Mulch CalculatorHow much mulch do I need?
Multiply the length and width of the bed in feet to get square feet. Multiply that by the depth in inches, then divide by 324. That gives cubic yards. A 20 by 10 foot bed at 3 inches deep needs 200 x 3 / 324 = 1.85 cubic yards, or 17 bags of 3 cubic feet.
The number 324 comes from 27 cubic feet in a yard times 12 inches in a foot. You can also do it in two steps. First turn the depth into feet: 3 inches is 0.25 feet. Then 200 x 0.25 = 50 cubic feet, and 50 / 27 = 1.85 cubic yards. The tool shows both cubic feet and cubic yards, so you can buy bags or bulk.
How deep should mulch be?
Most extension services say 2 to 4 inches. Use the low end for fine mulch and heavy clay soils, and the high end for coarse bark on soil that drains well. Keep mulch a few inches away from tree trunks and stems. Deeper is not better, since thick layers can starve roots of air.
| Source | Depth | Space from trunk |
|---|---|---|
| University of Illinois | 2 to 4 in (compost 2 to 3 in, coarse 3 to 4 in) | At least 4 in |
| Iowa State | 3 to 4 in on well drained soil, 2 to 3 in on heavy soil | About 6 in |
| Cornell | 2 to 4 in | 3 to 5 in for young plants, 8 to 18 in for mature trees |
| UF/IFAS | 2 to 3 in after settling | 12 to 18 in from trees, 1 to 2 in from shrubs |
| University of Maryland | 1 to 3 in | 3 in |
Sources: Illinois Extension, Iowa State, Cornell, UF/IFAS and UMD Extension.
UC extension adds that fine mulch should be 2 inches deep or less, while coarse mulch can go 4 to 6 inches (UC ANR). The USDA Natural Resources Conservation Service sets a floor: wood chips, bark or shavings at least 2 inches thick, with fine mulches no thicker than 2 inches. It also says to keep mulch 3 to 6 inches from plant stems (USDA NRCS Code 484). If you are not sure, 3 inches is inside almost every range above.
How many square feet does a yard of mulch cover?
This table divides 324 by the depth in inches. It shows how far one cubic yard goes and how many bags it would take to match it.
| Depth | Sq ft per cubic yard | 2 cu ft bags per 100 sq ft | 3 cu ft bags per 100 sq ft |
|---|---|---|---|
| 1 in | 324 | 5 | 3 |
| 2 in | 162 | 9 | 6 |
| 3 in | 108 | 13 | 9 |
| 4 in | 81 | 17 | 12 |
Bag counts are rounded up. UC extension gives the same rule in rounder numbers: one yard covers about 325 square feet at 1 inch, 160 at 2 inches, 100 at 3 inches and 80 at 4 inches.
How to measure beds of any shape
- Rectangle: length x width.
- Circle: 3.1416 x radius x radius. The radius is half the width across.
- Ring around a tree: the area of the outer circle minus the bare circle you keep clear at the trunk.
- Triangle: base x height / 2.
- Odd shapes: split the bed into rectangles and triangles, then add them up. Or use the "I know the square feet" option.
Illinois Extension suggests a mulch ring with a radius of up to 3 feet for most trees. That is a 6 foot circle, which is about 28 square feet. At 3 inches deep, that ring takes about 7 cubic feet, or 3 bags of 3 cubic feet.
Why add extra mulch?
Fresh mulch is fluffy and settles after rain. Beds are rarely perfect shapes, and some mulch gets lost when you spread it. The tool adds 10% by default. You can set it to 0 if you measured with care. UF/IFAS gives its depth target after settling, which is one more reason to buy a bit extra.
Mulch mistakes to avoid
- Mulch volcanoes. Piling mulch up against the trunk can cause rot and roots that wrap around the trunk. UVM Extension warns against layers over 4 inches. See our mulch volcano guide.
- Mulch near the house. UC extension suggests keeping organic mulch 5 feet from the house.
- Too much on top of old mulch. Rake the old layer and measure it first. Then only top it up.
More tools
Related guides
FAQ
How many bags of mulch make a yard?
One cubic yard is 27 cubic feet. That makes 13.5 bags of 2 cubic feet, or 9 bags of 3 cubic feet. Round up, since you cannot buy half a bag. The tool above does this for you and adds a little extra for settling.
How many square feet does a yard of mulch cover?
One cubic yard covers 324 square feet at 1 inch deep. At 2 inches it covers 162, at 3 inches 108, and at 4 inches 81. UC extension rounds these to 325, 160, 100 and 80 square feet.
Is it cheaper to buy mulch in bags or in bulk?
It depends on local prices, so compare the cost per cubic yard. Take the bag price and multiply by 13.5 for 2 cubic foot bags, or by 9 for 3 cubic foot bags. Then compare that number with the bulk price plus any delivery fee.
How often should I add new mulch?
Cornell says to add organic mulch every 2 to 3 years. UC extension notes that bark mulch lasts 2 to 4 years. Check the depth first. If the old layer is still 2 inches or more, add only enough to bring it back to your target depth.
References
- Conservation Practice Standard: Mulching, Code 484 (PDF). USDA Natural Resources Conservation Service, 2024
- Applying Mulch: Where, When and How Much. UC Marin Master Gardeners, University of California Agriculture and Natural Resources
- Proper Mulching Techniques (PDF). University of Illinois Extension
- How deep should the mulch layer be around trees and shrubs? Iowa State University Extension, Yard and Garden
- Mulch. Cornell University CALS, Turfgrass and Weed ID
- Principle 4 Guidance: Mulch. Florida-Friendly Landscaping Program, UF/IFAS
- Mulching Trees and Shrubs. University of Maryland Extension
- Beware the Mulch Volcano. University of Vermont Extension