Measure your garden beds in square feet, then divide by the depth you want

To find out how much mulch to buy, you need three numbers: the length of your bed in feet, the width in feet, and the depth of mulch you want in inches. Multiply length times width to get square feet. Then use a straightforward formula to convert that to cubic yards, which is how mulch is sold.

The math works like this: (square feet × depth in inches) ÷ 324 = cubic yards. That 324 is a fixed number that accounts for converting inches to feet and feet to cubic yards. If you are buying mulch by the bag instead of in bulk, most bags cover 8 to 10 square feet at a 3-inch depth, so you can count bags instead.

Start by sketching your beds on paper or taking photos with measurements written on them. Irregular shapes are easier to handle if you break them into rectangles, calculate each one, and add them together.

Key Takeaways

  • Multiply the length and width of your bed in feet to get square feet, then use the formula (square feet × depth in inches) ÷ 324 to find cubic yards.
  • Most mulch is sold by the cubic yard in bulk or by the bag, with each bag covering roughly 8 to 10 square feet at 3 inches deep.
  • A 2-inch depth works for established beds and pathways; use 3 to 4 inches for new beds or areas where you want strong weed suppression.
  • Irregular beds are easier to calculate if you break them into separate rectangles, measure each one, and add the results together.
  • Add 10 percent extra to your total to account for settling and bare spots that show through after a few months.

Decide on the depth you need before you measure

The depth of mulch matters because it changes how much you need to buy. A shallow layer of 2 inches works fine for established beds where you are just refreshing old mulch. A 3-inch depth is standard for new beds and gives better weed control. If you have a serious weed problem or are mulching around new trees, 4 inches is reasonable, but anything deeper than that wastes money and can trap moisture against the soil.

Keep mulch a few inches away from tree trunks and plant stems. Mulch piled against wood or bark can rot it and invite insects. A ring of bare soil around the base of a tree is normal and healthy.

Measure length and width in feet, then calculate square footage

Use a measuring tape to find the length and width of each bed. If your bed is 10 feet long and 4 feet wide, multiply 10 × 4 = 40 square feet. Write this number down for each bed you have.

For beds that are not rectangular—curved edges, odd corners, or L-shapes—break them into two or more rectangles. Measure each rectangle separately, calculate the square feet for each, and add them together. A bed that is L-shaped, for example, can be split into a long rectangle and a short rectangle. This method is close enough for mulch calculations; you do not need to be exact to the inch.

If you have a circular bed, measure across the widest point (the diameter) and divide it by 2 to get the radius. Then use the formula: radius × radius × 3.14 = square feet. A bed with a 6-foot diameter has a radius of 3 feet, so 3 × 3 × 3.14 = 28.26 square feet.

Convert square feet to cubic yards using the depth you chose

Once you have your total square feet and you have decided on a depth, plug both numbers into this formula: (square feet × depth in inches) ÷ 324 = cubic yards.

Here is a real example. You have 200 square feet of beds and you want 3 inches of mulch. The math is (200 × 3) ÷ 324 = 600 ÷ 324 = 1.85 cubic yards. Round up to 2 cubic yards to be safe.

Another example: 150 square feet at 2 inches deep. (150 × 2) ÷ 324 = 300 ÷ 324 = 0.93 cubic yards. You would order 1 cubic yard.

Add 10 percent extra for settling and coverage gaps

Mulch settles as it breaks down and as rain compacts it. Bare spots also appear after a few weeks where the mulch has shifted or washed away. Adding 10 percent to your total order means you will have extra on hand to top off these areas without a second trip to the supplier.

If your calculation came to 2 cubic yards, add 10 percent: 2 × 0.10 = 0.2, so order 2.2 cubic yards. Most suppliers will round this to 2.25 or 2.5 cubic yards, which is fine. For smaller orders under 1 cubic yard, buying one extra bag of mulch covers the settling buffer.

Use the bag method if you are buying bagged mulch instead of bulk

Bagged mulch is sold by volume—usually 2 cubic feet per bag—and each bag covers roughly 8 to 10 square feet when spread 3 inches deep. The coverage varies slightly depending on how densely you pack it, but 8 to 10 is a reliable middle ground.

To find how many bags you need, divide your total square feet by 8 (the conservative estimate). If you have 200 square feet, 200 ÷ 8 = 25 bags. If you use 10 as the estimate, 200 ÷ 10 = 20 bags. Buying 22 to 24 bags gives you a comfortable margin.

Bagged mulch costs more per cubic yard than bulk mulch, but it is easier to handle for small projects and you can buy only what you need without a delivery fee. For beds totaling more than 300 square feet, bulk mulch is usually cheaper.

Account for different mulch types and their settling rates

Shredded hardwood mulch and pine bark mulch break down at different rates. Hardwood mulch lasts longer—usually 2 to 3 years before it needs refreshing—while pine bark breaks down faster, typically in 1 to 2 years. Both settle noticeably in the first month, so the 10 percent buffer applies to both.

Dyed mulch and natural mulch have the same volume and settling behavior, so your calculation does not change. The color is cosmetic. Rubber mulch does not settle or break down, so you can skip the 10 percent buffer, but it is more expensive and does not improve soil the way wood mulch does.

Frequently Asked Questions

What if my beds are oddly shaped or have multiple sections?

Break the whole area into rectangles, measure each rectangle separately, calculate square feet for each, and add them together. For curved edges, measure the longest and widest points and treat it as a rectangle—you will have a small amount of extra mulch, which is fine.

Do I need to remove old mulch before spreading new mulch?

No. If the old mulch is not matted down or moldy, you can spread new mulch over it. This saves work and money. If the old layer is thick and compacted, rake it loose first so water and air can reach the soil. Your depth measurement should account for what is already there plus what you are adding.

How deep should mulch be around trees?

Use 3 to 4 inches around the base of newly planted trees, keeping it a few inches away from the trunk. For established trees, 2 to 3 inches is enough. Deeper mulch does not help and can trap moisture and invite pests.

Can I calculate mulch needed for a sloped garden bed?

Measure the bed as if it were flat—the slope does not significantly change the volume of mulch needed. If the slope is very steep, you may lose some mulch to runoff in heavy rain, so add an extra 15 percent instead of 10 percent.

What if the mulch supplier only sells by the bag, not by the cubic yard?

Divide your square footage by 8 to find the number of bags. If you calculated 2 cubic yards, that is roughly 27 bags (2 cubic yards = 54 cubic feet, and each bag is 2 cubic feet, so 54 ÷ 2 = 27 bags). Round up slightly to account for settling.