French Drain Gravel

A DIY French Drain around the outside of your house or property is an efficient way to divert water eliminating the effects that water damage can have and opening up a new space that you didn’t think you had before and maybe improving your real estate’s value. How Much Gravel Do I Need for a French Drain?

Hard washed Gravel used drainage stone that is a Gray granite approximately 1/2 to 3/4 inch in size on average gravel is most commonly used in French drains, if your French drain will be 30 feet long and 1 foot wide, you would use this formula:

30ft x 1 x 0.25 = 7.5 cubic feet of Gravel per/30ft.

Groundwater always finds its way to the lowest points, and the slope in the pipe helps to channel water diverting away from the area you are trying to protect making a French Drain with the right size gravel a perfect tool for water diversion.

How Much Gravel Do I Need for a French Drain


The hard type gravel used on the bottom layer of the trench for the French Drain should fill it one-third of the way if using a perforated pipe, or use two-thirds in the trench if not using any perforated piping. The gravel can act as an underground path or waterway for flowing groundwater. Gravel used for this layer is typically 1/2 inch to 1 inch across theProfessional gutter and Drain | French Drains | Columbus Ohio  larger it is, the better the water flow and less chance of clogs and blockages.

Animation of a French drain cross-section showing how rising groundwater gets into the French drain.For a French drain without a perforated pipe, pick for even larger gravel, such as 1 1/2 inches across is best for drainage. Smaller gravel such as pea-type gravel would not allow sufficient water flow. No matter what the gravel or stone, look for washed varieties of gravel, otherwise fine dust and particles may hinder or even stop water flow.

Hard gravel rocks used for French drain projects such as granite or river gravel are proven a reliable option for a trench drain like a French Drain because they won’t break down over time as much as other rock varieties, such as sandstone and limestone.

River gravel won’t lose its size because of the fact that it has already spent a good amount of time eroding down to its current size, as it is acquired from dried river beds before being collected cleaned, and sold to the public.

Drainage stone that is a Gray granite approximately 1/2-3/4 inch in size average gravel. Mostly used in French drains, the most common size used around drainage boxes and pipes. Often used as base stone for driveways and patio applications. The size of the gravel although very small can make a huge difference in work application.

#57 – Contains particles about ¾ inch in diameter

#67 – Contains particles ¾ inch or less in diameter



How Deep Should a French Drain Be?

About 8-24″ deep should be sufficient for many H2O diverting projects
About 18-24 ” minimum depth for foundations-this depth helps capture most of the H2O that’s an issue to protect your foundation or keep H2O out of the basement.
For a slab foundation, French Drain is about 18-24 ” below the slab. ………………………………………………………… Read more

Amount of Gravel You Need


To figure out how many cubic feet you’ll need for your French drain, multiply the length by the width by the depth. For French drains, the depth will be 3 inches, which is 0.25 feet. For example, if your French drain will be 30 feet long and 1 foot wide, you would use this formula:

30 x 1 x 0.25 = 7.5 cubic feet.

The illustration of the cross-sectional view of a french drain with an embedded drain pipe.
French Drain Width x Depth x Outside Diameter

If you’re looking at gravel that comes in pounds, you’ll need an additional step. 1 cubic foot of gravel is equal to about 96 pounds, so you’ll multiply your cubic feet by 96 to figure out how many pounds you need. For the example above, you’d multiply 7.5 by 96 to get 720 pounds of gravel. 

Lay the perforated pipe in the trench on top of the gravel. All you need to do is lay the pipe on top of the gravel bedding with the perforation holes facing the ground otherwise, the top layer of crushed stone and soil will clog the pipe. Look for a 4-inch diameter pipe. Any smaller and you risk clogs. Skip corrugated pipes and go for a rigid PVC drain pipe for durability and crush resistance. 

Gravel delivered for a French drain project costs about $270-$350 for a 20-foot drainage system most projects run about (100 ft. x by 5 approximately $1500.00)

This French drain calculator will help you find how much gravel you need for your French drain installation. In other words, this tool is our Gravel Calculator version for French drains.



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JimGalloway Author/Editor



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