How much mulch to buy: measure the layer you need to add

Measure the old layer before choosing a quantity

Move loose mulch aside at several points until you can distinguish it from the underlying soil, then measure the layer without digging into roots. Check a thin patch, the middle of the bed and a place where mulch has accumulated. Faded color alone does not tell you how much material is missing. If the depth varies substantially, calculate the thin and thick areas separately.

Depth depends on the material and drainage. Penn State gives two to four inches for organic mulch, less on poorly drained soil, and one to two inches for fine or double-shredded mulch. Maryland limits organic mulch to three inches. Pick the guidance that fits your material and site. The worked bed below uses a three-inch finished depth.

Work uneven beds in zones

Consider an 18-by-6-foot bed: 108 square feet. If it already has an even one-inch layer and the chosen finished depth is three inches, it needs two additional inches everywhere. Its volume is 108 × 2 ÷ 12 = 18 cubic feet. But an uneven bed of the same size can require much less. The table shows a different measured condition, with the same three-inch target.

These zones total 108 square feet but need only 10 cubic feet of new mulch. Adding two inches everywhere would overfill the deeper zones. Mark the areas with stakes or a sketch before spreading. Add nothing where the existing mulch already reaches the target, and remove excess where it is too deep.

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Convert the area you will actually cover

For a rectangle, multiply length by width. Divide an L-shaped bed into rectangles without counting their overlap twice. Subtract paving and areas you will leave bare. For a tree ring, use π × (outer radius² − inner radius²), with both radii in feet. An example ring with a five-foot outer radius and one-foot inner radius covers about 75.4 square feet. Choose the actual inner clearance for your tree before calculating.

One cubic yard is 27 cubic feet. The even-depth 18-cubic-foot example therefore needs two-thirds of a yard: nine 2-cu-ft bags or twelve 1.5-cu-ft bags. Check the actual bag volume, sum all zones first, then round the total bag count up.

For the uneven 10-cu-ft bed, 1.5-cu-ft bags require 10 ÷ 1.5 = 6.67, so buy seven bags containing 10.5 cu ft. Allocate 6 cu ft to the thin zone and 4 cu ft to the middle zone; the deepest zone gets none. The remaining 0.5 cu ft belongs in another suitable area or storage, not as an extra layer over a bed already at its target.

Compare a bag order with a delivered bulk order

For an 18-cubic-foot job, the sample prices below show how minimum orders and delivery affect the choice. The bulk supplier’s one-yard minimum supplies 27 cubic feet, leaving nine cubic feet beyond the job’s need. Compare the full delivered bill with the bag total. These prices are hypothetical calculation inputs.

The result can reverse as the job grows. At the same hypothetical prices, 72 cubic feet needs 36 two-cubic-foot bags costing $162. A supplier selling whole-yard increments would deliver three cubic yards for $105 plus $45 delivery, or $150, leaving nine cubic feet over. Add actual tax, transport charges and any handling cost before deciding. Compare the same mulch type and grade, not just two products with the same color name.

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Spread the correct layer without building a trunk mound

Begin with the thin zones and distribute material over the intended soil area. Check the combined old-and-new layer in several places as you go. Keep mulch off tree trunks and leave the root flare visible. A decorative cone against the trunk is excess material in the wrong place, not a use for the last bag. If the flare is buried in soil, get an arborist's advice before attempting excavation around roots.

Before ordering, check unloading access and the route from the delivery pile to the bed. If mulch runs short while spreading, remeasure the zone areas and the combined layer thickness before buying more.