Mulch Calculator – How Much Mulch Do I Need?

This mulch calculator tells you exactly how many cubic yards of bulk mulch or how many bags to buy – just enter your bed dimensions and chosen depth.

How I calculate this

The formula is just volume math, but the constants are where most people get tripped up, so here they are straight.

I multiply your bed area (length times width) by your chosen depth in feet. So a 12 by 8 foot bed at 3 inches deep is 12 x 8 x 0.25 = 24 cubic feet. Divide by 27 to convert to cubic yards (there are 27 cubic feet in one cubic yard). If you want bags instead, divide cubic feet by the bag size. Standard bags are 2 cubic feet, and one cubic yard breaks down to exactly 13.5 of them.

The coverage constants I use are published and consistent across the industry: 1 cubic yard covers 324 square feet at 1 inch deep, 162 square feet at 2 inches, and 108 square feet at 3 inches. These come from the same 27 ft3/yd3 conversion — no magic, just division. I verified them against the CalculatorSoup mulch calculator and The Calculator Site.

For soil and compost the math is identical — same formula, same cubic yard conversion. The only thing that changes is how you use the result.

A worked example

I recently helped a neighbor figure out mulch for three flower beds: 10 x 6 feet, 8 x 4 feet, and 15 x 3 feet. Total area is 60 + 32 + 45 = 137 square feet. She wanted 3 inches, which is 0.25 feet. Volume = 137 x 0.25 = 34.25 cubic feet. Divide by 27 and you get 1.27 cubic yards. Round up to 1.5 yards when ordering bulk — landscaping suppliers usually round to the nearest half. For bags: 34.25 / 2 = 17.1, so 18 bags. She bought 18 bags and had just a little left over, which is exactly right.

Depth guide and practical tips

The standard recommendation is 2 to 3 inches, and I think 3 is the better default for most situations. At 2 inches you’ll get reasonable weed suppression, but I’ve seen beds dry out faster in summer heat. At 3 inches, moisture retention and weed control are noticeably better. More than 4 inches is a problem — it can suffocate plant roots and cause water to shed off the surface rather than soak in.

Don’t pile mulch against plant stems or tree trunks. The “mulch volcano” around tree bases looks tidy but it traps moisture against the bark and causes rot. Keep mulch a few inches clear of any trunk or stem.

If you’re refreshing existing mulch rather than starting from scratch, you probably only need to add 1 inch to bring it back up to depth. Probe the old layer with a stick to see what’s actually there before ordering. I’ve seen people order 3 yards for a refresh when 1 yard would have done it.

Bulk delivery by the cubic yard is almost always cheaper than bags per cubic foot once you need more than about 3 yards. The break-even is roughly 40 to 50 bags. Below that, bags from a home center are more convenient and often competitive on price. The calculator shows both so you can compare.

Want to measure your beds accurately before ordering? See how to measure a room — the same length-times-width method applies to outdoor beds. If you’re also ordering gravel or stone for pathways, the gravel calculator uses the same volume math.

FAQ

How much does 1 cubic yard of mulch cover? At 2 inches deep, one cubic yard covers 162 square feet. At 3 inches, it covers 108 square feet. The formula is straightforward: 1 yd3 = 27 ft3, and coverage = 27 / depth in feet.

How many bags of mulch equal 1 cubic yard? If your bags are 2 cubic feet (the standard size at most home centers), you need 13.5 bags to equal one cubic yard. Round up to 14 bags to be safe.

Is 2 or 3 inches of mulch better? Three inches for most garden beds. Two inches is fine if you’re maintaining an established layer or mulching vegetables where you don’t want to block too much light. I default to 3 in this calculator and let you dial it down if needed.

Can I use this calculator for soil or compost? Yes. The volume formula is the same — area times depth divided by 27 gives cubic yards regardless of material. Just enter your area and depth and the result applies equally to topsoil, compost, or wood chip mulch.

Should I remove old mulch before adding new? Not usually. If the old layer is less than 2 inches, just top it up. If it’s compacted or matted and preventing water penetration, rake it loose or remove the top layer first. Decomposed mulch at the bottom of the bed is actually good for soil — leave it.

Built and maintained by Sam Walsh. Last updated July 2026. This is a planning estimate for purchasing purposes — actual coverage can vary slightly depending on material density and how the mulch settles.

Need to measure your garden beds? See how to measure a room. Ordering gravel or stone for the same project? Try the gravel calculator.