Quick answer: multiply the bed area (square feet) by the depth (feet) and divide by 27. Illinois Extension recommends mulch 2 to 3 inches deep, and one cubic yard covers about 162 square feet at 2 inches or 108 square feet at 3 inches. A 10 × 4 ft bed at 3 inches deep needs 0.37 cubic yards, or 5 bags of 2 cubic foot mulch.
The Mulch Calculator does this for any bed. Below is the math, a coverage chart, the weight of a yard of mulch, and the mistakes that harm trees.
How deep should mulch be?
According to Horticulture Educator Martha Smith of University of Illinois Extension, mulch should be 2 to 3 inches deep. It can look deeper when you first spread it, but it settles. The same source warns against going deeper: thick layers can leave the root zone too wet and lead to root rot, and fine mulch that mats down can block water and air and invite insect and disease problems.
So the depth you calculate is the settled depth, 2 to 3 inches, not the height of a fresh pile.
The formula
Cubic feet = area (ft²) × depth (in) ÷ 12
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ bag size (ft³), rounded up
Mulch coverage per cubic yard
| Depth | Area covered by 1 yd³ | Cubic yards per 100 ft² |
|---|---|---|
| 2 in | 162 ft² | 0.62 |
| 3 in | 108 ft² | 0.93 |
| 4 in | 81 ft² | 1.23 |
North Bay Materials quotes a rougher figure of about 155 square feet at 2 inches because of compaction and texture. Use the exact geometry for your estimate and a small allowance for settling.
Worked example 1: a 10 × 4 ft flower bed
- At 3 inches deep: 10 × 4 × 0.25 = 10 ft³, or 0.37 cubic yards. With 2 cubic foot bags that is 5 bags (10 ÷ 2); with 3 cubic foot bags it is 4 bags.
- At 2 inches deep: 6.67 ft³, or 0.25 cubic yards, which is 4 bags of 2 cubic feet.
Worked example 2: 500 square feet of garden beds
At 3 inches: 500 × 0.25 = 125 ft³ = 4.6 cubic yards. That is the point where bulk delivery usually beats bags: 4.6 yards (125 ft³) is about 63 bags of 2 cubic feet.
Worked example 3: a ring around a tree
Say you want a circle of mulch 6 feet across, but keep the mulch clear of the trunk by leaving a 1 foot circle bare:
- Ring area: π × (3² − 0.5²) = 27.5 ft²
- At 3 inches: 27.5 × 0.25 = 6.9 ft³, or 0.26 cubic yards, about 4 bags of 2 cubic feet.
The bare circle matters more than the bag count. See below.
How many bags are in a yard?
A cubic yard is 27 cubic feet:
- 13.5 bags of 2 ft³
- 9 bags of 3 ft³
- 18 bags of 1.5 ft³
Always check the bag, because sizes vary.
How much does a cubic yard of mulch weigh?
North Bay Materials lists mulch, bark and wood chips at about 500 to 800 pounds per cubic yard, around 0.33 tons. That is a fraction of the 2,500 to 2,900 pounds a yard of gravel weighs. A 3-yard delivery is therefore about 1,500 to 2,400 pounds, which is a heavy pickup load but easy to spread. Moisture and wood type change the number, and fresh or wet wood chips can be heavier.
Don’t build a mulch volcano
Illinois Extension describes the “mulch volcano” as a popular but harmful practice of piling mulch against a tree trunk. Bark is made to live in air and light, and burying it in dark, moist mulch makes it rot, which weakens its protection against insects and diseases. Heavy mulch can also deprive roots of oxygen, produce heat as it breaks down, and encourage secondary roots that circle and choke the trunk.
Practical takeaways:
- Spread mulch in a wide, flat layer that tapers thin near the trunk.
- Leave a gap so mulch does not touch the bark.
- Don’t add a fresh layer every year without checking how much is left underneath. The extension specialist notes that people prefer the fresh look of new mulch and top-dress annually without checking the existing depth.
Choosing and ordering
- Organic mulches decompose at different rates. Wood chips, pine needles, bark, cocoa hulls, leaves and compost mixes all work. Faster-rotting types need more frequent replenishing, while some, like cypress, last for years.
- Bulk vs bags: use bags for a few beds; compare bulk delivery pricing once you pass about 2 to 3 yards, including any delivery minimum.
- Order a little extra. Your edges, curves and settling will use more than the perfect rectangle.
Common mistakes
- Too deep. Going beyond 3 inches rarely helps and can harm roots.
- Mixing up depth units. Convert inches to feet by dividing by 12 before multiplying.
- Forgetting existing mulch. If you already have 1 inch down, a 3 inch target means adding only 2 inches.
- Covering the trunk. Keep mulch off the bark.
Frequently asked questions
How many square feet does one yard of mulch cover?
162 square feet at 2 inches, 108 at 3 inches and 81 at 4 inches.
How many bags of mulch make a cubic yard?
13.5 bags if the bags are 2 cubic feet, or 9 bags if they are 3 cubic feet.
How much mulch for 100 square feet?
At 3 inches deep: 0.93 cubic yards, or 12.5 bags of 2 cubic feet.
Can I just top up old mulch?
Check how much is left. Add only what brings the total to the 2 to 3 inch target.
How often should I replace mulch?
It depends on the type, since organic mulches decompose at different rates and the faster-decomposing ones need more frequent replenishment.
Sources
- Martha A. Smith, University of Illinois Extension, >Too much of a good thing, mulch volcanoes slowly kill trees, 2020: recommended depth, effects of deep mulch, mulch volcanoes.
- North Bay Materials, >Landscape Materials Density Chart: weight per yard for mulch, coverage notes.
- NIST, >Guide to the SI, Appendix B.8: unit conversions.