Mulch is simple arithmetic with one real decision in it — depth — and one crossover point where bags stop making sense. Here is both, with a worked example.
Step 1: the volume
Length × width × depth, with the depth converted from inches to feet, then divided by 27 for cubic yards.
A bed 20 feet long and 10 feet wide at 3 inches deep: 20 × 10 × (3 ÷ 12) = 50 cubic feet, which is 1.85 cubic yards.
For a curved or irregular bed, break it into rectangles and circles. A circle is radius × radius × 3.14.
Step 2: the depth is the decision
- 2 to 3 inches — the standard for an established bed. Enough to suppress weeds and hold moisture.
- 3 to 4 inches — a new bed, or one with a persistent weed problem.
- 1 to 2 inches — a top-up over mulch that is still in reasonable condition.
Deeper is not better. Past about 4 inches mulch starts working against you: water struggles to reach the soil, and the lower layer can go anaerobic and sour.
Step 3: bags or bulk
A standard bag holds 2 cubic feet, so a cubic yard is 13.5 bags. Our 1.85 yard bed is 25 bags.
The crossover is around 2 cubic yards. Below that, bags are easier and the delivery fee is not worth paying. Above it, bulk is dramatically cheaper and you are not left flattening 40 plastic sacks.
Mulch Calculator
Enter the bed size and depth — get cubic yards and the bag count in seconds.
Open the mulch calculator →The mistake that kills trees
Mulch piled up against a trunk — the shape people call a mulch volcano — holds moisture against the bark, which rots it and invites disease and rodents. It is one of the most common ways a healthy young tree is slowly killed by good intentions.
Keep mulch 3 to 6 inches clear of trunks and stems, and taper it down as it approaches. The correct shape is a doughnut, not a cone.
How often to replace it
Organic mulch breaks down — that is part of the point, since it feeds the soil as it goes. Most beds need a top-up once a year rather than a full replacement. Rake the old material to break up any crust, check the depth, and add only what is needed to bring it back to 3 inches.
Stripping and replacing the whole bed every spring wastes both money and the soil improvement the old mulch was busy making.
Which mulch to buy
Volume is the same for every type, but how long it lasts and what it does for the bed are not. A quick comparison:
| Type | Lasts about | Best for |
|---|---|---|
| Shredded hardwood | 1 to 2 years | Most beds; knits together and stays put on slopes |
| Pine bark nuggets | 2 to 3 years | Flat beds; large nuggets float away in heavy rain |
| Pine straw | About 1 year | Acid-loving shrubs and slopes; sold by the bale |
| Cedar or cypress | 2 to 3 years | Beds where you want a slower breakdown |
| Dyed wood mulch | About 1 year (color fades) | Color; check that it is made from clean wood |
| Rubber or stone | Many years | Paths and play areas; does not feed the soil |
Pine straw is the exception to the cubic yard math: it is sold by the bale, and a bale typically covers somewhere around 50 square feet at a normal depth, depending on the bale size. Check the coverage the seller quotes.
When to put it down
Mid to late spring, after the soil has warmed, is the usual time. Mulch put down on cold, wet soil keeps it cold longer and slows early growth. A second, lighter application in fall helps protect roots through winter in colder climates. Water the bed before you mulch: mulch holds moisture in, but it also slows light rain from reaching dry soil underneath.
Stone mulch is a different calculation
Decorative stone is ordered by volume like mulch, but sold by weight. Most landscape rock runs around 1.3 to 1.5 tons per cubic yard, so a bed that needs 2 cubic yards of stone is close to 3 tons. That weight matters if you are hauling it yourself. Under stone, a landscape fabric is worth using, because pulling weeds out of gravel is miserable. Under organic mulch, skip the fabric: it stops the mulch from feeding the soil and weeds root into the mulch on top of it anyway.