French Drain Calculator
Find the gravel and pipe needed for a French drain, from your trench length, width and depth.
Reviewed by the ToolNestr Editorial Team — July 2026
How the French drain calculator works
A French drain moves groundwater away from a wet area through a gravel-filled trench with a perforated pipe running along its bottom. Sizing the materials starts with the trench volume — length times width times depth — then subtracts the volume the pipe itself occupies, since gravel only fills the space around it, not where the pipe sits.
Typical residential trenches run 12 to 18 inches wide and 18 to 24 inches deep, sloped at a minimum of about 1% — roughly 1/8 inch of drop per foot — so water flows by gravity toward an outlet or dry well rather than sitting stagnant. A 4-inch perforated pipe is standard for most home drainage, running the full trench length. The calculator also estimates the fabric needed to line the trench, since fabric wraps around the gravel to keep surrounding soil from washing in and clogging the system over years of use.
This is a materials estimate for a straightforward, gravity-fed residential drain. Sizing for standing water problems, foundation drainage, or anywhere water quantity is significant should follow a site-specific drainage design, since pipe size, slope, and outlet capacity all need to match the actual water volume the drain must handle.
Trench slope and foundation drainage guidance referenced here follow the International Residential Code (IRC) Section R405 requirements for foundation drainage.
Plan the trench
Length, width, and depth of the run.
Pick the pipe size
Usually 4-inch perforated pipe for homes.
Order materials
Gravel, pipe length, and fabric.
The formula explained
Trench volume
= L × (W⁄12) × (D⁄12). A 50 ft trench, 12×18 in: 50 × 1 × 1.5 = 75 ft³.
Pipe volume (subtract)
= π × (dia⁄24)² × L. A 4 in pipe over 50 ft: π × 0.167² × 50 ≈ 4.4 ft³.
Net gravel
75 − 4.4 = 70.6 ft³, about 2.6 cubic yards.
Worked example
A 30 ft trench, 12 in wide, 24 in deep, 4 in pipe.
Use cases for the French drain calculator
Downspout and runoff correction
Redirect water pooling near a downspout or low corner of the yard with a short, shallow trench. These runs are typically 15 to 30 feet, 12 inches wide, and require only a small trailer's worth of gravel.
Foundation perimeter drainage
A curtain drain around a foundation intercepts water before it reaches the basement or crawlspace walls. These runs are longer and deeper, often 18 to 24 inches deep, and typically tie into a sump pump or daylight outlet.
Yard low-spot and lawn drainage
Chronic soggy patches in a lawn often need a shallower, narrower trench than a foundation drain. Combine the calculator's gravel estimate with a catch basin at the low point for standing-water problem areas.
Retaining wall backfill drainage
Retaining walls need a drain line behind the wall to relieve hydrostatic pressure. Use the calculator to estimate the gravel backfill volume along the base of the wall, in addition to the drain pipe itself.
Real-world context
A downspout runoff fix is one of the smallest, most common jobs this calculator handles. A 25 ft trench, 12 inches wide and 18 inches deep, with a 4-inch pipe, has a trench volume of 25 × 1 × 1.5 = 37.5 cubic feet. Subtracting the pipe's own volume of about 2.18 cubic feet leaves roughly 35.3 cubic feet of gravel, about 1.31 cubic yards — small enough to haul in bagged gravel from a home center rather than ordering bulk delivery.
A foundation perimeter drain sits at the other end of the scale. An 80 ft run around a home's foundation, 18 inches wide and 24 inches deep with a 4-inch pipe, has a trench volume of 80 × 1.5 × 2 = 240 cubic feet. After subtracting about 6.98 cubic feet for the pipe, that leaves roughly 233 cubic feet of gravel — about 8.63 cubic yards, enough that most contractors order it delivered by the dump-truck load rather than bagged.
A small yard low-spot repair falls in between. A 15 ft trench, 12 inches wide and 12 inches deep, with a smaller 3-inch pipe, has a trench volume of 15 cubic feet; the thinner pipe displaces only about 0.74 cubic feet, leaving about 14.26 cubic feet of gravel, roughly 0.53 cubic yards — under a single cubic yard, which is the rough threshold homeowners use to decide between bagged gravel and a bulk delivery minimum.
Common misconceptions
"A French drain is just a gravel-filled ditch." Slope is what actually makes it work. Gravel and pipe alone do nothing without a consistent downhill grade toward an outlet — a minimum of about 1%, or roughly 1/8 inch of drop per foot. A flat or backward-sloped trench simply becomes an underground reservoir that overflows in a heavy storm.
"More gravel is always better, so skip the pipe to save money." The pipe does most of the work. Gravel alone drains water slowly through the spaces between stones; a perforated pipe collects water along the trench and moves it efficiently to the outlet. Without the pipe, a French drain still works, but far more slowly and with less capacity during heavy rain.
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Frequently asked questions
How much gravel do I need for a French drain?
Multiply the trench length, width and depth to get the trench volume, then subtract the space taken by the perforated pipe. A 50 ft trench, 12 in wide, 18 in deep, needs about 55 cubic feet of gravel after subtracting a 4-inch pipe.
What size trench does a French drain need?
A typical residential French drain trench is 12 to 18 inches wide and 18 to 24 inches deep, sloped at a minimum of 1% (about 1/8 inch per foot) toward the drainage outlet. Deeper, wider trenches move more water but cost more to dig and fill.
How much perforated pipe do I need?
Generally one length of perforated pipe equal to the trench length, typically 4-inch diameter for residential use. The calculator also estimates the volume the pipe itself displaces so the gravel estimate is more accurate.
Do I need landscape fabric?
Yes, in most designs. Fabric lines the trench before the gravel and pipe go in, and wraps back over the top, to keep soil and silt from migrating into the gravel and clogging the drain over time. Estimate fabric by the trench length times roughly its wrapped perimeter.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.