How much mulch do you need? The math, step by step

Mulch is sold by the cubic yard, and a cubic yard is 27 cubic feet. So the math is: square feet of the bed, times the depth in feet, divided by 27.

Texas A&M AgriLife Extension in Travis County writes it this way: total cubic yards equals length in feet, times width in feet, times depth in inches divided by 12, all divided by 27. Their example is a 10-by-10-foot bed at 3 inches. That's 100 square feet times a quarter foot, which is 25 cubic feet. Divide by 27 and you get about 0.93 yards. Round up to 1 yard.

AgriLife also gives a shortcut: square feet times depth in inches times 0.0031. For the same bed, 100 times 3 times 0.0031 is 0.93 yards. Same answer, fewer steps.

Depth matters more than people think. Too thin and weeds come right through. Too thick and you've paid for mulch you didn't need.

Texas A&M AgriLife Extension horticulturist David Rodriguez recommends checking that mulch is 3 to 4 inches deep, and mulching the whole bed at that depth once plants are established. For shrub beds and new trees, he suggests a 4-foot circle of mulch. AgriLife's WaterSmart guide says a layer of coarse mulch 3 or more inches deep helps with weed control, and warns not to pile mulch against tree trunks.

AgriLife's Easy Gardening sheet on mulching adds one more point: put down organic mulch thick enough to leave a 4-inch layer after it settles. Fresh shredded mulch is fluffy. It packs down after a few rains.

Different materials behave differently, too. Coarse shredded wood stays put and breaks down slowly. Finer mulch and compost break down faster and need topping up sooner.

Most beds aren't neat squares. Break the space into simple shapes, find the square feet of each, and add them up. A rectangle is length times width. A circle is 3.14 times the radius times the radius.

Say you have a front bed 20 feet long and 4 feet deep, plus a ring around a young tree. The bed is 80 square feet. A 4-foot circle has a 2-foot radius, so it's about 12.6 square feet. Together that's about 92.6 square feet.

At 3 inches, multiply by 0.25 to get about 23 cubic feet. Divide by 27 and you need about 0.86 yards. Order 1 yard. The 0.0031 shortcut gives the same number: 92.6 times 3 times 0.0031 is about 0.86.

Bagged mulch is labeled in cubic feet. Because a cubic yard is 27 cubic feet, it takes 13.5 bags of the common 2-cubic-foot size to equal one yard. Three-cubic-foot bags take 9.

Bulk also makes it easier to hit the right depth. With bags, people tend to spread what they bought instead of what the bed needs, and the layer ends up thin. With a pile on the driveway, you spread to depth and stop when the bed is done. Lay a tarp under the pile if it's sitting on grass, and spread it within a few days.

For one small bed, bags are fine. For a whole yard of beds, the bag count climbs fast, and so does the trash from all that plastic. That's usually the point where people switch to bulk.

Before you order, push a finger or a trowel into the old mulch and see how deep it still is. AgriLife notes that mulch settles and slowly rots, so you add more during the season. If an inch is left, you may only need to top up 2 or 3 inches, not lay down a full new layer.

Pull mulch back off tree trunks while you're at it. If the old beds are full of weeds, dead shrubs or limbs, clear that first. Our yard debris removal page covers that side of the job.

Some of the mulch around here started as someone's curb pile. College Station brush goes to Twin Oaks, where it's shredded into compost and mulch that the public can buy.

ME3 Junk Solutions offers mulch and material delivery: mulch, topsoil, gravel, sand, compost, fill dirt, landscape rock and other bulk materials, for homeowners, landlords, property managers and contractors throughout Bryan, College Station, Caldwell, Hearne, Navasota and surrounding communities. Call or text the material, roughly how much you need from the math above, and the address. If you're also building up a bed, read topsoil vs. fill dirt before you order.