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are my calculations correct?


i am going to need over 500 cubic feet of drain rock for this french drain?

 sorry for the sloppy picture.

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Posted 2012-09-04T17:45:43+0000  by NerdrageIMO NerdrageIMO

Hi NerdrageIMO,


Thank you for your question and welcome to our community.  From looking at your photo, it is not clear as to the amount of area you area dealing with.  If you follow this simple formula, I think it will help you in doing your calculaions:


The formula for Cubic feet is; length x width x (height in feet)


Once you have calculated the square feet, multiply that with the (Height in feet) your trench is.  That will then equal the cubic feet of drain rock you will need.


So if you wanted to put the rock  6 inches deep, you would need 12 x .50 =  6.  You would need 6 cubic feet of drain rock.


Conversion Calculator


Please let us know if we can be of furhter assistance.  Good luck with your project.

Posted 2012-09-04T19:57:48+0000  by Rick_HD_OC

i have a 20'x20' deck...
2 ' out from the deck i am digging 2' and 2' out again, surrounding the parameter of the deck.

in one corner i am running the drain off to a hole which is 7'x7'

all of this done at a 2% decline
Posted 2012-09-04T20:40:05+0000  by NerdrageIMO

Hey NerdrageIMO,


Picture is just fine no worries...


Assuming that the 10' foot trench is going to be 2' feet wide i'm getting aprox. 505 cu. feet...


This is the breakdown:   2'7"D x 24'L x 2' W x 2 = 249.6 cu feet ...   that's for two 2'7" deep sides +

                                           2'D x 24' L x 2' W x 2  = 192 cu feet ... that's for two 2' side of the deck 

                                           3'2"D x10'L x 2'W = 64 cu feet that's the culvert ...


                                           TOTAL  505.6 cu3. feet




Here's the chart  with area per ton coverage ...




I'd suggest to get two different sizes of can do "coarse" fill in combination with top 3" to 4" of roadgrade   type A grade...may I ask why are you going so deep with the trench?


Hope this helps,







Posted 2012-09-04T20:45:52+0000  by George_HD_CHI

Hi again NerdrageIMO,


Thank you for the additional information.  Ok, let’s do the calculations together. 


Your trench is going to be 2ft wide by 2ft deep and is going to be 80 ft long ( 4 sides of 20 feet each)  80 x 2x 2 = 320 cu ft, right?   ( 80 ft  x 2 ft = 160 sq ft  x 2 ft =  320 cu ft.  of drain rock.   


The next calculation is the trench to the drain hole that is 7x7.  How wide is the trench, how deep will it be and how long?  If the trench is the same depth as the area around the deck then we know the depth is going to be 2 ft.  The unknown is the width of this trench and the length of the trench, assuming you are going to put drain rock in this trench as well.  The hole where the water is going to drain to is 7x7?  Is that 7ft square x 7ft deep?  If so that will be another 490 cu ft of drain rock need to fill the drain area.  


If the trench to the drain area is 6 inches wide the your calculation will be: .5 x 2 x length (which is the unknown)


I hope this helps.  Steel Toes has a good chart  and some great info too!  Check it out.

Posted 2012-09-04T21:07:34+0000  by Rick_HD_OC

the far corner is the only 2' deep part.. its at a 2% decline going both directions from that corner.

this means its going to be ~2'7" at two corners and ~3'2" near the discharge area.

im doing this so deep because the area i am building is a bowl where water gathers.. i am just making a place for it to run so that it doesnt settle under my deck and heave during winter.
Posted 2012-09-05T19:31:44+0000  by NerdrageIMO

I haven't read through the whole post, but I'll provide some input. Google Sketchup will provide you volume measurements for your shapes. Just the deck area alone is 507 cubic feet.

french drain calculation.jpg



Typing the following into a Google search bar...



Posted 2012-09-05T22:50:13+0000  by Paul

thanks for the reply with the sketchup


i didnt know it could do that...





do you guys think this is overkill?


the soil is pretty dry right now due to the lack of rain.. but i need to get this drain in... i was told i could go down to 16" at the starting point...


and save 3" for soil with grass on it (i dont want the drain to show)

Posted 2012-09-06T16:37:51+0000  by NerdrageIMO


what if i were to cut it down to this size?

starts at 1.5' deep at one corner with a decline of 2%.

the outer measurements of teh trench would be 26x26 @ 1' wide


the 25' measurement is one 1x1 corner plus the middle which is 24x1

again.. give 3" depth for soil and grass


not sure if this is confusing... lmk if i need to clarify anything!

Posted 2012-09-07T15:02:19+0000  by NerdrageIMO

Hi again NerdrageIMO,


Remember that your objective is to supply a means of escape for the water.  A normal drain pipe is six inches in diameter.  You slope coupled with the amount of drain gravel you are using should be more than adequate as SteelToes pointed out.  When I put my French drain in, I wrapped the gravel in a professional grade weed mat to help keep the finer sand out of the gravel. 


It is always better to be over prepared than it is to be under prepared.  Ultimately, the amount of preparation needed will be up to you.  Only you know what the inclement weather is like in your area and you will need to prepare accordingly. 


Be sure to take some pictures of your project as it progresses along and share them with us.  Best of luck with your project.


Ricks Picks

Posted 2012-09-07T18:04:09+0000  by Rick_HD_OC

You don’t have to trench all around the deck, you could just make one centralized trench to collect the water and direct wherever you wish to...


This would work better and it involve much less work and materials...also all of the dirt you got from the footings and trench you are going to dig … you can use to grade the ground  under the deck towards the trench...No brainer.nideck.JPG

Hope this helps,




Posted 2012-09-11T16:00:10+0000  by George_HD_CHI
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