Mulch Calculator

How to use this mulch calculator

1. Measure each bed in feet. Long, straight beds are rectangles. Use a circle for an island bed and a triangle for a corner. Odd shapes can be split into a few pieces, with one entry for each.
2. For a ring around a tree, enter the outer diameter of the ring and the inner diameter of the bare opening at the trunk. Make the opening the width of the trunk plus a few inches on each side, since mulch shouldn’t touch the bark.
3. Pick the kind of mulch. The depth fills in with Clemson Extension’s recommended maximum for that material, and you can change it.
4. If the beds already have mulch, choose “Top up old mulch.” Rake the old layer flat, push a ruler down to the soil in a few spots, and enter the average. The calculator subtracts it from the depth you want.
5. Choose the bag size you plan to buy. Most garden centers sell 2 cubic foot bags, and some brands also come in 1.5 or 3.
6. Open “Extra and prices” to enter the price of a bag, the bulk price per cubic yard at a landscape supplier, and the delivery fee. The defaults are only examples.

The result shows whichever way of buying costs less, and the line below compares both. Use “Copy link” to save the estimate or send it to whoever is picking up the mulch.

How the calculator works

area          = length × width, or π × diameter² ÷ 4 for a circle
depth to add  = depth you want − old mulch depth
cubic feet    = area × depth to add ÷ 12
cubic yards   = cubic feet ÷ 27, the same as area × depth ÷ 324
bags          = cubic feet ÷ bag size, rounded up
bulk          = cubic yards, rounded up to the next half yard

The 324 comes from Clemson Extension’s mulch fact sheet: one cubic yard spread 1 inch deep covers 324 square feet. Spread the same yard 3 inches deep and it covers a third of that, 108 square feet.

The depth presets come from the same fact sheet. Fine-textured shredded wood and bark mulch should be no more than 2 to 3 inches deep, because a thicker layer can starve roots of air. Coarse mulch such as pine bark nuggets lets air through and can go to 4 inches. Grass clippings and shredded leaves mat together, so they stay at 2 inches or less.

The bags vs bulk comparison is plain arithmetic. A bag price becomes a price per cubic yard: $4.50 for 2 cubic feet is $60.75 a yard. Bulk is cheaper per yard but adds a delivery fee, so the rough break-even point is the delivery fee divided by the difference in price per yard. At our example prices that’s $65 ÷ ($60.75 − $38), about 2.9 cubic yards. Whole bags and half-yard orders blur that line, so between about 2.9 and 3.6 cubic yards either one can come out ahead. Above 3.6, bulk always wins, and that’s the number the result shows. We round bulk orders up to the next half yard, which is our assumption. Ask your supplier what steps and minimum they sell.

A worked example

Say you have a 4 × 30 foot bed along the front of the house and a young maple in the lawn. You want a ring around the maple 6 feet across, with a 1 foot opening at the trunk, and 3 inches of shredded hardwood everywhere.

• Foundation bed: 4 × 30 = 120 square feet
• Tree ring: π × (6² − 1²) ÷ 4 = 27.5 square feet
• Total: 147.5 square feet
• Volume: 147.5 × 3 ÷ 12 = 36.9 cubic feet, or 1.37 cubic yards
• Bags: 36.9 ÷ 2 = 18.4, so 19 bags of 2 cubic feet, $85.50 at $4.50 each
• Bulk: 1.5 cubic yards at $38, plus $65 delivery, is $122

Bags win by about $36. The job is well under 2.9 cubic yards, where bags always win at these prices, so the delivery fee eats the bulk discount.

Next spring the old mulch has broken down to about 1 inch. In top-up mode you only add 2 inches: 24.6 cubic feet, or 13 bags. Buying a full 3 inches again would have meant 6 extra bags and a layer 4 inches deep.

Mulch for common bed sizes

At 3 inches deep, plus the bags needed to top up an existing 2 inch layer by 1 inch:

At our example prices, bags are cheaper through the 300 square foot row and bulk delivery wins from 500 square feet up. A big top-up stays a bag job much longer, since it needs only a third of the mulch.

Tips for spreading mulch

• Pull weeds first. Mulch slows new weeds down, but established ones grow right through it.
• Keep mulch 2 to 3 inches away from the stems of shrubs and never pile it against a tree trunk. Wet mulch against bark invites rot and gives rodents cover in winter.
• Leave a gap of 6 to 12 inches next to the house. Clemson points out that subterranean termites can use mulch as a bridge across the treated soil around a foundation.
• Mulch new plantings right after you plant them. Around established beds, early spring is the best time, before weed seeds sprout.
• Expect bark and pine needle mulch to last 2 to 4 years before it needs more. Leaves and clippings break down fast and need topping up more often.
• For a bulk load, pick the drop spot before the truck arrives. A tarp on the driveway makes cleanup easier, and the closer the pile is to the beds, the fewer wheelbarrow trips you’ll make.
• Spread from the bag in place. Cut the bag along one side, tip it where the mulch is going, and rake it out. It’s faster than filling a wheelbarrow and keeps the edges of the bed clean.