Gardening By the Numbers: How to Calculate Cubic Feet and Cubic Yards

What seems like simple math may not be so simple for some, but it's crucial when ordering soil, mulch or compost. Get answers without anyone knowing you asked.

Earlier this spring, we installed a new raised garden bed at our house, bringing in additional soil and compost to get a nutrient-rich garden. While I’ve learned a thing or two about gardening over the past few growing seasons, I still have trouble figuring out how much soil I need to fill a raised bed or even a container. Cubic feet. Cubic yards. It all makes my two-dimensional head spin a little bit!

Here’s what I did. I measured my new garden bed at approximately 150 square feet, considered adding about 6 inches of soil, so calculated that as 75 cubic feet. To translate to yards, I divided by 3 (as there are 3 feet in a yard) and got 25 cubic yards.

That is SO wrong.

Some of you might be laughing, wondering how this simple math could be hard for anyone. My husband certainly makes fun of me for my lack of third-dimension visualization, but I know I’m not the only one. Here, let’s try your knowledge — and no peeking ahead.

Do you have your answer?

OK, now you can look below and see if you were right.

You may have noticed that we rounded up to 1/5 (.2) from .185 to make the amount more usable, so if you got 1.185, you’re right. But if you didn’t get anywhere close on the first try, don’t feel too bad. Maybe they just said it to make me feel better, but the landscaping company around the corner from our house told me that people get it wrong all the time, often doing what I originally did and WAY overestimating the amount of soil needed in cubic yards. Thankfully, they stopped me before I bought 25 cubic yards of compost, which may well have covered our entire yard.

So that you (and I) don’t ever make that mistake again, see below for a lesson on how to calculate cubic feet and cubic yards. We’ll use a standard size for a basic raised bed garden: 4 x 8 x 1 feet.

Step 1. Start by taking the measurements of your bed or container in increments of feet: width, length and depth (aka height). If you want to prep an in-ground bed by working in 6 inches of soil or compost, you can count that as your depth measurement, but you’ll need to factor 6 inches as .5 feet. (Note: Square footage would just be the length x width, not including the depth.) 

Step 2. Now multiply width x length x depth to get cubic feet. Bagged soil and compost are often measured in cubic feet, so this measurement should help you purchase the right number of bags for your bed or container. 

Step 3. Now, moving on to cubic yards. If you have a larger garden bed, it’ll be more cost effective to buy soil and compost in cubic yards. You can see here how a cubic foot fits into a cubic yard. Essentially, a cubic yard is made of 27 (3 x 3 x 3) cubic feet. (P.S. This whole cubic yard diagram reminds me think a lot of a Rubik’s cube.)

Step 4. So to calculate the amount of cubic yards, take your cubic feet measurement (length x width x depth) and divide by 27. (See my earlier mistake here? I just divided by 3.)

Step 5. For reference, it’s been my experience that a cubic yard or so of soil will fill the bed of a small pickup truck (+/- depending on the water content and fluffiness of the soil and the size of your bed). 

Step 6. Getting the soil right goes a long way toward growing a successful garden, so it’s important to get your measurements right. Good luck! 

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