Mulch Calculator

How to use this calculator

1. Pick the shape. Use Rectangle for most beds, Circle for a round bed and Tree ring for mulch around a tree. For an L-shaped or curved bed, split it into rectangles and circles and add the results.
2. For a tree ring, enter the outer diameter of the ring, the trunk diameter at the ground and the bare gap you will leave around the trunk. The calculator subtracts that bare circle from the ring.
3. Enter how many. Five identical trees or three equal beds? Enter the count instead of repeating the calculation.
4. Choose the mulch type and the finished depth. The depth advice under the results changes with the type.
5. Measure the old mulch. Push a ruler down to the soil in a few spots and enter the average under Existing mulch. You only order the difference.
6. Pick a bag size and open Prices. The prices are only examples, so enter your supplier’s. If you leave a price blank, the calculator won’t compare bulk with bags.

The formula

Mulch is sold by volume: cubic feet in bags, cubic yards in bulk.

Area (sq ft) = length × width, or π × radius² for a circle

Tree ring area = π × (outer radius² - bare radius²), where the bare radius is half the trunk diameter plus the gap

Cubic feet = area × (finished depth - existing depth) in inches ÷ 12

Cubic yards = cubic feet ÷ 27

Clemson Extension gives the same rule in one step: one cubic yard covers 324 sq ft one inch deep, so cubic yards = square feet × inches ÷ 324. Bags are the cubic feet divided by the bag size, rounded up. Bulk is rounded up to the next half yard, a common way suppliers sell it (ask yours). The bag count, bulk amount, costs and shopping list all use the same rounded numbers.

Jolly Gardener’s coverage chart agrees: a 2 cu ft bag covers 12 sq ft at 2 inches and 6 sq ft at 4 inches, and a 3 cu ft bag covers 18 sq ft at 2 inches and 9 sq ft at 4 inches. The formula gives exactly the same coverage.

Worked examples

A 20 × 10 ft shrub bed, 3 inches deep (the calculator’s default):

• Area: 20 × 10 = 200 sq ft
• Volume: 200 × 3 ÷ 12 = 50 cubic feet, or 50 ÷ 27 = 1.85 cubic yards
• In 2 cu ft bags: 50 ÷ 2 = 25 bags, $113 at $4.50 a bag (25 × $4.50 = $112.50, shown in whole dollars)
• In bulk: 2 yd³, $155 at $40 a yard plus $75 delivery, so bags win here

The same bed with 1 inch of old mulch: you add only 2 inches, which is 33.3 cubic feet or 17 bags.

Five established trees, each with a 3-inch trunk, a 6 ft ring and a 3-inch bare gap. Clemson’s 2 ft of ring per inch of trunk gives 6 ft for a 3-inch trunk.

• Bare circle radius: 1.5 in (half the trunk) + 3 in = 4.5 in, or 0.375 ft
• Ring area: π × (3² - 0.375²) = 27.8 sq ft per tree, 139.2 sq ft for five
• At 3 inches: 139.2 × 3 ÷ 12 = 34.8 cubic feet, which is 18 bags of 2 cu ft

1,000 sq ft of beds at 3 inches: 250 cubic feet, or 9.26 yd³. Bulk is 9.5 yd³ for $455 delivered at the example prices, against 125 bags for $563 (125 × $4.50 = $562.50).

How deep should mulch be?

Every inch of depth takes 3.09 yd³ per 1,000 sq ft: 6.17 yd³ at 2 inches, 9.26 yd³ at 3 inches and 12.35 yd³ at 4 inches.

Deeper is not better. Penn State Extension warns that more than 4 inches may harm a tree’s root system, because under too much mulch, air may not reach the soil. Thick layers of wet mulch may also heat up as they decompose, like compost. Temperatures within mulch piles may reach 140 degrees, and that heat may kill the inner bark of young trees. Voles and mice may also tunnel under deep layers and gnaw the bark. Clemson adds that too much fine-textured mulch can suffocate plant roots, which shows up as yellow leaves and poor growth.

Topping up. Clemson says pine needle and bark mulches typically last around 2 to 4 years before you need to add more. Penn State recommends checking the depth before you add any and removing some old mulch if needed. Applying new mulch over old mulch year after year is the same as applying one layer that is too deep. If the old layer has only matted or faded, break it up with a rake instead.

If the old layer is already deeper than the range in the table, don’t add more. Penn State’s advice is to remove some, or spread the excess evenly toward the drip line until the total is back within that range, and never past the 4 inches Penn State gives as the limit for organic mulch. The calculator flags this and orders nothing, even if you enter a deeper target.

Mulching around trees: no volcanoes

Piling mulch up against a trunk is so common that it has a name: volcano mulching. Penn State Extension says it is never recommended and can cause major tree health problems. Instead:

• Leave the root flare visible. That’s the swelling where the trunk meets the soil (Penn State).
• Keep a bare gap. Clemson says to keep mulch 2-3 inches away from the stems of woody plants, which prevents decay from wet mulch and rodent damage in winter. The calculator defaults to 3 inches.
• Mulch wide, not deep. Clemson suggests a circle 3-4 ft in diameter for newly planted trees. For established trees in lawns, it suggests about 2 ft of diameter per inch of trunk diameter, enlarged as the canopy widens. Penn State recommends mulching toward the drip line (the outer edge of the canopy) where possible.

Bags or bulk?

Bagged mulch has no delivery fee, and you buy exactly the number of bags you need. Bulk mulch costs less per yard, but adds a delivery fee and usually comes in half-yard steps. The calculator compares both at your prices and shows the amount above which bulk is always cheaper: 122 cubic feet (about 4.52 yd³) at the example prices. Below that point, the cheaper option can switch back and forth as the bag count and the half-yard steps change. Ask suppliers about minimum delivery amounts, and have bulk dumped on a tarp near the beds.

Tips

• Weed first. Clemson recommends weeding the area before applying any type of mulch.
• Keep it off the house. Clemson advises keeping mulch 6-12 inches away from building walls so termites can’t use it as a bridge across treated soil.
• Nuggets float. Clemson notes that large pine bark nuggets float and may not stay in place during a heavy rain. Pine needles interlock and stay in place better than most mulches, especially on a slope.
• Follow the bag. Scotts’ directions for its Nature Scapes mulch are to spread it 3 inches deep and keep it several inches from tree trunks and house foundations.
• Building new beds? Use the topsoil calculator for the soil underneath, then come back for the mulch.

Last reviewed Oct 9, 2026. See how we calculate. Results are planning estimates - confirm with your supplier and local code.