Mulch Calculator

How to use the mulch calculator

Measure each bed at its longest and widest points and work in feet. Garden beds are rarely tidy rectangles, so for a curved or tapering bed take the widest and the narrowest measurement and use the average of the two; for a bed that wraps a corner or snakes along a fence line, break the shape into two or three rough rectangles, run the calculator once per section, and add the cubic-foot results together.

Precision to the inch is wasted effort here, because mulch spreads, settles, and gets raked toward the edges. Rounding each dimension up to the nearest half foot is close enough, and it errs in the direction you want: a quarter yard left over goes under a tarp for the autumn top-up, while a quarter yard short means a second trip and often a second delivery fee.

The result reads two ways on purpose. Cubic yards is the bulk-order number, the unit a landscape supplier’s order form and delivery driver both speak, and the one to quote on the phone. The bag count is the same volume expressed in store bags, defaulting to the standard two-cubic-foot bag with a three-cubic-foot option under Advanced for the larger sacks some stores carry.

Because a cubic yard is 27 cubic feet, it is exactly 13.5 standard bags or 9 large ones, which is the conversion to hold in your head while you compare a shelf price against a bulk price plus delivery. Bulk usually wins somewhere around a full yard, though a delivery fee on a small order can flip that — and this is a shopping estimate, not a landscaper’s takeoff.

How deep should the mulch actually go?

Depth is the setting that actually drives the number, and it is where most orders go wrong. University of Illinois Extension states the recommendation plainly — mulch should be two to three inches deep. Three inches is the working default for shrub and perennial beds, deep enough to shade out germinating weed seed and slow evaporation from the soil surface.

Two inches is right for an annual refresh over a layer that has not fully broken down, and for shallow-rooted annuals that resent being buried. Past four inches you are buying problems rather than benefits: a thick mat holds water against stems, keeps oxygen away from roots, and gives voles somewhere to overwinter. Change the depth before you change anything else and watch the cubic-yard figure move.

The formula, worked line by line

Mulch is a volume problem wearing a landscaping hat: the area you are covering multiplied by how deep you want the cover to be. The only thing that makes it awkward is that the three measurements arrive in three different units. Your bed is measured in feet, your depth is chosen in inches, and bulk mulch is sold in cubic yards, so two conversions sit between the tape measure and the order form.

Dividing the depth by 12 turns inches into feet, so the multiplication produces cubic feet rather than a meaningless hybrid. Dividing that result by 27 turns cubic feet into cubic yards, because a yard is three feet on every side and three cubed is 27. Skipping either division is where nearly every wildly wrong mulch order comes from, and the error is not subtle: forget the 27 and you order twenty-seven times too much.

cubic feet = length ft × width ft × (depth in ÷ 12)
cubic yards = cubic feet ÷ 27
bags = cubic feet ÷ bag size, rounded UP to a whole bag
sq ft covered by one cubic yard = 324 ÷ depth in inches

The single number worth memorising is 324. One cubic yard is 27 cubic feet, and spreading 27 cubic feet one inch deep covers 27 × 12 = 324 square feet, so coverage at any depth is just 324 divided by the depth in inches.

UGA Extension publishes the same figures in its horticultural conversion tables: 324 square feet at one inch, 162 at two inches, 54 at six. At the recommended three inches a cubic yard covers 108 square feet, which is the fastest way to sanity-check any quote you are given.

Two things the arithmetic does not know about. Bulk mulch is measured loose and settles once it is spread and rained on, so a bed laid at a full three inches in April reads closer to two and a half by midsummer; that is normal, not a shortfall.

And mulch is sold by volume while trucks are limited by weight — the same cubic yard of bark weighs considerably more soaked through than it does dry, so if you are hauling it yourself, ask the yard what a wet yard weighs before you load your own pickup and find out the hard way.

Questions people ask