Easy French Drain Gravel Calculator

This is a free gravel volume calculator for your french drain trench. Calculate how much gravel you need to fill your ditch without the pipe.

French Drain Gravel Calculator

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FEET METERS

Trench Length


Trench Width


Trench Depth


Pipe Diameter


Pipe Length


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The Formula for calculating the gravel needed for your french drain

The trench for your french drain can be represented mathematically by a cuboid.

The pipe in your french drain can be represented mathematically by a cylinder.

How to Calculate the Volume of a Cuboid

To calculate the volume of a cuboid, we use the following formula:

l * w * h

Where l represents the length, w the width and h the height of the cuboid.

How to calculate the Volume of a Cylinder

To calculate the volume of a cylinder, we use the following formula:

V = Π r ² h

Where h represents the height (or, since it’s vertical, the length) of the pipe, and r represents the radius, which is half the diameter (the width) of the pipe.

Calculating the Volume of gravel

To calculate the amount of gravel needed to fill the space between the pipe and the walls of your french drain, we simply subtract the volume of the pipe from the volume of the trench. We now get the volume of the remaining space, which you want to fill up with gravel. Our french drain gravel calculator above does the entire calculation for you.

Keep in mind that you don’t have to stack the gravel tightly, or else water will not flow through the rocks. Generally, river gravel or granite works well, as it lets water through and does not deteriorate over time.

Common units

• 1 foot = 30.48 centimeters
• 1 inch = 2.54 centimeters
• 1 inch = 1/12^th of a foot = 0.83333 feet
• 1 foot = 1/3^rd of a yard = 0.33333 yards
• 1 cubic foot = 1728 cubic inch
• 1 cubic yard = 27 cubic feet
• 1 mile = 1760 yards = 5280 feet

Tips for measuring your french drain

In order to accurately calculate the amount of gravel you’ll need to fill up your french drain trench, you’ll need to calculate two things:

• The length, width and depth of the trench
• The volume of the pipe itself

Once we have these two numbers, we’ll deduct the volume of the pipe from the volume of the trench. The number we end up with is the amount of gravel you’ll need.

Measuring the dimensions of the french drain trench

You want to note and write down the length, width and depth of your trench. It’s important to note that your trench will not have uniform dimensions throughout its span, and it may vary due to small anomalies made during the digging.

In order to make sure you don’t order too much or too little gravel, measure the width and depth at regular intervals. You can take the average of the data points to determine the average width and depth. In the case that there are big differences between the width and depth of the french drain trench, we reccomend to either make the trench more uniform, or to calculate the volume of uniform sections separately and add them together at the end to find the volume of gravel you’ll need.

Measuring the pipe

When you buy a pipe for your french drain, the dimensions are included in the name of the pipe. For accuracy, we reccommend that you use these measures, instead of measuring the pipe itself. Check your receit if you don’t remember these dimensions. Alternatively, you can always measure the length and diameter itself. To do this, we recommend you lay the pipe flat on a table, and measure its width at a 90 degree angle from the table, to get the most accurate measuring result.