Home›Calculators›Construction›French Drain Calculator

French Drain Calculator

Construction

Calculate the cubic yards of gravel backfill needed for a French drain trench based on length, width, depth, and pipe diameter instantly and free.

Reviewed by the thecalcu.com team · Last updated August 2, 2026

11,000
436
660
212

Gravel Needed

2.62
Perforated Pipe Length
50

This calculator computes your Gravel Needed, Perforated Pipe Length from the values you enter.

Inputs
Trench LengthTrench WidthTrench DepthPipe Diameter
Outputs
Gravel NeededPerforated Pipe Length

What is a French Drain?

A French drain calculator estimates the volume of gravel backfill needed for a drainage trench, accounting for the space taken up by the perforated pipe that runs through it. A French drain redirects water away from a foundation, yard, or low-lying area by combining a gravel-filled trench with a perforated pipe that channels water to a safe discharge point.

Because the pipe occupies real volume inside the trench, simply calculating trench volume alone overestimates how much gravel you actually need, especially with larger diameter pipe in a narrower trench. This calculator subtracts the pipe's cylindrical volume from the total trench volume to give an accurate gravel figure in cubic yards, the standard unit gravel is sold and delivered by.

This is a common step in yard drainage and foundation waterproofing projects, where getting the material order right the first time avoids either a mid-project supply run or paying for gravel you don't use. If you're using crushed limestone instead of standard gravel, the Limestone Calculator applies the same logic with a limestone-specific density.

Why Use a French Drain Calculator?

Gravel is typically delivered by the cubic yard, and delivery minimums or per-yard pricing make it expensive to guess wrong in either direction. A French drain calculator gives you a precise volume figure based on your actual trench dimensions and pipe size, so you can order exactly what you need.

This is especially useful when comparing trench width or depth options: widening a trench from 12 to 18 inches significantly increases gravel volume over a long run, so knowing the exact difference helps you balance drainage capacity against material cost. It's also valuable when quoting a drainage job for a client, where an accurate gravel estimate is a key line item in the total project cost.

Who Should Use This Calculator?

  • Homeowners installing a DIY French drain around a foundation, downspout, or low spot in the yard.
  • Landscaping contractors quoting drainage installation jobs who need an accurate gravel material line item.
  • Foundation waterproofing specialists designing perimeter drains around a home's footing.
  • Civil and site contractors planning drainage for larger commercial or multi-lot developments. Pair this with the Gravel Calculator for any additional fill or base material needed beyond the drain trench itself.

What Insights Does the French Drain Calculator Give You?

  • Gravel Needed, the primary result, showing the volume of gravel backfill in cubic yards after subtracting the pipe's volume. This is the figure to use when ordering gravel delivery.
  • Perforated Pipe Length, the total linear footage of perforated pipe needed, which matches your trench length, useful for ordering pipe alongside the gravel.

Together, these outputs give you the two main material quantities for a French drain: gravel by volume and pipe by length, both needed to complete the trench.

How to use this French Drain calculator

  1. Enter your Trench Length in feet, the total straight-line run of your planned drain.
  2. Enter your Trench Width in inches, 12 inches is a common width for residential trenches.
  3. Enter your Trench Depth in inches, 18 inches is typical, though foundation drains may go deeper.
  4. Set your Pipe Diameter in inches, 4 inches is standard for most residential French drains.
  5. Read the Gravel Needed result at the top of the result card, this is your gravel order in cubic yards.
  6. Check the Perforated Pipe Length figure to confirm how much pipe to order alongside the gravel.
Show formula & methodology ↓Show less ↑

Formula & Methodology

Trench volume:
Vt = (L × 12) × W × D

Pipe volume:
Vp = π × (d ÷ 2)² × (L × 12)

Gravel volume:
Vg = max(0, Vt − Vp) ÷ 46,656

Where L is trench length in feet, W is trench width in inches, D is trench depth in inches, d is pipe diameter in inches, and 46,656 converts cubic inches to cubic yards.

