Mulch Calculator

How much mulch a bed needs

Mulch is bought by volume — by the bag or by the yard — so «how much» is a volume question. For a rectangular bed it is one multiplication and one division:

The 324 is not a rule of thumb. A cubic yard is 3 × 3 × 3 feet = 27 cubic feet; spread one inch deep it becomes a sheet 27 × 12 = 324 square feet across. Divide 324 by the depth you want and you have the coverage of one yard. Nothing in that chain depends on what the mulch is made of, which is why the volume is exact and the weight is only an estimate.

Step by step

1. Measure the bed in square feet. Width × length for a rectangle; split a border or an island bed into rectangles and add them up.
2. Choose the depth in inches. Extension services recommend 2–4 inches of organic mulch; 3 inches is the middle of that range and the default here.
3. Multiply and divide by 324. That is cubic yards. Multiply the square feet by the inches and divide by 12 instead if you want cubic feet, which is what bags are labelled in.
4. Divide the cubic feet by the bag size. A standard bag is 2 cubic feet, so a cubic yard is 13.5 bags — and part bags always round up.

How much one cubic yard covers

This is the second question the topic asks, and the answer only depends on depth — not on the material:

What a cubic yard of mulch weighs

Weight is the second thing people ask about mulch, usually because they are deciding between a delivery and several trips in a car. Here the honest answer is a band, not a figure:

• Dry bark or wood chips: roughly 400 to 800 lbs per cubic yard — a fifth to two fifths of a ton. A 2 cubic foot bag is therefore around 30 to 60 lbs.
• Soaked with rain: two to three times that. Mulch is a sponge; a load that fitted in the car dry may be over the axle rating wet.

Bags or bulk

One cubic yard is 27 cubic feet, so it is 13.5 bags of 2 cubic feet. Bagged mulch is priced per bag and bulk mulch per yard, which is why the two are hard to compare in the aisle: convert both to the same volume first. Bags win on small beds and on anything you have to carry through a house; bulk wins from roughly two cubic yards up, where the delivery charge is spread over enough material to matter.

What people are actually working out

• Beds and borders: the everyday case — measure the bed, take 3 inches, order.
• Around trees: a ring, not a volcano. Mulch piled against the trunk keeps the bark wet and invites rot; leave a hand's width of clear ground around the stem.
• Weed suppression: the depth is doing the work. Under 2 inches light reaches the soil and weeds come through anyway.
• Playgrounds and paths: deeper than a bed, and a different product — engineered wood fibre rather than bark.
• Topping up: last year's mulch has not vanished, it has composted down. Measure what is left and add the difference, not the full depth.

Where the formula stops working

• The bed must be flat and rectangular. Split irregular shapes into rectangles; for a ring around a tree, take the outer circle and subtract the inner one.
• Deeper is not better. Over 4 inches of organic mulch keeps water and air from the roots, which is the opposite of what mulching is for.
• Mulch settles and decomposes. A 3 inch layer will not be 3 inches next spring, so plan on topping up rather than on a one-off.
• The weight is an estimate. Use it to decide whether a load will fit in the car, not to settle a delivery invoice.

Doing it in your head

Remember one number: 324. One cubic yard, one inch deep, covers 324 square feet — so at the recommended 3 inches a yard does about 110 square feet, and 100 square feet is very nearly one yard, or about 13 bags. Most home beds land between half a yard and two yards.

Mulch for common areas

At the recommended depth of 3 inches, in bulk and in bags: