The proper formula for calculating an amount of stone needed is as follows:
Length (feet) x Width (feet)x Depth (feet)x 100 (or 118 - I will explain the difference in a moment) / 2000. That is length times width times depth times 100 or 118 divided by 2000 (if in area where the metric system is used use meters and/or centimeters just keep all of the measurements the same).
The length and the depth are found by simple measurement in feet. The depth is the depth in inches converted into feet, ie. 1" = .083 feet (divide 1" by 12"(number of inches in a foot) When calculating for gravel all of the units need to be the same - generally feet because the areas are large that one is usually trying to fill with gravel
|1" = .083
2" = .167
3" = .250
| 4" = .333|
5" = .417
6" = .500
Back to the choice of either 100 or 118- This has to do with the weight of a particular type of stone. In our area we use a crusher run or just regular stone of different sizes. The crusher run is what is left over after the various different stone sizes have been screened out. A motley collection of odd small sizes and sand like material that has absolutely no uniformity at all. This weighs approx 118 lbs /Square Foot. Regular Stone weighs approx. 100 lbs per square foot. This 100 lbs per square foot applies to all stone because the larger the stone the fewer you get in a given area so it all kind of averages out. Most folks who want to use rock for a run or the like would use 100 in the above calculation.
For example you have a 60 foot long dog run 8 feet wide that you want to fill with 5 " of gravel.
The calculation is as follows-60 feet x 8 feet x .417 x 100 (regular stone) / 2000
The answer would be 10.008 Tons of rock necessary.
The first part of the calculation is 60 x 8 x .417 x 100 = 20016. This is the number of pounds of rock.
Divide by the 2000 to find the number of tons so 20,026/2000 = 10.008 Tons
One "yard" is equal to aprox 1.35 Tons - just do 3 x 3 x 3 x 100 / 2000.
A "yard" refers to a cubic yard which is a cube 3 feet on each side. So it is 3 feet long, 3 feet wide, and 3 feet thick.
Don't forget when ordering rock that sometimes the tax and the delivery charge (dump truck cost) are not figured in.
I hope I haven't given too much detail here I was just trying to answer the question thoroughly. My qualifications are - I develop subdivisions and contract out road construction for them so I need to do these calculations frequently. If any of you are mathematically challenged I will do the calculations for you. Just send me your dimensions and I will e-mail the results back to you.