Mulch Calculator

The formula

Mulch is a volume problem, and volume is area times depth. The only trap is the units: you measure the bed in feet and the depth in inches, and you buy in cubic yards.

• Cubic feet = area in sq ft × (depth in inches ÷ 12)
• Cubic yards = cubic feet ÷ 27
• Bags = cubic feet ÷ the bag's cubic feet, rounded up

A cubic yard covers 108 sq ft at 3 inches, 162 sq ft at 2 inches, and 324 sq ft at 1 inch. Those three numbers are worth memorising — they let you sanity-check any quote in your head.

Worked example

A foundation bed 30 ft long and 6 ft deep, mulched at 3 inches, with 5% for settling.

1. Area = 30 × 6 = 180 sq ft 2. Depth in feet = 3 ÷ 12 = 0.25 ft 3. Volume = 180 × 0.25 = 45 cu ft, × 1.05 = 47.3 cu ft 4. Cubic yards = 47.3 ÷ 27 = 1.75 cu yd 5. In 2 cu ft bags = 47.3 ÷ 2 = 24 bags

Twenty-four bags at around $4.50 is roughly $108. Two yards of the same shredded hardwood delivered is about $76 plus a delivery fee. On a bed this size it is close, and the delivery fee decides it.

Depth: 2 to 3 inches, and no more

This is the part worth taking seriously. Two to three inches of mulch does everything mulch is for — it holds moisture, moderates soil temperature and smothers weed seedlings. Four inches does not do it better. Past about three inches the layer starts shedding water rather than letting it through, and the soil underneath stays dry while the mulch stays wet.

The specific failure is the mulch volcano: mulch piled in a cone against a tree trunk. Bark above the root flare is not adapted to being wet. Keep it wet for a season or two and it rots, and the tree loses its ability to move water and sugar up and down the trunk. It also invites voles, which chew bark under cover, and it encourages roots to grow up into the mulch where they girdle the trunk. The tree does not die that year. It dies five or eight years later and nobody connects it to the mulch.

Pull mulch back so you can see the flare where the trunk widens into the roots. A doughnut, never a volcano.

Bulk versus bagged

Bags cost roughly two to three times what the same volume costs in bulk, and you pay for the plastic and the handling. The crossover is around 12 to 15 bags — under that, bags are fine and you avoid a delivery fee and a pile on the driveway. Over that, order bulk.

Bulk has its own friction. It arrives as a heap, it has to go somewhere the truck can reach, and it needs moving with a wheelbarrow the same day if it is sitting on a driveway you use. Fresh bulk mulch also heats — a big pile will steam, and if it is left more than a few days it can go sour and needs airing before it goes near plants.

Pine straw is different

Pine straw is sold by the bale, not by volume, and bales vary between suppliers. A bale typically covers 35 to 50 sq ft at a settled depth of about 3 inches, so budget around 2 to 3 bales per 100 sq ft. It is lighter, it knits together on slopes where shredded mulch floats away in a storm, and it breaks down faster — expect to top-dress annually rather than every second year.

Rubber mulch does not break down at all, which is the argument for it and against it. It adds nothing to the soil, it holds heat, and it costs several times what wood does. It earns its place under playground equipment and almost nowhere else.