Depth is the input everyone guesses, and it decides the yardage.
By David Liu, founder of DIY9lives. Last updated November 2, 2026.
Mulch is sold in 2 cubic foot bags at the garden centre and by the cubic yard from a landscape supplier, and the two prices are not comparable until you convert. This guide does the conversion for a typical bed and then spends most of its length on the part that matters more: how deep the mulch should actually be.
Volume is the bed's square footage multiplied by the depth in feet. The bed's shape does not matter; only its area does. A 20 by 8 foot rectangle is 160 square feet. A curved bed is measured by breaking it into rectangles and triangles, or estimated from a rough length times average width.
Depth is where the arithmetic goes wrong, because mulch is discussed in inches and the formula wants feet. Three inches is 0.25 feet. Two inches is 0.167. Four inches is 0.333, and that one third is easy to remember.
For the 20 by 8 bed at three inches: 160 × 0.25 is 40 cubic feet. Add 5% for the edges you cannot see and the wheelbarrow spill, and it is 42 cubic feet.
| Unit | Conversion | For 42 cubic feet |
|---|---|---|
| 2 cubic foot bags | divide by 2 | 21 bags |
| Cubic yards | divide by 27 | 1.56 yards |
| Truckload (typical pickup) | about 2 yards | most of one load |
Twenty-one bags is a lot of trips from the car, and 1.56 yards is one bulk delivery. The break-even is usually around half a yard: above that, bulk is cheaper per cubic foot and dramatically faster, provided the supplier will deliver and your driveway can take the truck.
If you order bulk, order the nearest quarter or half yard above your figure. Suppliers sell in increments, and a yard of mulch is easier to give away than to buy a second time for a 1 cubic foot shortfall.
Two to three inches is the useful range for a planting bed. Less than two does not suppress weeds or hold moisture; more than four starts to cause problems. The reason is oxygen: roots and soil organisms need air, and a thick blanket of mulch slows the exchange enough to hurt shallow-rooted plants.
Three inches is the common recommendation for a new bed over bare soil. On an established bed that gets topped up each year, one inch may be enough, because the old layer is still there. Measure the existing depth before adding to it — accumulated mulch over several seasons is how beds end up with six inches and unhappy shrubs.
Two places need care. Keep mulch a few inches away from trunks and stems, so it is not piled against bark where moisture sits and rot starts. And do not bury the crown of a perennial; a mound over the growing point is a slow way to kill it.
| Bed | Area | At 2 in | At 3 in |
|---|---|---|---|
| 4 by 20 ft foundation strip | 80 sq ft | 13 cu ft (7 bags) | 20 cu ft (10 bags) |
| 20 by 8 ft bed | 160 sq ft | 27 cu ft (14 bags) | 40 cu ft (20 bags) |
| 30 by 10 ft bed | 300 sq ft | 50 cu ft (25 bags) | 75 cu ft (38 bags) |
| Ring around one tree, 6 ft across | about 28 sq ft | 4.7 cu ft (3 bags) | 7 cu ft (4 bags) |
A tree ring is worth doing properly. Piling mulch against the trunk to make a volcano is the most common mulch mistake there is, and a shallow, wide ring is both better for the tree and easier to keep tidy.
Bagged mulch is convenient, consistent, and priced per cubic foot at a premium. Bulk is cheaper and comes wet. A cubic yard of bark mulch weighs somewhere in the region of 800 pounds according to the figure this site uses, and wet mulch weighs more than dry — which matters if you are buying by weight rather than volume, and matters a great deal if you are loading it yourself.
Do not carry the mulch weight figure over to gravel or topsoil. Those are far heavier per yard, which is why the mulch, soil and gravel calculator keeps separate weights for each material.
Spring is the usual time, before the beds dry out and before weeds get established. A fresh layer over last year's mulch works if the total depth stays in range. If the old layer has broken down into the soil, that is fine — decomposition is the point, and it feeds the bed slowly.
Do not mulch over dry soil in a heatwave without watering first. The mulch will hold the dryness in as effectively as it holds moisture. Water the bed, then mulch, then water again.
For the bed in this guide, the answer is 42 cubic feet: 21 bags, or about a yard and a half delivered. The number is easy; the depth is the decision that makes it right or wrong.
The volume calculation is the same for every material, but what you put down changes what happens in the bed. Shredded bark breaks down slowly and knits together, which makes it stay put on a slope. Pine bark nuggets last longer and float in heavy rain. Straw is cheap and short-lived, which suits a vegetable bed you will turn over. Compost is not really mulch — it is a soil amendment that happens to suppress weeds for a season.
Colour matters less than people think. Dyed mulches hold their colour for a season and then fade; the dye does not harm the bed in the amounts used, but it also does not improve it. Undyed bark will grey out in the sun, which is normal rather than a sign that it has failed.
The one material to be careful with is fresh wood chips worked into the soil rather than laid on top. As they decompose, the microorganisms doing the work draw nitrogen from the soil, which can leave plants short. On the surface, that is not an issue; dug in, it can be.
For a 20 by 8 bed at three inches, the 21 bags or 1.56 yards is the same whichever material you choose. Choose on how long you want it to last, not on the bag count.
A bed on a slope needs mulch that knits together, or it will be at the bottom of the hill after the first storm. Shredded bark holds far better than nuggets or straw, and on a steep bank a light application of a coarse, fibrous material stays put where a thick layer slides.
An edge makes the bed easier to keep tidy. A spade-cut edge costs nothing and lasts a season or two; steel or aluminium edging costs more and lasts decades. Either is cheaper than replacing mulch that has washed into the lawn.
Delivery is the practical limit on bulk. A cubic yard of mulch is a heap roughly three feet on each side, and the supplier will usually tip it on the driveway rather than in the bed. Budget the wheelbarrow trips before ordering: 1.5 yards is around twenty loads, which is an afternoon, and it is why bagged mulch is still popular for small jobs.
Last updated: November 2, 2026. Estimates only. Not tax, legal, financial, or professional advice. See the terms.