How to calculate how much mulch to order

How much mulch should I order for my garden bed?

When ordering bulk materials, you typically need to order by the cubic yard. Here’s how to calculate how many cubic yards of soil or mulch you will need for your project. I’ll also show you how to convert your calculation to cubic feet if you decide to purchase bags of mulch (by the cubic foot).

Before you start:

Contact your local bulk bark and soil supplier. Find out:

If there are any minimum amounts for delivery

What they charge for delivery (fees? Additional cost per mile for certain distances?)

That your address is within their delivery range

If there is a certain amount to order to receive free delivery

If they offer half cubic yards (this makes it easier to order the exact amount you need)

The price per cubic yard of the material you desire to use (learn more about mulch materials, here)

Call them back to place your order once you have calculated the amount of materials you will need

How to measure the area (and take notes)

Next, measure the length and width of the area (in feet or meters).
If you have an area that’s a complicated shape? Break it down into multiple, smaller, simpler shapes (like rectangles or circles). If a space is complex, I prefer to make a quick sketch on paper so I can make note of measurements and keep better track of calculations.

Here's a pretend example...
Area 1: Length 15 feet, width 3 feet
Area 2: Diameter 3 feet
Area 3: Length 30 feet, width 10 feet

Shapes don’t need to be perfect, just get as accurate as you reasonably can. For example, if you have a long oval shape, essentially “cut off the ends” and make two half circles, and measure just the rectangle in the middle (subtracting the space the two half circles take up).

Or if the corners are rounded but not to the point that it's a circle, consider the shape a rectangle and just leave a few inches off the measurement.

When measuring a circle, you just need the diameter (measure across the center of the circle, one end to the other) or radius (halfway across the center of the circle, from the center point to the end. When measuring a rectangle, you need the length and the width.

How deep to mulch?

Once you have the length and width of each area, it’s time to decide how deep you want each area to be.

For example:
Area 1: Length 15 feet, width 3 feet, 1 inch deep
Area 2: Diameter 3 feet, 1 inch deep
Area 3: Length 30 feet, width 10 feet, 4 inches deep

If weeds are fully and properly removed, and if you plan to plant in the area, you should mulch 2-4 inches (5-10 cm) deep (on top of the soil) the first time you mulch. I personally aim 3 inches. The material settles a bit over the first few months. Four inches is a lot of mulch, and you’ll need to taper the depth down between plants and around the edges of beds (Four inches of arborist chips is also much easier to work with than 4 inches of something like small fir bark nuggets, which are round and spill all over the place, making 4 achieving inches deep around sidewalks and plants impractical in most yards).

If you are “topdressing” an area that is already mulched, 1 inch (~3 cm) is often plenty. Although, if mulch has thinned notably, there are lots of weeds coming through, or it has been a long time, topdress with 2-3 inches (5-8 cm). Pull any weeds before you spread mulch!

If you are using mulch to smother out a weedy area, 5-8 inches (13-20 cm) or more may be needed, depending on the weeds you are smothering and the material you use (note that free arborist chips are best for this method, mostly because they are free. Or a mix of mulch and other organic materials like grass clippings, plant trimmings, leaves, etc. But this is a topic for another blog post!)

How to calculate the amount you’ll need

The next step is to type your numbers into this calculator. Pay attention to your units! Length should be feet, width should be feet, and depth should be in inches. I personally like type in one area at a time, write down the resulting amount for each in my notes, and then add up the total amount last. This will tell you how many cubic yards of materials you need.

This gives me context: how much will be used in each area. If I decide to mulch deeper in one area, I can just make the change to this one area and re-add the total.

Area 1: Length 15 feet, width 3 feet, 1 inch deep = 0.1481 cubic yards
Area 2: Diameter 3 feet, 1 inch deep = 0.0262 cubic yards (see circle calculation below)
Area 3: Length 30 feet, width 10 feet, 4 inches deep = 3.7037 cubic yards
TOTAL = 3.878 cubic yards

How to calculate the amount for a circular area

To calculate the area of a circle? Remember pi? The fancy symbol, 𝞹? Which means this random number: 3.14159265359? The area of a circle is pi times the radius times the radius, or πr^2. The radius is one half the length of the diameter.

