French Drain Calculator uses net gravel volume = trench volume – pipe volume to estimate gravel weight, fabric area, excavation volume, and cost from trench length, width, depth, pipe size, and price.
The Material Most People Forget to Calculate
Ordering gravel for a French drain is straightforward. What catches people out is the landscape fabric — because it doesn’t just cover the trench bottom, it wraps the entire interior: down one wall, across the base, up the other wall, and folds over the top with an overlap. For a typical 50-foot trench that’s 12 inches wide and 18 inches deep, you don’t need 50 square feet of fabric. You need 250. This calculator figures all four material quantities at once: gravel tonnage, fabric area with wrap geometry, excavated dirt volume with swell, and a gravel cost estimate broken down per linear foot.
How the Calculations Work
Gravel Volume and Weight
The trench is treated as a rectangular prism minus the cylindrical pipe running through it.
Trench Volume = Length × Width × Depth Pipe Volume = π × (Pipe Radius)² × Length Net Gravel Volume = Trench Volume − Pipe Volume
In US mode, note that length is entered in feet while width, depth, and pipe diameter are in inches — the tool converts those internally before calculating. The net cubic volume is then converted to cubic yards (divide by 27) and multiplied by a bulk density of 1.35 US tons per cubic yard to get the gravel order weight. For metric, the density used is 1,600 kg/m³ (1.6 tonnes/m³), which is a standard bulk density for clean washed crushed stone.
Setting pipe diameter to zero is valid — the tool simply skips the pipe displacement calculation and treats the trench as a solid gravel fill. This is useful for simple rubble drains or infiltration trenches without a perforated pipe.
Landscape Fabric
This is the output that consistently surprises people. The required fabric width isn’t the trench width — it’s the full interior wrap distance:
Fabric Width = Trench Width + (Trench Depth × 2) + 1 ft overlap Fabric Area = Fabric Width × Trench Length
The overlap allowance is 1 foot (US) or 30 cm (metric), representing the fabric folded over the top of the gravel before backfilling. The “Minimum Roll Width” shown in the results tells you the narrowest roll you can buy for this trench — order anything narrower and you’ll be piecing fabric together.
Excavation and Soil Swell
Excavated soil expands once it’s out of the ground. The tool applies a 25% swell factor:
Solid Earth Volume = Trench Length × Width × Depth Loose (Hauled) Volume = Solid Volume × 1.25
The “Dirt Excavation Volume” card shows the loose volume — the figure that matters when ordering a dump truck or calculating how many wheelbarrow loads leave the site. The swell breakdown shows exactly how much expansion you’re accounting for.
Cost Estimate
Material cost is gravel tonnage multiplied by your entered price per ton (or tonne). The per-linear-distance breakdown divides that total by trench length, which makes it easier to compare quotes or scale the project if the run gets extended or shortened.
The Fabric Calculation Nobody Gets Right the First Time
Most DIY and contractor estimates for landscape fabric on a French drain are based on surface area — the footprint of the trench. That’s wrong. Fabric is installed as a U-shaped liner, draped to cover the walls and base continuously before gravel is placed, then folded over the top. The required width scales with depth, not just the opening.
A trench that doubles in depth doesn’t need twice the fabric length — it needs roughly twice the fabric width. A 12-inch-wide, 12-inch-deep trench needs a 4-foot-wide roll. The same 12-inch-wide trench at 24 inches deep needs a 5-foot-wide roll. Buying fabric based on trench surface area alone, as many people do, will leave you short mid-job. The roll width output in Card 2 is specifically there to prevent that order mistake.
A Worked Example
Backyard drainage problem: water pooling along a 65-foot fence line after rain. Plan is a 4-inch perforated pipe in a 12-inch-wide, 24-inch-deep trench, daylighting to a dry well at the low end. Local quarry quotes $38 per ton for washed river rock.
Trench volume: 65 ft × 1 ft × 2 ft = 130 cu ft = 4.81 cu yd. Pipe displacement: π × (0.167 ft)² × 65 ft = 5.7 cu ft. Net gravel: 130 − 5.7 = 124.3 cu ft = 4.60 cu yd. At 1.35 tons/yd³, that’s 6.21 tons of gravel — so ordering 7 tons with a bit of waste margin makes sense.
Fabric width: 1 ft + (2 ft × 2) + 1 ft = 6 ft. Fabric area: 6 × 65 = 390 sq ft. That’s a full 6-foot-wide roll at 65 feet of run — important to know before calling the supplier.
Excavation: 130 cu ft / 27 = 4.81 cu yd solid, swelling to 6.01 cu yd loose. That’s a meaningful difference when deciding whether a single truck haul covers it.
Gravel cost: 6.21 tons × $38 = $236, or about $3.63 per linear foot — useful for comparing against a contractor’s per-foot quote that includes the gravel material component.
Frequently Asked Questions
Why are width, depth, and pipe entered in inches but length in feet in US mode?
French drain dimensions are almost universally specified this way in practice — trench runs are described in feet while cross-section dimensions are given in inches. The tool matches that field convention. Internally it converts everything before calculating, so there’s no need to convert your measurements before entering them. If you switch to metric, all dimensions shift to meters and centimeters respectively.
What happens if I enter a pipe diameter larger than the trench width or depth?
The tool validates this and returns an error rather than producing a nonsensical result. A pipe that doesn’t fit inside the trench cross-section is physically impossible, so no outputs are shown until you correct the pipe diameter or increase the trench dimensions. The pipe diameter must be smaller than or equal to both the trench width and trench depth.
When I switch measurement systems, do my entered values convert automatically?
Yes — unlike some unit converters that just relabel fields, this tool actively converts the values when you change systems. Length converts between feet and meters; width, depth, and pipe diameter convert between inches and centimeters. If you’ve already entered values in one system and switch, check that the converted numbers look correct before running the calculation, particularly for small pipe diameters where rounding can be noticeable.
The gravel cost shows zero even though I entered a price. What’s wrong?
Check that the pipe diameter isn’t equal to or larger than the trench dimensions — if the tool is in a validation error state it won’t update any outputs including cost. Also confirm the price field contains a positive number. A price of zero is accepted (useful for estimating volume without a cost figure), but entering zero intentionally or accidentally will show a $0 result regardless of tonnage.
Does the excavation volume account for the soil I’ll be replacing with gravel, or just the total dig?
The excavation figure represents the full trench volume converted to loose (swelled) soil — essentially everything you’re removing. It doesn’t subtract the gravel that goes back in, because excavated soil and backfill gravel are separate material flows: the dirt is leaving the site, and the gravel is coming in. Plan removal capacity based on the excavation output and delivery based on the gravel tonnage output independently.