Mulch Calculator
Work out how much mulch your beds need: in cubic yards and bags.
Mulch does three jobs at once: it holds moisture in the soil, suppresses weeds, and keeps soil temperature steady. It only works at the right depth, though. Too thin and weeds push through, too thick and roots suffocate. This calculator tells you exactly how much to buy, in cubic yards for bulk orders and in bags for the garden centre.
Measure your bed, pick a depth, and check what is already on the ground first: new mulch goes on top of the old layer, so the old depth counts. Once the bed is mulched, use the Raised Bed Soil Calculator when you build the next bed, and the Plant Spacing Calculator to fill it.
How this is worked out
Cubic feet = area (sq ft) × depth (in) ÷ 12. Cubic yards = cubic feet ÷ 27. Bags = cubic feet ÷ bag size, rounded up. If you already have mulch down, the calculator subtracts the existing depth first. You only buy the difference.
Assumptions:
- Depth means settled depth, measured after rain. Fresh mulch settles about half an inch as it beds down.
- One cubic yard equals 27 cubic feet, and one cubic yard covers 324 sq ft one inch deep.
- Bag sizes are as printed on the bag; the common sizes are 2 cu ft and 3 cu ft.
- The area is the mulched area only. Paths and bare soil inside the bed outline are not counted.
Three worked examples
1. A 100 sq ft bed at 3 in. 100 × 3 ÷ 12 = 25 cu ft, which is 25 ÷ 27 = 0.93 cubic yards. 13 bags of 2 cu ft. This is the most common case: one cubic yard is about right for a small bed.
2. Topping up a 200 sq ft bed. You want 3 in total but 1 in is already there, so you add 2 in: 200 × 2 ÷ 12 = 33.3 cu ft, or 1.23 cubic yards. 17 bags of 2 cu ft.
3. A mulch ring around a tree. A ring 3 ft out from the trunk covers about 28 sq ft. At 4 in deep that is 28 × 4 ÷ 12 = 9.4 cu ft, or 0.35 cubic yards. 5 bags of 2 cu ft. Keep the mulch a few inches clear of the trunk itself.
How much one cubic yard covers
| Depth | Sq ft per cubic yard | 2 cu ft bags per 100 sq ft |
|---|---|---|
| 2 in | 162 sq ft | 9 bags |
| 3 in | 108 sq ft | 13 bags |
| 4 in | 81 sq ft | 17 bags |
Depth guidance follows Penn State Extension (2 to 4 in of organic mulch, 1 to 2 in for fine or double-shredded mulch) and Clemson Extension HGIC 1604 (fine mulch 2 to 3 in maximum, coarse pine bark nuggets up to 4 in).
Common mulching mistakes
- Piling mulch against the trunk. The mulch volcano. Mulch held damp against bark causes rot and gives rodents cover to chew the stem in winter. Pull it back 3 to 5 in from young trees and 8 to 10 in from mature ones, so the root flare shows.
- Topping up without measuring the old layer. New mulch over old mulch, year after year, is the same as one layer that is too deep. Measure first; if the total would pass 4 in, remove some or spread the excess toward the drip line.
- Going thicker to stop weeds. Beyond about 4 in, mulch starves roots of oxygen. The damage shows up years later as yellow leaves and poor growth. Weed control comes from coverage, not depth.
- Using fine mulch too deep. Finely textured or double-shredded bark lets less air through, so its ceiling is 2 in, not 4 in. Use coarse chips where you want a deeper layer.
- Mulching a soggy bed thick. A thick layer keeps wet soil wetter. On poorly drained soil, use the thin end of the range.
What to do next
Order bulk mulch by the cubic yard for large beds. It is usually cheaper per yard than bags. And buy bags for small top-ups. Spread it evenly, keep it clear of stems, and re-check the depth after the first rain, when it has settled. For the soil underneath, the Raised Bed Soil Calculator works out the mix your next bed needs.
Putting the result to work
Buy for the area you actually have, not the area you wish you had. Measure each bed separately. Two beds that look the same size often differ by 20 square feet, and that is a quarter of a cubic yard at 3 inches. If you are ordering bulk, round up to the nearest half cubic yard: a little extra mulch is useful for topping up paths and pots, and a short delivery means a second delivery fee.
Time the job for a calm, dry day after rain, when the soil is moist but not muddy. Water the bed first if the soil is dry, spread the mulch evenly, and check the depth with a ruler in three or four places. Eyes lie about depth, especially on a dark bed. Re-check after the first heavy rain, when the mulch has settled, and top up the thin spots. In autumn, a fresh layer also protects soil from winter rain compaction; in spring, wait until the soil has warmed before mulching warm-season beds, or the mulch will hold the cold in.
How much mulch do I need?
- Measure the bed: length x width in feet = area in square feet.
- Choose a depth: 2-4 in of organic mulch; 3 in is the sweet spot for most beds.
- Measure the old layer first. New mulch goes on top of it, so subtract the existing depth.
- Volume in cubic feet = area x depth in inches / 12. Cubic yards = cubic feet / 27. Bags = cubic feet / bag size, rounded up.
- Spread evenly and keep it a few inches clear of stems and trunks.
How deep should mulch be?
Two to four inches for most beds, with 3 inches as the sweet spot. Less than 2 inches lets sunlight through and weeds germinate. More than 4 inches can suffocate plant roots and trap excess moisture. Fine-textured mulch has a lower ceiling of 2 to 3 inches, while coarse bark nuggets can go up to 4 inches.
How much does 1 cubic yard cover?
One cubic yard covers about 162 square feet at 2 inches deep, 108 square feet at 3 inches, and 81 square feet at 4 inches. One cubic yard equals 27 cubic feet, which is 13.5 bags of 2 cubic feet or 9 bags of 3 cubic feet.
Should I remove old mulch first?
Measure it first. If the old layer plus the new layer would pass 4 inches, remove some old mulch or spread the excess evenly toward the drip line until the total is back inside the range. Never add a full new layer on top of a layer that is already too deep.
What is a mulch volcano, and why is it bad?
A mulch volcano is mulch piled high against a tree trunk. Bark held damp against the trunk rots, and the pile gives rodents cover to chew the stem in winter. Always pull mulch back from the trunk so the root flare is visible.
Does mulch type change the depth?
Yes. Fine or double-shredded bark should be 1 to 2 inches deep because it lets less air through. Coarse wood chips and pine bark nuggets can be 3 to 4 inches. Pine needles and pine straw sit in the middle at 2 to 3 inches.
Depth guidance follows Penn State Extension (2-4 in of organic mulch, 1-2 in for fine or double-shredded mulch) and Clemson Extension HGIC 1604 (fine mulch 2-3 in max, coarse pine bark nuggets up to 4 in). Formula: cubic feet = area (sq ft) x depth (in) / 12; cubic yards = cubic feet / 27.
