Much Ado about Mulch

by Blackthumb Bob

Last Saturday, I went down to my local "earth products" retailer to buy some mulch for my beds. When I got there, the clerk said, "How much?" I mumbled something like how much do I need if my beds are so long by so wide. That was where I really messed up. I overestimated the width of my beds by a good third. As a result, I have enough mulch for all the beds I meant to mulch as well as the ones I did not want mulched too.

Don't let this happen to you! Here's how.

First, figure out how big your beds are BEFORE you get to the retailer. At least pace it off, if not out right measure them. This is the single most important thing you need to do.

Next comes the math part. OK, get the calculator. Got it? OK, let's go on.

First we have to calculate the AREA of the beds. Length times Width for beds that are pretty much rectangular.

Circles are a bit harder. Remember that "Pie are Square" thing? First, how WIDE is that bed anyway? The width is the diameter. Take half the width. That's the radius. Now get the calculator. Take the radius times the radius (the "are Square" -- r² - thing). Next multiple this number by 3.2 (the "Pie" thing -- actually pi is an irrational number approximately equal to 3.141592654..., but this is gardening, not brain surgery. If you really want to know what pi equals, some dude has it out to 10000 decimal places. Personally I think this is a bit much.)

Area Examples:

You have a bed that follows your driveway for about 45 feet. It is about 3 feet wide. Even though the drive curves, you can treat this bed, pretty much like a rectangle.

AREA = Length X Width

AREA = 135 square feet.

(International viewers, please forgive the use of English measures. Metric measures can be substituted and the same principles apply.)

Suppose you have a round bed that is 10 feet across. Half the diameter is the radius 5.

AREA = radius X radius X 3.2 (approximately)

AREA = 5 X 5 X 3.2

AREA = 80 square feet.

One more. Suppose you have a bed that is a 15 foot half circle against the fence. To do this, you have to calculate the area of the whole circle, then take half.

AREA = radius X radius X 3.2 (approximately)(Take half)

AREA = 15 X 15 X 3.2 (Take half)

AREA = 720 square feet. (Take half)

AREA = 360 square feet

Next, decide how deep you want the mulch to be. Most people spread it 3 or 4 inches. Maybe 6 inches in some place. This is good, because these number work well with feet and inches. Here's how.

If you want your mulch 3 inches deep, that is equal to 1/4 of a foot deep. Four inches is 1/3 foot and 6 inches is 1/2 foot. Since VOLUME is AREA X DEPTH, you multiply again. HOWEVER those pesky fractions are involved. Remember that "multiplying by a fraction is the same as dividing by the reciprocal" rule? That means multiplying by 1/4 is the same as dividing by 4. So depending on how deep you want your mulch, you divide (get the calculator) the AREA by 2, 3, or 4.

Volume Examples:

Take that first bed along the driveway. If you want mulch 3 inches deep, you need 33 3/4 cubic feet of mulch. (135 divided by 4 equals 33.75.)

If you want it 4 inches deep you need to divide by 3 so you need 45 cubic feet. (135 divided by 3 equals 45.)

Now you know what volume of mulch you want. Next, you have to get it home. If you buy it by the bag, remember that most bags come in 3 cubic feet sizes. Divide (math again) the total you want by 3 to know how many bags you need.

If you buy it from some place with dump trucks, they will sell it my the "yard" which is 27 cubic feet. In such a case, divide your total need by 27.

Final example.

You have a bed along the front and side of your house that is 120 feet long and averages about 5 feet wide. This is an area of 600 square feet. If you want mulch 4 inches deep you have need of 200 cubic feet of mulch. At 27 cubic feet per yard, you need 7.4 yards. At 3 cubic feet per bag you need 66 2/3 bags.

Any questions?

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