Most, if not all, of the people that would visit this site more than likely have some form of a garden... probably. But how many of you know how many cubic yards of materials are needed in said garden?

If we use my drawing as an example, I'll show you how to determine the amount of dirt, in cubic yards, that is needed to put three inches of dirt in all six of the raised planters.

In a nut shell, and to give you a full measure of the math, we are going to go, mathematically, from square feet to cubic feet to cubic yards... if you can multiply and divide you're good to go.

Square Feet: There are 4 planters that measure 6' long (length) by 4' wide (width) and there are 2 planters that measure 4' long (length) and 4' wide (width). NOTE: The math below does not account for the potato planter as it is filled continuously as the plants grow.

6' x 4' = 24 sq ft x 4 (planters) = 96 sq ft (subtotal #1)
4' x 4' = 16 sq ft x 2 (planters) = 32 sq ft (subtotal #2)
96 sq ft (subtotal #1) + 32 sq ft (subtotal #2) = 128 sq ft (total #1)

Unfortunately, this information is, basically, only useful for the purpose of communicating the sheer size of your garden when speaking with friends, neighbors, family, and colleagues.

Cubic Feet: To find cubic feet, you need to add the dimension of depth. So, say I wanted 3" of dirt in each planter. To get to cubic feet, I simply multiply L x W x D. However, I first have to convert any dimension that is measured in inches over to feet. As a result, I need to divide my depth (3") by 12". Therefore, 3" / 12" = 0.25'.

6' x 4' x 0.25' = 6 cu ft x 4 (planters) = 24 cu ft (subtotal #3)
4' x 4' x 0.25' = 4 cu ft x 2 (planters) = 8 cu ft (subtotal #4)
24 cu ft (subtotal #3) + 8 cu ft (subtotal #4) = 32 cu ft (total #2)

OK, so now we're getting somewhere. We now know that we have 32 cu ft (total #2) in the six raised planters.

Cubic Yards: Now that I have cubic feet, I need to divide total #2 (32 cu ft) by the number of cubic feet in a cubic yard, which is 27.

32 cu ft / 27 = 1.19 cu yds

If I round up the purchase to make sure I have enough dirt due to settling, I'll purchase 1.5 cu yds of dirt. Well, technically I won't be purchasing the dirt as the dirt is already available back at my house... that's a completely different problem

Amount of Pathway Rock Needed

Calculating the amount of rock needed is a bit more complex because, while the pathways are contiguous, you have to add up the length and width and depth (2") for each pathway. What I did for this calculation was to determine the vertical paths (4) and the horizontal paths (2)... look at the image above again and you should quickly see what I am referring to. Here's what the math looks like for that:

Overall Pathway Depth: The entire rock path will be 2" deep. So I need to convert that to feet --> 2" / 12" = 0.16' (D).
Overall Pathway Width: The path is 1' 6" all the way around for both the vertical and horizontal paths. As a result, I need to convert the 6" in the 1' 6" dimension to feet. That's easy --> 6" / 12" = 0.5', then simply add the 0.5' to the original 1' dimension --> 1.5'. The 1.5' (W) dimension will be used hereafter.
Vertical Pathway Length: Each vertical path in the image is 4' long (L1).
Horizontal Pathway Length: I have two pathways that measure 13' 6" in length. Just like the Overall Pathway Width, I need to convert the 6" to feet. Since we know that number is 0.5', I'll just that to the original 13' dimension which gives me 13.5' (L2).

Calculations:

Vertical --> 4' x 1.5' x 0.16' = 0.96 cu ft x 4 (vertical paths) = 3.84 cu ft / 27 = 0.14 cu yds
Horizontal --> 13.5' x 1.5' x 0.16' = 3.24 cu ft x 2 (horizontal paths) = 6.48 cu ft / 27 = 0.24 cu yds
Total Rock Needed: 0.14 cu yds (vertical paths) + 0.24 cu yds (horizontal paths) = 0.38 cu yds total

Rounding up the 0.38 cu yds indicates that I'll need about 1/2 yard of rock.

Easy peezy... now you know how to estimate.