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.
ft
in
in
in
$
Required Gravel Weight
3.53 US Tons
Estimated mass needed to backfill around the drainage pipe.
Required Gravel Volume
2.62 cu yd
Net Gravel Space
70.6 cu ft
Pipe Displacement
-4.4 cu ft
Exact cubic volume of gravel required to fill the trench minus the space taken by the pipe.
Landscape Fabric Area
250 sq ft
Min. Roll Width
5.0 ft Wide
Overlap Allowance
50 sq ft
Total area of non-woven filter fabric required to line the trench, including the necessary top overlap.
Dirt Excavation Volume
3.47 cu yd
Solid Earth Volume
2.78 cu yd
Swell Amount
+0.69 cu yd
Total volume of loose dirt. Swell represents the expansion of dirt once excavated.
Estimated Material Cost
$141
Gravel Order Wgt
3.53 Tons
Cost per Distance
$2.83 / ft
Estimated cost of gravel based on tonnage, broken down to a per-linear-distance cost.
Drainage Setup Note
Use about 1% slope, which is 1.2 inches of drop per 10 feet of run. Using clean, washed round stone (like 1 to 1.5 inch river rock) provides the best longevity and water movement.
The French Drain Calculator estimates the materials needed to build a subsurface drainage trench. Enter your trench dimensions, pipe diameter, and local gravel price to get gravel weight, gravel volume, landscape fabric area, excavation volume, and a material cost estimate — all in one calculation.
Results are planning estimates based on trench geometry. Actual quantities depend on soil type, compaction, gravel gradation, and site conditions. Use the outputs to size your material order and compare costs before you dig.
What the French Drain Calculator Estimates
Gravel Weight
The estimated mass of drainage stone required to fill the trench around the pipe, expressed in US tons or metric tonnes. Use this figure to place your supplier order — aggregate is sold by weight, not volume.
Gravel Volume
The net cubic volume the gravel must fill — trench volume minus pipe displacement. Shown in cubic yards (US) or cubic metres (metric). Useful for cross-checking a supplier's bulk volume quote against calculated need.
Landscape Fabric Area
The total square footage (or square metres) of non-woven filter fabric needed to line the trench sides, base, and top overlap. Also shows the minimum roll width so you can buy the right fabric width without trimming waste.
Excavation Volume
How much dirt will come out of the ground, including the 25% swell expansion that occurs once soil is loosened. Knowing the loose volume helps you plan spoil removal — whether that means hauling away, spreading on site, or renting a skip.
Material Cost Estimate
Gravel cost calculated from tonnage and your entered price per ton or tonne. The result also breaks down to a cost per linear foot (or metre), which makes it easy to compare quotes across different trench lengths or adjust the estimate when your supplier's price changes.
French Drain Gravel Formula
Step 1 — Gross Trench Volume
Trench Volume = Length × Width × Depth
All dimensions must be in the same unit before multiplying. The calculator converts inches to feet (US) or centimetres to metres (metric) before this step.
Step 2 — Pipe Displacement
Pipe Volume = π × (Pipe Diameter ÷ 2)² × Length
The pipe is treated as a solid cylinder. Its cross-sectional area is subtracted from the gross trench volume to find the actual gravel space.
Step 3 — Net Gravel Volume
Net Gravel Volume = Trench Volume − Pipe Volume
Step 4a — Unit Conversion (US Customary)
Cubic Yards = Cubic Feet ÷ 27
Step 5a — Gravel Weight (US Customary)
Gravel Weight (US tons) = Cubic Yards × 1.35
The 1.35 factor (approximately 3,645 lb/yd³ ÷ 2,000) is an estimating assumption for clean, dry drainage gravel. Actual density varies by stone type, gradation, moisture content, and compaction.
Step 5b — Gravel Weight (Metric)
Gravel Weight (tonnes) = Cubic Metres × 1.6
The 1.6 t/m³ figure is an estimating assumption for loose, clean drainage aggregate. Stone density varies widely — use supplier data where available.
Density note: The weight factors used here are estimating assumptions, not universal material standards. Crushed limestone, pea gravel, and river rock all have different bulk densities. Verify with your aggregate supplier for precise tonnage if over-ordering is costly.
