MathBench

Mulch Calculator

Find out how much mulch you need for a rectangular bed or a circular ring, in cubic yards, cubic feet, and 2 or 3 cubic-foot bags.

You need

—cubic yards
—cubic feet
—2 cu ft bags
—3 cu ft bags

How this calculator works

Choose your bed shape, enter its size in feet, and pick a mulch depth in inches. The calculator finds the bed's area, multiplies by the depth to get a volume, and reports it as cubic yards (how bulk mulch is sold), cubic feet, and the equivalent count of 2 and 3 cubic-foot bags, the two sizes stacked at every garden center.

For a ring around a tree, use the circular mode with the ring's outer diameter. The trunk in the middle is small enough that skipping it just gives you a little healthy margin. Mulching several similar rings at once? Set the count and the tool multiplies for you. For odd-shaped beds, break the area into rectangles and add the results.

The formula

Rectangle: area = length × width
Circle:    area = π × (diameter ÷ 2)²

Volume (ft³) = area × (depth ÷ 12)
Cubic yards  = volume ÷ 27
2 cu ft bags = volume ÷ 2   (rounded up)
3 cu ft bags = volume ÷ 3   (rounded up)

There's no waste factor here: mulch doesn't compact like gravel, and the bag rounding already gives you a small cushion. If your beds have lots of edges and curves, add one extra bag per 50 ft² of bed.

Worked example

Say you're mulching a 12 ft × 4 ft foundation bed, 3 inches deep:

  1. Area: 12 × 4 = 48 ft²
  2. Volume: 48 × (3 ÷ 12) = 12 ft³
  3. Cubic yards: 12 ÷ 27 = 0.44 yd³
  4. Bags: 12 ÷ 2 = 6 two-cubic-foot bags (or 12 ÷ 3 = 4 three-cubic-foot bags)

Six bags fit in any car trunk, so this is a bag purchase, not a bulk order. A 10-foot tree ring at the same depth works out almost identically: 78.5 ft² × 0.25 ft ≈ 19.6 ft³, ten 2 cu ft bags.

Assumptions & tips

  • Three inches, not six. Piling mulch deeper doesn't suppress weeds better. It suffocates roots and sheds water. If last year's layer is still partly there, only add the difference.
  • Keep it off the trunks. "Mulch volcanoes" against tree bark hold moisture where rot and rodents thrive. Leave a 3-inch gap around trunks and woody stems.
  • Do the bulk math before checkout. A yard is 13.5 small bags. When your bag count crosses about 14, price a bulk delivery. It's often a third cheaper, and better-quality mulch.
  • Measure beds, don't eyeball them. Walk the bed with a tape: paced-off guesses run 20–30% small, and a mid-project second trip for two more bags costs more than the tape measure did.
  • Weed first, water after. Mulch stops weed seeds from sprouting. It does nothing about established weeds underneath. Pull them, mulch, then water to settle the layer in place.

Frequently asked questions

How many bags of mulch make a cubic yard?

A cubic yard is 27 cubic feet, so it takes 13.5 of the standard 2-cubic-foot bags, call it 14, or 9 of the larger 3-cubic-foot bags. That number is the quickest way to compare bag prices against bulk: multiply the bag price by 13.5 and see how it stacks up against a yard delivered.

How deep should mulch be?

Three inches is the sweet spot for beds: deep enough to suppress weeds and hold moisture, shallow enough that water and air still reach the soil. Use 2 inches around shallow-rooted perennials, and never pile mulch against tree trunks or wooden siding. Leave a few inches of clearance.

How much does a cubic yard of mulch cover?

One cubic yard covers 162 square feet at 2 inches deep, 108 square feet at 3 inches, and 81 square feet at 4 inches. Divide 324 by your depth in inches to get square feet of coverage per yard.

How often should mulch be replaced?

Organic mulch breaks down and settles, typically losing an inch or more per year. Most beds want a 1-to-2-inch top-up annually and a full refresh every two to three years. Run the calculator with just the top-up depth to size the annual order. It is usually about half the original amount.

Sources

  1. Proper Mulching Techniques (Tree Care Basics). University of Illinois Extension. extension.illinois.eduThe 2-to-4-inch recommended depth range that this page's 3-inch default sits inside, and the instruction to keep mulch several inches clear of the trunk.
  2. Mulching Landscape Trees. David R. Jackson, Penn State Extension. extension.psu.eduSource for the warning against “volcano” mulching piled against the trunk, and for checking the depth of last year's layer before topping up, which is how the annual top-up amount is sized.
  3. Mulching Trees and Shrubs. University of Maryland Extension. extension.umd.eduDepth guidance by mulch material and the expectation that organic mulch decomposes and needs replacing, behind the FAQ's replacement interval.

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