If an 18-inch excavation includes 4 inches of soil or sod cover, the stone envelope could be only 14 inches deep. Use that stone depth in the quantity model. This is an illustration, not an approved cross-section. Bedding, cover, pipe clearance, outlet elevation and actual trench shape must follow the site design.
NDS describes a fabric-lined gravel trench with perforated pipe directed to an outlet, plus additional fabric and topsoil/sod above the gravel.1 Its general article gives several possible trench dimensions, not a universal design. Do not choose a foundation-drain excavation simply from a web size range. Conditions near footings, retaining walls or underground rooms need qualified assessment.
Gravel per 10 feet of run
Our table uses an approximate 4-inch pipe displacement. Width and depth describe stone, not total excavation. The right column applies a 10% example allowance, not a guaranteed waste or compaction conversion. These are quantity examples, not recommended trench sizes.
| Stone width × depth | Net gravel | Net cubic yards | With 10% |
|---|---|---|---|
| 6 × 12 in | 4.127 cu ft | 0.153 cu yd | 0.168 cu yd |
| 12 × 12 in | 9.127 cu ft | 0.338 cu yd | 0.372 cu yd |
| 12 × 14 in | 10.794 cu ft | 0.400 cu yd | 0.440 cu yd |
| 12 × 18 in | 14.127 cu ft | 0.523 cu yd | 0.576 cu yd |
| 12 × 24 in | 19.127 cu ft | 0.708 cu yd | 0.779 cu yd |
| 18 × 24 in | 29.127 cu ft | 1.079 cu yd | 1.187 cu yd |
Three checked examples
1. 50-foot run with 12 × 18-inch stone envelope
Envelope: 50 × 12 × 18 / 144 = 75.000000 cu ft. Approximate pipe displacement: 50 × π × 2² / 144 = 4.363323 cu ft. Net gravel: (75.000000 – 4.363323) / 27 = 2.616173 cu yd. With 10% example allowance: 2.877791 cu yd. Ask the supplier about delivery increments before rounding an order.
2. 50-foot run with 12 × 14-inch stone envelope
Envelope: 50 × 12 × 14 / 144 = 58.333333 cu ft. Approximate pipe displacement: 50 × π × 2² / 144 = 4.363323 cu ft. Net gravel: (58.333333 – 4.363323) / 27 = 1.998889 cu yd. With 10% example allowance: 2.198778 cu yd. The shallower stone envelope illustrates reserving part of the excavation for cover; it is not an instruction to use that cover depth.
3. 100-foot run with 18 × 24-inch stone envelope
Envelope: 100 × 18 × 24 / 144 = 300.000000 cu ft. Approximate pipe displacement: 100 × π × 2² / 144 = 8.726646 cu ft. Net gravel: (300.000000 – 8.726646) / 27 = 10.787902 cu yd. With 10% example allowance: 11.866692 cu yd. Ask the supplier about delivery increments before rounding an order.
Convert cubic yards to short tons using supplier density
Short tons = cubic yards × pounds per cubic yard / 2,000. There is no one density for every drainage stone. The cited material-weight chart lists dry 3/4-inch gravel at 2,565 lb per cubic yard and wet 3/4-inch gravel at 3,375 lb per cubic yard.2 Those are chart references, not measurements of your supplier’s washed stone.
Using the dry chart value solely as an illustration, the first example’s net 2.616173 cu yd weighs 6,710.48 lb, or 3.355242 short tons. With the 10% quantity allowance, 2.877791 cu yd weighs 3.690766 short tons at the same assumed density. Do not quote the net tonnage as the allowance-inclusive order. Ask for the bulk density and moisture basis of the exact stone you will buy.
An added-volume allowance is not a compaction conversion. If the supplier specifies delivered loose volume differently from the design volume, use the appropriate product and installation guidance rather than assuming 10% converts between them. The gravel calculator can use a supplier-provided density.
Clean stone and a compatible filter
NDS’s general guide discusses aggregate ranging from pea gravel to larger river rock and describes surrounding fabric as helping prevent soil mixing with gravel.1 That does not justify a blanket rule that every French drain must avoid pea gravel or use one named stone size. Match the aggregate and filter to the pipe, soil and design. Avoid substituting a compacting road-base mix or generic weed barrier without checking suitability.
The drainage-stone comparison guide discusses supplier recommendations. Treat those as source-specific, not permission to override a product or engineered detail. The stone-size chart helps decode names, but the supplier’s gradation determines what is actually in the load.
Fabric: count both the bottom and top
For a full rectangular wrap, strip width = 2 × stone depth + 2 × trench width + closure overlap. With our example 12-inch overlap, an 18-inch-deep, 12-inch-wide envelope needs 72 inches of strip width: 18 + 12 + 18 + 12 + 12. A 50-foot run therefore needs 300 sq ft before end closures and seams. This is our full-wrap geometry assumption, not a universal required overlap. Follow the selected filter and assembly instructions.
Check roll width as well as area. A 5-foot roll does not automatically cover a 6-foot wrap, even if it has enough total square feet. Multiple strips or seams need the approved overlap detail and added material. The calculator now includes the second width in this full-wrap count.
Slope: calculate the fall, then verify the design
1% slope = 0.12 inch of drop per foot. A 50-foot run at 1% drops exactly 6 inches; one inch per 8 feet is about 1.0417%, not exactly 1%. NDS gives the grade formula (rise / run) × 100.3 Its article mentions a 1% guideline and some 2% advice for corrugated pipe, while its quick-review sheet describes different corrugated-pipe guidance and recommends expert design in critical applications.1,3
Do not simply choose the steeper number “to be safe.” Slope changes outlet elevation, available cover and excavation depth. Confirm continuous fall, pipe requirements, outlet permission and site elevations with the designer or installer. A calculator’s drop result does not check drainage capacity or make an outlet possible.
Before buying or digging
- Confirm the stone envelope, actual pipe profile, filter detail and any soil cover.
- Verify a permitted outlet and the necessary elevations before excavation.
- Contact 811 and follow your state’s locate, waiting and safe-digging rules; private utilities may require separate locating.
- Keep collecting pipe separate from solid outlet runs and fittings.
- Confirm supplier density, minimum order and delivery increments; do not use a generic tonnage as a quote.
FAQ
How much gravel for a 50-foot French drain?
For the example 12-inch-wide, 18-inch-deep stone envelope and approximate 4-inch pipe, 2.616 cu yd net or 2.878 cu yd with 10% example allowance. If some depth is soil cover, use the smaller stone envelope. Those dimensions are not a drainage design.
How many tons is that?
It depends on the supplier’s bulk density. At the cited dry chart’s 2,565 lb/cu yd, the example is about 3.36 short tons net or 3.69 with the 10% quantity allowance. Treat those as arithmetic illustrations, not your delivery weight.
What depth should I use?
This page cannot determine it. Soil, water source, cover, pipe, outlet elevation, utilities, structure and local rules affect the design. Enter the chosen stone zone only after the section has been checked.
Related tools
French drain calculator | Gravel calculator | Drainage-stone guide
Sources
- NDS: French Drain Installation Guide (general gravel, pipe, outlet and cover guidance; not a site-specific design)
- Material Weight, Pounds per Cubic Yard (reference chart hosted by TRORC; dry/wet 3/4-inch gravel weights)
- NDS: Principles of Exterior Drainage – Quick Review (grade formula, slope caveats and critical design guidance)