# Gravel backfill - Check my math please

#### oh-DIY-io

##### Well-known member
Could someone check my math?

I'm coming up with about 50 tons of #57 gravel to back fill my overdig. That seems like a lot...

I've got a True L that's 18x43x28 with a mostly 30" overdig. That would make the hole roughly, 23x48x33 which would be 152' of perimeter (I think).

So my trench is
• 152'
• 30" wide
• 32" deep (I'm figuring 42" minus collar and decking)
I also did the math this way
• 1334 - sq ft of hole
• 954 - sq ft of pool
• =380 difference that needs back filled
Using both methods of calculation, the gravel calculators are telling me I need 45-50 tons of #57 gravel.

Sound right?

#### d_red1

##### Well-known member
Using the info provided above, I calculate 49cy in both cases. I have always used a conversion factor of 1.5 tons/cy for 57stone so that would equate to 74tons + 5-10% waste/compaction (although 57stone does not compact much as there are no fines). Call your local quarry and ask what they use for a conversion factor for the 57stone to be sure.

EDIT. I saw your post with the conversion calculator links. All those links yielded 62-67tons (neat, no waste) of 57stone based off of 380sf and the 42" depth you noted.

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#### oh-DIY-io

##### Well-known member
Using the info provided above, I calculate 49cy in both cases. I have always used a conversion factor of 1.5 tons/cy for 57stone so that would equate to 74tons + 5-10% waste/compaction (although 57stone does not compact much as there are no fines). Call your local quarry and ask what they use for a conversion factor for the 57stone to be sure.

EDIT. I saw your post with the conversion calculator links. All those links yielded 62-67tons (neat, no waste) of 57stone based off of 380sf and the 42" depth you noted.
Appreciate the input! I'm assuming the calculators are right so it was the way I came up with the inputs as much as anything.

It seems like subtracting the area of the pool from the area of the hole the best way to figure the area I need.

It's the assumption that 32" of height that I wasn't sure of. (I figured 32" would be the wall height minus concrete collar and decking.) I'll need 2.5 loads at 32". I'll round it up to 3 loads and use any extra with my decking or something.

I didn't think through compaction well. Good tip on asking the quarry!

Thanks!

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