Mulch Calculator

How mulch quantity is worked out

Every mulch order comes down to one sum: area times depth. Measure the bed in feet, convert the depth from inches to feet by dividing by twelve, multiply the two, and you have cubic feet. Divide by 27 and you have cubic yards, which is how bulk suppliers sell it. Divide by the bag size instead and you have the number of bags to load into the car. The calculator above runs exactly that chain, including the circle formula for tree rings and island beds.

Worked example: a 20 ft by 4 ft border

The bed is 80 square feet. At the standard 3 inch depth, that is 0.25 feet, so 80 x 0.25 = 20 cubic feet of mulch. In cubic yards that is 20 / 27 = 0.74, and in retail bags it is 10 bags of the two cubic foot size or 7 bags of the three cubic foot size. Note how the bag counts round up: you cannot buy 0.74 of a yard or 6.7 bags, so always order the whole unit above.

Change the depth and the answer scales in a straight line. The same border at 2 inches needs 13.3 cubic feet — 7 small bags. At 4 inches it needs 26.7 cubic feet, or 14 small bags. That is the whole reason the depth selector sits next to the measurements: an inch either way moves the bill by a third.

Worked example: a circular tree ring

A 10 foot diameter ring is pi x 10² / 4 = 78.5 square feet, near enough the same as the border above. At 3 inches it takes 19.6 cubic feet, so 10 two cubic foot bags again. If you have four such rings around the front lawn, set the bed count to 4 and the tool returns 78.5 cubic feet, or 2.9 cubic yards — which has just crossed into bulk delivery territory.

Bags or bulk: where the price flips

One cubic yard equals 13.5 bags of two cubic feet. In the US, bagged hardwood mulch typically runs $4 to $6 a bag, so a yard's worth in bags costs roughly $55 to $80. The same yard delivered in bulk is commonly $35 to $50 plus a flat delivery fee of $50 to 00. Below about one cubic yard, bags win because the delivery fee dominates. Above two to three yards, bulk is usually half the price per yard even after delivery.

The catch is labour. A cubic yard of damp hardwood mulch weighs 600 to 1,000 pounds and arrives as a pile on your driveway, which you then move by wheelbarrow. Bags cost more per cubic foot but can be carried one at a time and stored until the weekend. In the UK, where bulk is sold in cubic metres and bags in 50 to 70 litre sacks, the same crossover logic applies — one cubic metre is roughly 1.31 cubic yards, or about 20 of the 70 litre sacks.

Never build a mulch volcano

The single most common and most damaging mulching mistake is piling mulch in a cone against a tree trunk. Bark is not root tissue and is not adapted to constant moisture. Buried under mulch it stays wet, softens, and gives insects, fungi and rodents a protected route into the cambium. Trees mulched this way often look fine for five years and then decline for reasons the owner never connects to the mulch. Roots also grow into the mulch cone and circle the trunk, eventually girdling the tree they belong to.

The fix is trivial: spread mulch flat like a doughnut, not a volcano, and keep a bare gap of three to six inches around every trunk and stem. The same rule applies to shrub bases and perennial crowns. Depth over four inches anywhere in the bed carries a milder version of the same risk, which is why this calculator flags the six inch option.

Choosing the material

Shredded hardwood knits together and stays put on slopes, making it the default for most beds. Bark nuggets last longer but float in heavy rain and end up on the path. Pine straw is light, cheap in the southern US and excellent around acid-loving plants. Cedar and cypress resist decay and smell good but break down slowly, so they add little to the soil. Rubber mulch never decomposes, which sounds like an advantage until you want to change the bed or improve the soil; it also holds heat and offers no organic benefit.

What the numbers do not cover

Bulk mulch settles and compacts by roughly 10 to 15 percent in the first season, so ordering a little over is normal practice rather than waste. Existing mulch counts toward your target depth — measure what is already there and add only the difference. Uneven ground, deep hollows and the volume displaced by established plants all shift the real figure. Bag sizes also vary: 1.5, 2 and 3 cubic feet are all sold as standard, and some suppliers quote litres, so check the label before you convert. Treat the result as a shopping figure accurate to within about ten percent, which is exactly the tolerance the job needs.