Worked Example
Inputs used
50 ft trench · 12 in wide · 18 in deep · 4 in pipe · $40/ton
1
Convert width and depth from inches to feet
Width: 12 in ÷ 12 = 1.0 ft
Depth: 18 in ÷ 12 = 1.5 ft
Pipe: 4 in ÷ 12 = 0.333 ft
Depth: 18 in ÷ 12 = 1.5 ft
Pipe: 4 in ÷ 12 = 0.333 ft
2
Calculate gross trench volume
50 ft × 1.0 ft × 1.5 ft = 75.0 cu ft
3
Subtract pipe displacement
π × (0.333 ÷ 2)² × 50 ft ≈ 4.36 cu ft removed
Net gravel space = 70.6 cu ft
4
Convert net gravel volume to cubic yards
70.6 cu ft ÷ 27 = 2.62 cu yd
Required Gravel Volume = 2.62 cu yd
5
Estimate gravel weight
2.62 cu yd × 1.35 = 3.537 US tons
Required Gravel Weight = 3.53 US tons
6
Estimate material cost
3.53 tons × $40/ton = $141.20 → rounded to $141
Estimated Material Cost = $141
$2.83 / ft
$2.83 / ft
2.62
cu yd gravel volume
3.53
US tons gravel weight
$141
estimated gravel cost
Landscape Fabric Estimate
How fabric width is calculated
Filter fabric must line the trench base and both side walls, then fold back over the gravel at the top to seal out soil. The calculator adds the trench bottom width plus both side depths plus an overlap allowance.
Fabric Width Formula
Fabric Width = Trench Width + (2 × Trench Depth) + Overlap Allowance
US Customary: overlap allowance = 1 ft · Metric: overlap allowance = 0.3 m
Trench Width
1.0 ft
Both Sides (2 × 1.5 ft)
3.0 ft
Overlap Allowance
1.0 ft
Min. Roll Width
5.0 ft
Total Fabric Area
5.0 ft roll width × 50 ft trench length = 250 sq ft of filter fabric required for the default example.
Fabric Area
250 sq ft
Overlap Portion
50 sq ft
The minimum roll width tells you which standard fabric roll to buy so the sheet covers all trench surfaces without a longitudinal seam. Buy the next available roll width above the calculated minimum. The overlap allowance is not extra waste — it is a functional fold that prevents soil from migrating into the gravel bed over time.
Excavation and Soil Swell
Solid Earth Volume
The in-ground cut volume — the exact space the trench occupies. This is the same as the gross trench volume before any swell adjustment. For the default example: 2.78 cu yd.
Dirt Excavation Volume
Soil expands when disturbed. The calculator applies a 25% swell factor to solid volume to give the loose, spoil-pile volume. For the default example: 3.47 cu yd loose (+0.69 cu yd swell).
Swell Formula
Loose Excavation Volume = Solid Earth Volume × 1.25
The 25% swell multiplier is a general estimating assumption. Actual swell depends on soil type — clay and silty soils typically swell more than sandy or gravelly soils. Moisture content at the time of excavation also affects the result. Use this figure for skip hire and spoil removal planning, not for engineered earthwork calculations.
Drainage Slope Note
Slope reference: A 1% slope equals approximately 1.2 inches of drop per 10 feet of run. This is the commonly cited minimum for gravity-fed subsurface drainage to carry water to the outlet without ponding inside the pipe.
Factors that affect real installation decisions
Slope, outlet location, soil permeability, and local drainage regulations all influence how a French drain is designed and positioned. The calculator does not account for these factors — it estimates materials only. Consult a licensed contractor or your local authority where drainage affects property boundaries, foundations, or public infrastructure.
Calculator Assumptions and Limits
Gravel density is an estimating assumption (1.35 US tons/cu yd; 1.6 t/m³), not a fixed material standard. Actual density varies by stone type, gradation, and moisture.
Pipe volume is subtracted as a solid cylinder displacement. Perforations and pipe wall thickness are not modelled.
Pipe diameter must be smaller than or equal to both trench width and trench depth. The calculator flags inputs where this condition is not met.
Landscape fabric area includes trench sides, base, and the overlap allowance. It does not account for end caps or overlapping seams on very long runs requiring multiple rolls.
Excavation volume includes a 25% swell estimate for loose spoil. Real swell depends on soil classification and moisture content.
Material cost covers gravel only. Pipe, fabric, fittings, labour, equipment hire, and disposal costs are not included.
Results are planning estimates only. This calculator is not a substitute for engineered drainage design or site-specific professional assessment.
References and Calculation Notes
NIST — National Institute of Standards and Technology
Handbook 44 — Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices; and NIST Special Publication 811: Guide for the Use of the International System of Units (SI)
nist.gov/pml/special-publication-811
University of Minnesota Extension
Subsurface Drainage — Design and Installation of Tile Drainage Systems
extension.umn.edu/drainage/subsurface-drainage
RS Means / Gordian — Construction Cost Data
Soil and Rock Bulking / Swell Factors used as standard estimating assumptions in earthwork quantity takeoff
gordian.com/resources/rsmeans-data
ASTM International
ASTM C29 — Standard Test Method for Bulk Density ("Unit Weight") and Voids in Aggregate. Density of aggregate varies by stone type, moisture, and compaction state.
astm.org/c0029_c0029m-23.html