Short answer
A 20 ft × 4 ft flower bed at 3 inches deep needs 0.74 cubic yards of mulch, about 10 bags of 2 cu ft mulch. Work through the checklist below with your own bed size, or try the Bark Mulch Calculator for an instant result.
The formula
Cubic feet = length × width × (depth in inches ÷ 12). Cubic yards = cubic feet ÷ 27. Bags of 2 cu ft mulch = cubic feet ÷ 2, rounded up. A shortcut for yards is: area in square feet × depth in inches ÷ 324 = cubic yards.
Worked example: 20 ft × 4 ft bed, 3 inches deep
Step 1: area is 20 × 4 = 80 sq ft. Step 2: cubic feet are 80 × (3 ÷ 12) = 80 × 0.25 = 20 cu ft. Step 3: cubic yards are 20 ÷ 27 = 0.74 cu yd. Step 4: bags are 20 ÷ 2 = 10 bags of 2 cu ft mulch. That is the same result the Bark Mulch Calculator gives for these numbers.
Mulch buying checklist
1. Measure the bed. Measure length and width at the widest points. For a curved bed, split it into rectangles and add the areas, or use 3.14 × radius × radius for a round bed.
2. Pick the depth. Use 2 to 3 inches for flower beds after settling, 3 to 4 inches around trees in a donut shape, and only 1 to 2 inches when topping up an existing bed.
3. Convert to yards and bags. Use the formula above, or the shortcut: area × depth ÷ 324 = cubic yards. Multiply yards by 13.5 for 2 cu ft bags, or by 9 for 3 cu ft bags, and round up.
4. Add a little extra. Add 5 to 10% on a new install for settling, slopes and spillage. Mulch settles in the first months, so an exact minimum often leaves a bed short.
5. Choose bags or bulk. Bags suit small beds. From about 1 cubic yard upward, bulk delivery by the yard is usually cheaper per cubic foot, but check the delivery fee before you decide.
6. Keep it off trunks and stems. Pull mulch a few inches back from tree trunks and plant stems. Piled-up mulch traps moisture against bark and invites pests and rot.
How much mulch for 100 square feet
Bag counts are for 2 cu ft bags, rounded up to whole bags.
| Depth | Cubic feet | Cubic yards | 2 cu ft bags |
|---|---|---|---|
| 2 inches | 16.67 cu ft | 0.62 cu yd | 9 bags |
| 3 inches | 25 cu ft | 0.93 cu yd | 13 bags |
| 4 inches | 33.33 cu ft | 1.23 cu yd | 17 bags |
What changes the number
Depth matters more than area: doubling the depth doubles the mulch. Bag size matters too: a 3 cu ft bag covers half as many bags per yard as a 2 cu ft bag, 9 per yard instead of 13.5. Irregular beds, slopes and tree rings all add waste, which is why the checklist adds 5 to 10% on new installs.
Method and limits
This guide uses the standard volume formula for rectangular beds: length × width × depth, converted with 27 cubic feet per cubic yard. It assumes a flat bed measured at its widest points and a settled depth, not the height of a fresh fluffy pile. Real beds vary with shape, slope and settling, so treat the result as an estimate and confirm the final quantity with your supplier. For compost instead of mulch, see the Compost Calculator.
Try it with your own numbers
Open the Bark Mulch Calculator, enter your bed size and depth, and it gives cubic yards and bag counts. More garden tools are in Home & Garden calculators, and more guides are on the BuildCalc Blog. See How Our Calculators Are Checked for how the numbers behind these guides are verified.
Results are estimates and general information only. Always check your product's bag size and confirm final quantities with your supplier or a qualified professional.