Worked example: For a 50 ft trench, 12 in wide, 18 in deep, with a 4 in pipe:

- Trench volume: (50 × 12) × 12 × 18 = 129,600 in³
- Pipe volume: π × (4 ÷ 2)² × 600 = 7,540 in³
- Gravel volume: (129,600 − 7,540) ÷ 46,656 ≈ 2.62 yd³

Frequently Asked Questions

How much gravel do I need for a French drain?
The amount depends on your trench's length, width, and depth, minus the space taken up by the perforated pipe running through it. This calculator subtracts the pipe's cylindrical volume from the total trench volume, then converts the remaining space to cubic yards, the standard unit gravel is sold by. For a 50 ft trench that's 12 in wide and 18 in deep with a 4 in pipe, you'd need roughly 3.3 cubic yards of gravel.
What is a French drain and how does it work?
A French drain is a gravel-filled trench containing a perforated pipe that redirects surface and groundwater away from a foundation, yard, or low-lying area. Water percolates down through the soil and gravel, enters the pipe through its perforations, and flows by gravity to a safe discharge point away from the problem area. The gravel serves two purposes: it prevents soil and debris from clogging the pipe, and it speeds up water infiltration into the drain.
How deep should a French drain trench be?
Most residential French drains are dug 18 to 24 inches deep, though shallower trenches around 12 inches can work for simple surface water diversion. Deeper trenches are typically needed near foundations to intercept water before it reaches the footing. This calculator's Trench Depth input defaults to 18 inches but can be adjusted from 6 to 60 inches for different applications.
What size pipe should I use for a French drain?
A 4 inch perforated pipe is the most common choice for residential French drains, offering enough capacity for typical yard drainage without being oversized for a standard trench. Larger 6 inch pipe is sometimes used for high-volume drainage areas or commercial applications. This calculator lets you enter any pipe diameter from 2 to 12 inches to match your specific design.
Do I need landscape fabric with a French drain?
Yes, most French drain installations wrap the gravel and pipe in landscape fabric (geotextile fabric) to prevent surrounding soil from migrating into the gravel and clogging the drain over time. This calculator doesn't estimate fabric quantity directly, but you can measure the trench's total surface area (length times the sum of width and twice the depth) to estimate fabric needed for a full wrap.
What type of gravel is best for a French drain?
Washed, angular gravel between 3/4 inch and 1.5 inches in size, often called drain rock or clean stone, is the standard choice for French drains. Washed gravel is free of fine particles and dust that could clog the pipe's perforations over time, unlike unwashed crushed stone. Avoid rounded pea gravel alone, since it compacts more easily and offers less pore space for water to move through.
Can a French drain be too shallow to work properly?
Yes, a trench shallower than about 12 inches may not intercept enough subsurface water flow to be effective, especially in clay-heavy soils where water moves slowly downward. If your drainage problem is primarily surface water pooling, a shallower trench can still help, but foundation drainage generally needs greater depth to reach below the water-saturated zone near a structure.
How much does it cost to install a French drain?
DIY French drain installation typically costs $5 to $15 per linear foot for materials like pipe, gravel, and fabric, while professional installation can run $20 to $50 per linear foot including labor and excavation. Once you know your gravel volume from this calculator, multiply by your local price per cubic yard (often $40-$60 delivered) to estimate the gravel portion of your material budget.
Is fill-in gravel volume the same as trench volume?
This calculator subtracts the pipe's cylindrical volume from the total trench volume, since the pipe itself takes up space that gravel won't fill. Ignoring the pipe's volume, especially with larger diameter pipe in a narrow trench, can lead to over-ordering gravel by a meaningful margin on longer runs.
What other calculators help with drainage and landscaping projects?
The [Gravel Calculator](/gravel-calculator/) is useful for general fill and base material estimates beyond drainage trenches, and the [Limestone Calculator](/limestone-calculator/) helps if you're using crushed limestone instead of standard gravel. For the surrounding graded area, the [Square Footage Calculator](/square-footage-calculator/) helps plan total yard coverage.
Can this calculator be used for a curtain drain or dry well overflow trench?
Yes, the same volume formula applies to any trench-style drain that combines gravel backfill with a perforated pipe, including curtain drains and overflow trenches from a dry well. Just enter the specific trench dimensions and pipe size for your design to get an accurate gravel volume estimate.
Also known as
drain gravel calculatorhow much gravel for french draindrainage trench calculatoryard drainage calculator