I’m volunteering to assist in a local community gardening project that will introduce and teach 75 families about organic gardening and need to get some math done before we can order our topsoil for the project asap.
I need to figure out how much soil I would need if each raised bed is 3 feet wide by 12 feet long and 18 inches in height. The beds need not to be filled to the top but somewhere around 10 inches and then we need to supplement with some compost too.
How would I figure out how much topsoil it would take per raised bed and than out how soil I would need for all 45 raised beds.
Than I need to calculate what its gonna costs us for our community gardening project.
Local topsoil costs $350 per 12 cubic yards which is what comes in a dumptruck size load. How many loads would I need and what would the costs come too?
Please and thank you to anyone willing to help out here.
Comments
I didn’t know things like this actually happened IRL. 
The volume of each bed would be 3 feet * 12 feet * 10 inches (5/6 feet), or 30 cubic feet. For 45 beds, that would be 30*45, or 1350 cubic feet.
1350 cubic feet is 50 cubic yards, which you would need five loads for, or $1750.
Actually, the numbers are a bit too perfect for me to think this is an IRL problem.
To determine volume it’s Length*width*height.
Since you are only filling the beds to 10 inches, we need to know that 10 inches is 0.83 feet
12*3*0.83=29.88 cubic feet
1 cubic foot = 0.037037037 cubic yard
Therefore, you would need 1.106 cubic yards of soil per planter.
Since you need 1.106 cubic yards of soil per planter, for 45 planters you would need 49.77 cubic yards of soil.
49.77/12=4.1475, so for an exact dollar amount you would pay $1,451.63 in total.
If they will only give you full dumptruck loads and you end up having to round up to 5 dumptruck loads rather than 4.1475 then you would pay $1750.
OK, the short answer is that if you don’t mind compromising the height of each bed by about half an inch (3′ x 12′ x 9.6 instead of 3′ x 12′ x 10), then you can do it in four loads for a total of $1,400. Otherwise, you’d need no more than five loads which would cost $1,750. Here’s how I figured it out:
If each bed really only has to be a rough rectangular box with dimensions 3′ x 12′ x 10, then that’s the same as (1 yard) x (4 yards) x (10/36 yards), which is (1 yard) x (4 yards) x (5/18 yards), which is 20/18 cubic yards. That’s approximately 1.11 cubic yards of soil per bed. So for 45 beds, you would need exactly 50 cubic yards.
Buying four loads would give you 48 cubic yards, which is just shy of what you need. So assuming that they only sell the soil in 12 cubic yard loads, then you’d have to buy 5 loads, which would cost 5*350 = $1,750.
This DOES leave a lot of soil left over, though. If you don’t mind making the beds a little shorter than 10 inches, and just stuck with four loads, then you’d save yourself $350 and only have to spend $1400.
Let’s see how MUCH shorter that would mean. If h is the height of the new beds, then
45 * (1*4*h) = 48
45h = 12
h = 12/45 yards = 9.6 inches.
So basically, if you just made each bed about nine and half inches tall instead of ten inches tall, you wouldn’t need the fifth load. Again, I’m assuming that you can only buy the soil by the 12 yd^3 load.
First, lets convert cubic yards to cubic feet
1 yd³ = 1yd x 1yd x 1yd = 3ft x 3ft x 3ft = 27 ft³
12 yd³ = 12*27 ft³ = 324 ft³
Assuming beds will be filled to 10 inches = 10/12 foot
Each bed is 3 feet wide, 12 feet long, 10/12 feet deep
Volume = 3 x 12 x 10/12 = 30 ft³
Volume of 45 beds = 45 x 30 ft³ = 1350 ft³
How many loads of 324 ft³ do we need to fill 1350 ft³?
1350 ft³ / 324ft³/load = 4 1/6 loads
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You say beds need not be filled to the top, but only about 10 inches.
Unless you are going to use a ruler to make sure a bed will be filled exactly 10 inches, you may want to calculate using height of 12 inches or 1 foot.
Now each bed is 3 feet wide, 12 feet long, 1 foot deep
Volume = 3 x 12 x 1 = 36 ft³
Volume of 45 beds = 45 x 36 ft³ = 1620 ft³
How many loads of 324 ft³ do we need to fill 1620 ft³?
1620 ft³ / 324ft³/load = 5 loads
__________
So you probably need about 5 loads
Cost = 5 * $350 = $1750
