French Drain Calculator: Material and Cost

You have found the material amount so what is next?

Follow Aqviz Expert Guide

What is a French Drain Calculator?

French drain calculator is a calculation tool that is used to calculate the amount of rock, geotextile fabric, excavated soil volume, backfill soil volume, and perforated pipe length. The inputs of the French drain calculator are French drain length, width, depth, rock height, pipe diameter, and geotextile fabric size. Using these inputs French drain calculator can calculate the rock amount, excavated soil volume, pipe length, geotextile requirement, and backfill soil volume. More than there is an adjustable wastage factor. It helps to increase the accuracy of the calculation in real life.

How to Calculate Materials for a French Drain?

To calculate the French drain materials, you should find the French drain volume, pipe volume, and rock volume. By using those values, you can calculate the ideal values for the French drain materials as below.

To find the French drain soil volume: French drain volume × Swell factor
To find the total rock volume: Gravel height × Gravel width × Length of the trench
To find the net rock volume: Total rock volume – Pipe volume
To find the backfill soil volume: Total French drain volume – Total rock volume
To find the remaining soil volume: Total soil volume – backfill volume

How to Calculate French Drain Volume?

To calculate the French drain volume, you should find the suggested French drain length, width, and depth, then multiply them together. This gives you the total trench volume in cubic feet. To convert it to cubic yards, divide by 27. This volume helps determine how much gravel or drain rock you’ll need.

Formula:

Volume (ft³) = Length × Width × Depth
Volume (yd³) = Volume (ft³) ÷ 27

Example:

Calculate French drain volume for a trench 100 ft long, 1 ft wide, and 1.5 ft deep:
Hence French drain volume = 100 × 1 × 1.5 = 150 ft³ → 150 ÷ 27 = 5.56 yd³

How to Calculate French Drain Soil Volume?

To calculate French drain soil volume, measure the trench length, width, and depth, then multiply them to get the banked soil volume in cubic feet. Multiply that by the swell factor (1.3–1.4) to estimate the loose excavated soil.

Formula:

Banked Volume (ft³) = Length × Width × Depth
Loose Volume = Banked Volume × Swell Factor

Example:

For a 100 ft long, 1 ft wide, 1.5 ft deep trench:
French drain soil volume = 100 × 1 × 1.5 = 150 ft³ → 150 × 1.35 = 202.5 ft³ loose soil

How to Calculate French Drain Pipe Volume?

To calculate French drain pipe volume, measure the pipe diameter and trench length. Convert the diameter to radius in feet, then use the formula for the volume of a cylinder. This helps subtract pipe volume from total trench or gravel volume.

Formula:

Pipe Volume (ft³) = π × (Radius in ft)² × Length
Radius = Diameter ÷ 2 ÷ 12 (if diameter is in inches)

Example:

100 ft long pipe, 4 in diameter:
Radius = 2 ÷ 12 = 0.167 ft
French drain pipe volume = 3.14 × 0.167² × 100 = 8.76 ft³

How to Calculate French Drain Gravel Volume?

To calculate French drain rock volume, measure the trench length, width, and gravel depth, then subtract the pipe volume before converting to cubic yards. This gives you the net gravel needed after accounting for the space taken by the pipe.

Formula:

Gravel Volume (ft³) = (Length × Width × Gravel Depth) − Pipe Volume
Pipe Volume = π × (Radius)² × Length
Gravel Volume (yd³) = Volume (ft³) ÷ 27

Example:

100 ft long, 1 ft wide, 0.75 ft deep, 4 in pipe (0.33 ft):
French drain gravel volume = (100 × 1 × 0.75) − (3.14 × 0.165² × 100) = 66.45 ft³ → 2.46 yd³

How to Calculate French Drain Backfill Soil Volume?

To calculate backfill soil volume, find the total trench volume, then subtract both the gravel volume and pipe volume. The result is the amount of soil needed to refill the trench above or around the drain system.

Formula:

Backfill Volume (ft³) = Trench Volume − (Gravel Volume + Pipe Volume)
Backfill Volume (yd³) = Volume (ft³) ÷ 27

Example:

Trench = 100 × 1 × 1.5 = 150 ft³
Gravel = 75 ft³, Pipe = 9 ft³
Backfill = 150 – (75 + 9) = 66 ft³ → 2.44 yd³

How to Calculate French Drain Fabric Area?

To calculate French drain fabric area, measure the trench length, depth, and width. Add up the surface area of the bottom and two sides, then include extra for overlap on top. This ensures complete wrapping and filtration.

Formula:

Fabric Area (ft²) = Length × (2 × Width + 2 × Depth) + Overlap
Add 10-15% for trimming and overlapping.

Example:

Calculate the French drain fabric Area for a 100 ft trench, 1 ft wide, 1.5 ft deep.
Hence the Fench drain fabric Area = 100 × (1 + 2×1.5) = 100 × 4 = 400 ft² + 10% = 440 ft²

How to Calculate Cost for the French Drain Materials?

To calculate the cost for French drain materials, you should first find the amount of material with proper units. Then check the unit price of the material in the market. Then multiply the unit price and the amount of material to have the cost of the French drain material

Formula:

Material Cost = Unit Price × Total amount of material

Example:

Calculate the material cost for the French drain rock 2.44 yd³. Unit price=$55/yd³
Total cost for the French drain material = $55/yd³ × 2.44 yd³ = $134.2

*Remember to add the waste factor before calculating the total volume.

How to Install a French Drain?

To install a French drain in the basement or crawlspace, you should follow the 7 steps below.

Inspect the basement and plan the French drain layout
Gather relevant tools and materials for the French drain
Excavate the French trench
Set up a catch basin
Lay the geotextile fabric at the bottom of the French drain
Add a rock base and install a perforated pipe
Cover the perforated pipe and backfill

What Should You Avoid When Constructing a French Drain?

There are 12 common mistakes you must avoid if you want your French drain to perform efficiently and last long.

Avoid solid PVC or corrugated non-perforated pipes.
Avoid laying pipes flat in the French drain.
Don’t install perforated pipes with perforations facing up.
Don’t fill the trench with soil or sand only.
Avoid placing drains near trees or aggressive root systems like maple, willow, or bamboo.
Don’t release the water at your building’s footing or a neighbor’s yard.
Avoid trenches shallower than 6 inches (150 mm).
Avoid using native clay soil to backfill the French drain.
Don’t dig without calling utility locating services (Call 811 in the USA).

Can I Install a French Drain DIY?

Yes, you can install a French drain DIY. First, you should gather the relevant tools and then follow the expert guide explained by the expert. Then you should follow the recommended requirements for the French drains. More than that, you can clean, unclog a French drain DIY also.