VolunaroShort answer: multiply the bed's length by its width (in feet) to get square feet, multiply by the depth (in inches divided by 12) to get cubic feet, then divide by 27 to get cubic yards. A 12 × 10 ft bed mulched 3 in deep needs 30 cubic feet, or 1.11 cubic yards, before any margin. With a 5 percent margin it is 1.17 cubic yards, which is 16 bags of 2 cubic feet.
Pick the depth for your plants and site. Penn State Extension's guidance for home fruit plantings is 2 to 3 in, and it says a thicker layer tends to waste mulch. The numbers below use 3 in as an example.
For your own measurements, use the mulch calculator. Read on to check a quote, choose between bags and bulk or mulch around a tree.
The formula, step by step
- Area in square feet. Length × width for a rectangle. Split an irregular bed into rectangles and circles, and subtract anything that will not be mulched.
- Depth in feet. Inches ÷ 12. Three inches is 0.25 ft.
- Volume in cubic feet. Area × depth in feet.
- Margin. Multiply by 1.05 for settling and uneven edges.
- Cubic yards. Cubic feet ÷ 27.
- Bags. Cubic feet ÷ the volume printed on the bag, rounded up.
Worked example: a 12 × 10 ft bed, 3 in deep
| Step | Calculation | Result |
|---|---|---|
| Area | 12 ft × 10 ft | 120 ft² |
| Depth | 3 in ÷ 12 | 0.25 ft |
| Volume | 120 ft² × 0.25 ft | 30 ft³ |
| With 5% margin | 30 × 1.05 | 31.5 ft³ |
| Cubic yards | 31.5 ÷ 27 | 1.17 yd³ |
| Bags of 2 ft³ | 31.5 ÷ 2 = 15.75 | 16 bags |
Without the margin the bed needs 1.11 cubic yards and 15 bags. For bulk, round the 1.17 up to what your supplier delivers.
A rule worth remembering
One cubic yard covers 108 square feet at 3 in, because 108 × 0.25 = 27 cubic feet. At 2 in it covers 162 square feet, and at 4 in, 81. Divide your area by those numbers for a quick check of any quote.
Quick reference: mulch per 100 square feet
These values come from the same formula (area × depth ÷ 27) with the 5 percent margin, using 2 cubic foot bags. Scale them by your area divided by 100.
| Depth | Cubic yards (+5%) | Bags of 2 ft³ |
|---|---|---|
| 2 in | 0.65 | 9 |
| 3 in | 0.97 | 14 |
| 4 in | 1.30 | 18 |
Bag size changes the count
Bags are sold by volume and the volume varies. The same 12 × 10 ft bed at 3 in with the 5 percent margin needs 21 bags of 1.5 cubic feet, 16 bags of 2 cubic feet or 11 bags of 3 cubic feet. When you compare shelf prices, compare per cubic foot, not per bag, and read the volume on the label. The calculator has a field for it.
Mulching around a tree
Beds are rarely rectangles. Take a ring 8 ft across around a trunk with a 1 ft clear zone at the centre. Its area is π × 4² − π × 0.5² = 49.5 ft². At 3 in that is 12.4 cubic feet, or 0.46 cubic yards, and 0.48 cubic yards with the margin: 7 bags of 2 cubic feet.
Do not pile mulch against the trunk. LSU AgCenter recommends keeping mulch 2 to 3 in away from the base of tree trunks and shrub stems. Recommendations vary by plant and region, so ask your local extension office.
Common mistakes
- Mixing units. Entering 3 as feet instead of inches makes the answer twelve times too big.
- Counting bags without checking the size. A bag is not a standard volume.
- Going too deep. More is not better: a thicker layer tends to waste mulch.
- Ignoring the hole. Subtract trunks, stones and paths from the area.
- Forgetting settling. Fine or shredded mulch may need a larger margin than the default.
What this guide cannot tell you
- The right depth for your plants and climate. Ask your local extension office.
- Which mulch to use.
- The price. It varies by supplier and location.
The calculator gives planning estimates. If you are also laying turf, see the sod calculator. Our assumptions are listed on the methodology page.
Frequently asked questions
How much does a cubic yard of mulch cover?
108 square feet at 3 inches deep, or 162 square feet at 2 inches. A cubic yard is 27 cubic feet, so divide 27 by the depth in feet.
How many bags of mulch make a cubic yard?
13.5 bags if each holds 2 cubic feet, so 14. Bags of 1.5 or 3 cubic feet change that to 18 or 9.
Should I add extra to my mulch order?
A small margin for settling and uneven beds is normal. Our calculator defaults to 5 percent, which is a planning assumption you can change, not a standard.
How deep should mulch be?
Penn State Extension gives 2 to 3 inches for home fruit plantings and says a thicker layer tends to waste mulch. Other plants and sites differ, so check with your local extension office.