Mulch is sold by volume and spread by area, which is why the arithmetic catches people out. A bag that looks generous can disappear across a surprisingly small bed. This calculator turns the area you are covering and the depth you want into a number of bags, and tells you how far a single bag goes at that depth — the figure worth knowing before you get to the checkout.
How to use it
- Tell it what you know about the bed. Enter a length and width, a diameter for a round bed or tree ring, or type the area straight in if you already have it.
- Set the depth. Two to three inches suits most beds. The calculator will happily work at any depth, but it is worth reading the note above about why deeper is not automatically better.
- Choose your bag size. Two cubic feet is close to universal for bagged mulch in the United States; three cubic foot bags exist, and metric bags run 50 to 70 liters.
- Use the metric toggle if you work in metres and centimetres.
For an L-shaped or awkward bed, split it into rectangles, run each one and add the results — that is more accurate than guessing an average width.
How it is calculated
cubic feet = area (sq ft) x depth (in) / 12
cubic yards = cubic feet / 27
bags = cubic feet / bag size, rounded up
Depth arrives in inches and area in square feet, so the depth has to become feet before they multiply — hence the division by twelve. Forgetting it is the single most common error in this sum, and it produces an answer twelve times too big.
The coverage figure runs the same arithmetic backwards:
square feet per bag = bag size (cu ft) x 12 / depth (in)
A worked example
A bed twenty feet by ten, mulched three inches deep:
- Area: 20 x 10 = 200 square feet
- Volume: 200 x 3 / 12 = 50 cubic feet
- In cubic yards: 50 / 27 = 1.85 cubic yards
- At two cubic feet a bag: 50 / 2 = 25 bags
- Coverage: 2 x 12 / 3 = 8 square feet per bag
Coverage per two cubic foot bag
| Depth | Square feet per bag | Bags for 100 sq ft | Bags for 500 sq ft |
|---|---|---|---|
| 1 in | 24 | 5 | 21 |
| 2 in | 12 | 9 | 42 |
| 3 in | 8 | 13 | 63 |
| 4 in | 6 | 17 | 84 |
Tips and common mistakes
Measure the bed, not the lawn it sits in. Mulching to the edge of a border and then along a path adds up fast. Beds are usually smaller than they look from the house and larger than they look up close.
Keep it off stems and trunks. Leave an inch or two of clear ground around anything woody. Mulch against bark holds moisture where you least want it.
Do not just add three more inches every spring. Check what is already there first. Mulch breaks down slowly, and layers accumulate into a mat that sheds water rather than letting it through. Rake the old layer to break the crust, then top up to the depth you want in total.
Coarse mulch lasts longer; fine mulch feeds the soil. Bark chips and pine nuggets hold their structure for two or three seasons. Shredded leaves, compost and grass clippings disappear within one, but leave the soil visibly better for it. Neither is wrong — they are doing different jobs.
Buy a little extra for edges. Beds are rarely as tidy as their measurements, and sloped ground needs more at the bottom than the top.