So in our example above, where we have a diameter of 3 feet? The radius is half that, or 1.5 feet.

1.5 x 1.5 x 3.14159 = 7.0685 is the area of the circle in square feet. You can enter this amount into the second tab of the calculator above, plus the desired depth.

OR you can use this calculator, below:

Mulch Calculator

Cubic yards vs. cubic feet of mulch materials

(And how to convert from cubic yards to cubic feet)

When ordering bulk materials, you typically need to order by the cubic yard. When purchasing mulch by the bag, sometimes the materials will be in cubic feet, sometimes in cubic yards.

CUBIC yards. Not square yards. Cubic includes 3 numbers: length, width, AND depth/height. Don’t make the mistake of calculating just the square yards (length and width).

A cubic yard is 27 cubic feet (3x3x3 feet). NOT 9 (3x3), a common error.

If you want to convert from cubic yards to cubic feet, multiply by 27. Or from feet to yards, divide by 27.

If you don't want to use a calculator

If you don’t want to use the provided calculator, it is simple multiplication EXCEPT that you must convert inches to feet before you multiply, or your result will be incorrect.

So let’s say you have an area that’s 10 feet long, 3 feet wide, and you want to mulch 5 inches deep. Divide the 5 by 12 before multiplying by 10 and then again by 3. Then you will have your result in cubic feet. Convert it to cubic yards by dividing by 27.

10 x 3 x (5/12) = 12.5 cubic feet / 27 = 0.46 cubic yards

What to do when you get a weird random number (round up or down?)

Depends on the result, but I am often inclined to round up to the nearest cubic yard (or half cubic yard, if the soil supplier offers this). I like to make the mulch a bit deeper in any areas where there are no plants (or any large spaces between plants), and a bit deeper toward the center of the garden bed.

So our example, which came to 3.878 cubic yards? I'm definitely ordering 4 cubic yards of mulch and finding places for the extra.

Generally, though, round up! Mulch is worth the investment: the deeper you mulch, the lower maintenance your landscape will be. Just don’t mulch deeper than 4 inches (10 cm) around plants you want to keep alive. This includes under the drip line (under the branches) of trees.

Some suppliers will let you purchase a few 5 gallon buckets of materials if your project ends up a bit short.

Consider storing some extra mulch

You may want to plan to intentionally store extra materials. It can be nice to have an extra bit of mulch on hand in case you need to fill in a few spots (for example, if a little soil gets mixed into your mulch after transplanting something).

You can easily store small amounts of mulch in old potting soil bags, buckets, or a garbage can with a lid.

Or, consider a corner of your yard for a small pile of extra mulch. Larger amounts of extra mulch can be helpful if you are consistently making changes to the space (like planting trees or shrubs in a field of grass every, which will need a circle of mulch).

Before you order

Consider where you would like to have the materials dumped (street? driveway?). Do you want to put a large tarp down first? Or are you okay with sweeping materials off the driveway?

Think about other areas you might need mulch for. Want to smother an area of weeds or lawn in preparation for a future garden bed? Plan next year's garden now and order the mulch you'll need for that, too. Perhaps ordering more will save you the delivery fee and get you a head start on next year’s project. Don’t cover up any aggressive, invasive weeds, though!

A quick note on ordering arborist chips

When requesting free arborist chips in my area, you don’t get to choose how much you get. It's free, but the trade off is that the amount of material you end up with is often a mystery!

Luckily, arborist chips are amazing at decomposing, turning into soil, and revitalizing the soil below it. And if you end up with too much? Post on your favorite neighborhood social media platform, hang up a free sign, and let neighbors know they can have some for free, too. I’ve picked up free arborist chips from neighbors this way (picture me and a wheelbarrow going up the street).