Mulch & Topsoil Calculator

The Bed

For irregular beds, split them into rectangles and add the results.

Shape

Length

Width

Diameter

Depth3–4 in for mulch · 6–12 in for new topsoil

Material

Bag size sold locally

MATERIAL NEEDED

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Cubic yards—

Cubic feet—

Bags at your size—

Approximate weight—

Area covered—

Coverage per cubic yard—

Bags or bulk?

Recommendation—

Pickup loads (~2 yd³)—

Bags to fill one cubic yard—

Depth check—

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Depth is the decision

Mulch works by blocking light. Below about two inches enough light reaches the soil for weed seeds to germinate, so the layer does almost nothing. Above about four inches it starts holding too much moisture against stems and restricting air to the roots. Three to four inches is the working range for bark mulch, and it should be pulled back a few inches from trunks and stems rather than piled against them.

Bags versus bulk

A cubic yard is 27 cubic feet, so 13 or 14 bags of the common 2 cubic foot size. Bulk delivery is usually cheaper per yard from about two yards upward and saves a great deal of bag handling, but needs somewhere to tip it and a wheelbarrow. Below one yard, bags almost always win on convenience.

Topsoil is a different job

Mulch sits on top; topsoil is the growing medium and goes on much deeper — six to twelve inches for a new bed, which is two to four times the volume for the same area. Topsoil also settles noticeably after watering, so ordering about 10 per cent extra is normal rather than wasteful.

Depth does the work, and more is not better

Mulch suppresses weeds by denying light and conserves moisture by breaking the evaporation path, and both effects are largely achieved at 2 to 3 inches for shredded bark. Beyond about 4 inches the marginal benefit disappears and the drawbacks start: water sheds off a thick mat of fine mulch before it reaches the root zone, and a persistently damp layer against stems invites rot. One cubic yard covers roughly 108 square feet at 3 inches, or 162 square feet at 2, which is the conversion that decides bags versus bulk delivery.

The most damaging habit in the whole category is piling mulch against a trunk. Bark is not adapted to constant moisture, and a mulch volcano rots the collar, invites boring insects and encourages roots to circle in the mulch instead of the soil. Pull mulch back a few inches to expose the flare at the base of the trunk. Topsoil and compost are also not interchangeable: compost is an amendment to be mixed in, not a growing medium to be used alone at depth.

Frequently Asked Questions

How deep should mulch be?

Three to four inches. Less than two lets weeds through; more than four holds moisture against stems and restricts air to the roots.

How many bags of mulch are in a cubic yard?

Thirteen and a half bags of the common 2 cubic foot size, since a cubic yard is 27 cubic feet. Most people round to 14.

Should I buy bags or bulk?

Bulk is usually cheaper from about two cubic yards upward, if you have somewhere to tip it. Below one yard, bags are easier and often no more expensive.

How much topsoil for a new bed?

Six to twelve inches depending on what you are planting — two to four times the volume of a mulch layer over the same area. Allow about 10% for settling.

Where these numbers come from

Sources

Official publications only. Links open the original document in a new tab.

U.S. Department of Health and Human Services — ODPHP Dietary Guidelines for Americans USDA guidance on growing food at home
National Institute of Standards and Technology Office of Weights and Measures How bulk garden material is sold

Mulch & Soil Calculator