Calculator
French Drain Gravel Calculator
A French drain trench is not filled solid with stone — a perforated pipe runs through the middle of it, and that pipe displaces volume the stone would otherwise occupy. On a long run of narrow pipe the difference is small enough to round away, but on a short, deep trench with a wide solid pipe, the pipe can take up a meaningful share of the trench, and ordering stone by trench volume alone quietly over-orders every time.
This calculator subtracts the pipe's own volume from the trench volume before converting to stone, and separately works out how wide the filter fabric lining the trench actually needs to be — the trench's width, plus twice its depth for the two side walls the fabric climbs, plus an overlap allowance where it folds back over the top. That width is very often wider than the standard fabric roll most people assume will cover the job.
Calculate your quantity
Drainage stone
2.09 cu yd
Stone weight
2.93 tons
- Trench volume
- 60 cu ft
- Volume the pipe takes up
- 3.49 cu ft
- Stone volume
- 56.51 cu ft
- Bagged stone equivalent
- 114 50 lb bags
- Fabric width needed
- 60 in
- Filter fabric
- 200 sq ft
- Perforated pipe
- 42 ft
- Fabric has to be 60 in wide to wrap a 12 x 18 in trench with a 12 in overlap — a 36 in roll will not do it.
Shopping summary
- • 2.09 cu yd (2.93 tons) of washed drainage stone
- • 2 roll(s) of filter fabric, 200 sq ft total
- • 42 ft of 4 in perforated pipe
This is an estimate — confirm structural work with a professional.
How this calculation works
Trench volume is simply length times width times depth, but the pipe running down the middle of that trench is a cylinder, and its volume — pi times its radius squared times the trench length — comes off before the remainder converts to stone. A 4 in pipe on a long run barely registers against the total; a 6 in pipe in a short, narrow trench can be a tenth or more of the volume, which is exactly the situation where skipping this deduction over-orders stone by a noticeable margin.
Filter fabric wraps the trench on three sides — up one wall, across the bottom, up the other wall — and then needs extra width to fold over the top of the stone and overlap itself, keeping soil from migrating down into the stone from above. That total is the trench width plus twice the depth plus the overlap, which for a typical 12 by 18 in trench already exceeds the 36 in fabric rolls sold at most home centers, before the overlap is even added.
- Pipe displacement matters most on short, deep trenches with wide pipe — a long run of narrow pipe barely moves the stone total, but a short run of wide pipe can shift it by ten percent or more.
- Fabric width scales with depth twice as fast as it scales with trench width, because depth is counted on both walls of the trench while width is counted once across the bottom.
- Stone tonnage depends on the stone type selected, not just its volume — washed 3/4 in stone, crushed clean stone and river rock all pack to different weights per cubic yard.
- A wider trench needs proportionally less pipe-displacement correction relative to its total volume than a narrow one does, since the pipe's cross-section stays the same while the trench's grows.
The formula
trench volume = length × (width ÷ 12) × (depth ÷ 12); pipe volume = π × (pipe diameter ÷ 24)² × length; stone volume = trench volume − pipe volume; fabric width = trench width + 2 × trench depth + overlap
- trench volume
- The full excavated volume of the trench, in cubic feet, before anything is subtracted for the pipe running through it — length times width times depth, all in feet after converting from the inch measurements you enter.
- pipe volume
- The cylindrical volume the perforated pipe itself occupies along the trench's length, subtracted from the trench volume because that space is pipe, not stone, even though the pipe is itself surrounded by stone on every side.
- fabric width
- The total flat width of filter fabric needed to line both trench walls and the bottom, plus overlap at the top — not the trench's width alone, which only covers the bottom of a lined trench.
- overlap
- Extra fabric width held back to fold over the top of the stone and lap onto itself, closing off the path for surface soil to migrate down into the drainage stone from above.
- stone density
- Weight per cubic yard for the chosen stone type, used to convert the stone volume into a tonnage figure a supplier can actually quote and deliver against, since drainage stone is sold by weight, not by the cubic foot.
Where these numbers come from
- 1.35 to 1.5 tons per cu yd stone density
- Trade convention for common drainage aggregate types: washed 3/4 in stone runs lighter due to more voids between angular pieces, river rock runs heavier because rounded stone packs tighter. A specific quarry's product sheet, where available, should override these working figures.
- 50 lb bag ≈ 0.5 cu ft of stone
- Trade convention for bagged aggregate at typical drainage-stone density — used only to convert a bulk delivery figure into a bagged-equivalent count for smaller jobs where a bulk delivery minimum isn't worth ordering.
- 100 sq ft per fabric roll
- Trade convention for a common non-woven filter fabric roll size sold at home centers and landscape suppliers. Roll width and length both vary by manufacturer, so treat the roll count here as an estimate to confirm against the specific product being bought.
- 5% pipe length allowance
- Derived on this page as a working margin for connections, bends and a slightly longer run than the straight trench length implies — not a manufacturer figure, since actual pipe waste depends heavily on how many fittings a specific layout needs.
Worked examples
A 60 foot foundation perimeter drain, 12 by 18 in trench
| Trench length | 60 ft |
|---|---|
| Trench width | 12 in |
| Trench depth | 18 in |
| Pipe diameter | 4 in |
| Stone type | Crushed clean stone (1.4 t/cu yd) |
| Fabric overlap at the top | 12 in |
| Drainage stone | 3.14 cu yd |
|---|---|
| Stone weight | 4.4 tons |
| Trench volume | 90 cu ft |
| Volume the pipe takes up | 5.24 cu ft |
| Stone volume | 84.76 cu ft |
| Bagged stone equivalent | 170 50 lb bags |
| Fabric width needed | 60 in |
| Filter fabric | 300 sq ft |
| Perforated pipe | 63 ft |
The 4 in pipe running through this 90 cu ft trench displaces about 5.24 cu ft — under 6% of the total — which trims the stone order from 3.33 cu yd down to 3.14, a modest but real correction on a run this long. Skipping it would have over-ordered stone by roughly a fifth of a ton, small on its own but the kind of rounding error that adds up across a job with several drain runs.
The fabric width comes to a full 60 in — the 12 in trench width plus 36 in for the two 18 in walls plus a 12 in overlap — which needs a 60 in or wider fabric roll, or two narrower rolls overlapped down the length of the trench. A single 36 in roll, the width most commonly stocked, comes up well short here.
A short, deep window-well drain with 6 in solid pipe
| Trench length | 15 ft |
|---|---|
| Trench width | 9 in |
| Trench depth | 24 in |
| Pipe diameter | 6 in |
| Stone type | Washed 3/4 in stone (1.35 t/cu yd) |
| Fabric overlap at the top | 6 in |
| Drainage stone | 0.72 cu yd |
|---|---|
| Stone weight | 0.98 tons |
| Trench volume | 22.5 cu ft |
| Volume the pipe takes up | 2.95 cu ft |
| Stone volume | 19.55 cu ft |
| Bagged stone equivalent | 40 50 lb bags |
| Fabric width needed | 63 in |
| Filter fabric | 78.8 sq ft |
| Perforated pipe | 16 ft |
Here the pipe displacement is far more significant: 2.95 cu ft of pipe against a 22.5 cu ft trench is over 13% of the total, more than double the first example's share, because this trench is both shorter and narrower while the pipe is wider. Ignoring the pipe's volume on a short run like this over-orders stone by a proportionally larger amount than it would on a longer, wider trench — the deduction matters most exactly where it's most tempting to skip for a 'small' job.
Fabric width comes out even wider than the first example at 63 in, despite this trench being narrower, because depth drives fabric width twice as hard as width does — a 24 in deep trench alone accounts for 48 of those 63 inches. Deep, narrow trenches are the case most likely to catch someone out who assumed a standard roll width would be enough.
Common mistakes
- Ordering stone by trench volume alone, without subtracting the pipe's own volume — a small error on a long run of narrow pipe, but a real over-order on a short, deep trench with wide pipe.
- Buying a standard 36 in fabric roll without checking the calculated fabric width first — most trenches deeper than about a foot need more width than a 36 in roll provides once both walls and the overlap are counted.
- Using a stone density figure for a different aggregate than what's actually being ordered — washed stone, crushed stone and river rock don't weigh the same per cubic yard, and tonnage quotes are priced against actual weight, not volume.
- Skipping the pipe length allowance and ordering pipe cut exactly to the trench length, leaving nothing for a coupling, a bend around an obstruction, or a slightly longer actual run than the straight-line trench length suggested.
- Forgetting that fabric overlap has to close at the top of the stone, not just meet — a lapped seam that barely touches rather than genuinely overlaps gives soil a path down into the drain over a season or two of settling.
Shopping summary
Stone is ordered by weight, in tons, from the volume this page calculates — confirm the density figure against what your supplier is actually selling, since drainage stone products vary enough that a wrong density assumption changes the tonnage by a meaningful margin.
Fabric is bought by the roll at the calculated width, or as two narrower rolls overlapped along the trench length if a single roll wide enough isn't stocked locally — either way, buy to the fabric width figure rather than assuming a standard 36 in roll will do. Pipe rounds up to the next standard length sold, with the length allowance already built into the figure this page returns.
If the excavated soil from this trench needs to be hauled away rather than reused as backfill elsewhere on the property, the excavation spoil calculator works out how much material that actually is — trench volume and spoil volume are related but not identical once the stone and pipe are accounted for. Where the drain is collecting roof runoff rather than groundwater, check it against the gutter and downspout calculator to confirm the drain has enough capacity for what's actually being routed into it.
FAQ
Does the pipe volume deduction really matter on a typical residential drain?
It depends on the trench proportions. On a long run of standard 4 in pipe it's usually under 10% of the total and easy to round past; on a short, deep trench with 6 in solid pipe it can exceed 13%, as in this page's second example — the shorter and narrower the trench relative to the pipe's diameter, the more the deduction matters.
Why is the fabric width so much larger than the trench's own width?
Because fabric has to line both walls of the trench as well as the bottom, and depth is counted on both walls while width is counted only once. A trench twice as deep as it is wide needs far more fabric width than the raw trench width suggests — depth, not width, is usually what pushes a job past a standard fabric roll.
Can I use perforated pipe without any fabric at all?
You can, and some drains are built that way, but without fabric the stone eventually silts up as fine soil particles migrate into the voids between the aggregate over successive seasons, reducing how well the drain moves water. Fabric is what keeps the stone's void space open over the drain's working life, not just at installation.
Should the pipe sit at the very bottom of the trench or up in the stone?
Typically the pipe sits on a bed of stone rather than directly on soil, with more stone both beside and above it — this calculator treats the pipe as centered in the trench for the displacement calculation, which is a reasonable approximation for typical installations but not a substitute for the specific bedding depth a site's drainage plan calls for.
How does trench slope affect the volumes this page calculates?
It doesn't directly — the depth you enter is treated as constant along the trench length, while a real French drain needs a consistent fall toward its outlet, commonly around 1% grade. If the trench depth varies meaningfully from one end to the other to achieve that fall, running the calculation at the average depth gives a reasonable volume estimate, though the deepest section will need proportionally more stone than the shallowest.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.