Mulch Calculator

The arithmetic

Cubic feet = area in square feet × depth in inches ÷ 12. Cubic yards = cubic feet ÷ 27, which is the same as area × depth in inches ÷ 324, because a cubic yard is 27 cu ft and a foot is 12 in. Clemson’s HGIC 1604 gives the same rule: “Multiply your square footage by the depth desired (in inches) and divide by 324 square feet.”

Bags = cubic feet to order ÷ the bag’s volume in cubic feet, rounded up once, at the end; a division that comes out whole is not rounded up. One bag covers its volume in cubic feet × 12 ÷ the depth in inches, in square feet, and one cubic yard covers 324 ÷ the depth in inches. A litre is 0.001 m³ and 1 ft = 0.3048 m exactly, so a 56.6 L bag is 2 cu ft to three figures.

Lengths can be entered in feet, inches, yards or metres and depths in inches, centimetres or millimetres: everything is converted before it is multiplied.

Beds, circles and rings around trees

A bed is length × width; a round bed is π × diameter² ÷ 4. A ring around a tree is a circle with a smaller circle taken out of its middle: π × (outside diameter² − bare center²) ÷ 4, where the bare center is the trunk diameter plus the gap you leave on each side, twice. Identical areas multiply the first area, and up to three more rectangular beds are added to it, all at the same depth.

How far to keep mulch from the trunk, as the sources put it: Penn State, “pull mulch 3 to 5 inches away from young trees and 8 to 10 inches away from mature trees”; Clemson, “2 to 3 inches away from the stems of woody plants”; Maryland, “about 3 inches away from the base of tree trunks”; Iowa State and Colorado State, at least 6 inches; UF/IFAS, “about 1 foot from the base” for large plants and trees; K-State, “a 6-12 inch diameter circle of bare soil at the center”. The gap is yours to enter.

How big a ring, as the sources put it: ISA, “extend to the drip line, if practical”, and “Generally, a 3 foot (1 meter) radius ring is the minimum for most trees”; Clemson, “a circle of mulch 3 to 4 feet in diameter” for newly planted trees and “about 2 feet in diameter for each inch of trunk diameter” for established trees in lawns; the Morton Arboretum, the drip line on a younger tree and at least a four to five foot diameter on older, larger ones; K-State, “at least 3-6 feet in diameter”; Utah State, “at least three feet from a tree's trunk in all directions”; UF/IFAS, “at least 8 feet in diameter”. A radius is half a diameter: a ring 3 ft out from the middle of the trunk is 6 ft across, so enter 6.

Depth: what the sources say, and where they disagree

The calculator takes the depth you enter. The sources below give depths for particular mulches, soils and plantings, and they do not agree: their figures run from 1 in to 6 in. Six of them can be chosen from the depth list, each labelled with its source and condition. Every one is a printed range, so the calculator works it at both ends and never averages it, and quotes the source beside the result.

• ISA, “Proper Mulching Techniques” (©2021): “For well-drained sites, apply a 2–4 inch (5–10 cm) layer of mulch. Fine mulches, such as composed materials, should be applied in a 2–3 inch layer and coarse mulches, such as arborist wood chips, should be applied in a 3–4 inch layer.” On wet soils, it says more than 2 inches (5 cm) of organic mulch can reduce soil drying. All three ISA ranges are on the depth list.
• Penn State Extension, “Mulching Landscape Trees” (updated April 12, 2018): “Apply a 2- to 4-inch layer of organic mulch and no more; use less if the soil is poorly drained.” “If using finely textured or double-shredded mulch, use 1 to 2 inches”. “More than 4 inches may harm the tree's root system.” The 1 to 2 in is on the depth list.
• Iowa State University Extension and Outreach (last updated March 3, 2022): “A 3- to 4-inch-layer of wood chips or shredded bark would be appropriate for well-drained sites around trees and shrubs. On heavy soils, a 2- to 3-inch-layer would be suitable.” The 2 to 3 in for heavy soils is on the depth list.
• Washington State University Extension, FS160E (January 2015), on arborist wood chips: “Wood chips maintained at a depth of 4 to 6 inches will control weeds without detracting from appearances.” On the depth list.
• Clemson HGIC 1604 (revised June 20, 2016): fine-particle wood and bark mulches “should not be more than 2 to 3 inches deep”; coarse mulches such as pine bark nuggets “can be as deep as 4 inches”; grass clippings or shredded leaves “should never be deeper than 2 inches”.
• University of Maryland Extension (updated December 6, 2023): “Always keep mulch shallow, from one to three inches.”
• The Morton Arboretum: “Under trees and shrubs, spread mulch to a recommended depth of 3 to 4 inches for medium-to coarse-textured materials.” “Perennial beds need only a couple of inches of mulch.”
• UF/IFAS Gardening Solutions: “maintain a 2- to 3-inch layer around established trees, shrubs, and bedding plants and a 1-inch layer around seedlings or small vegetables and herbs.” “Florida-Friendly Landscaping™ recommends mulching to a depth of 3 inches.” “Don't apply more than 3 inches”.
• Colorado State University Extension, GardenNotes #245 (2015, reviewed 2023): “In order to suppress weed growth, wood chip, gravel, grass clipping, and other mulch should be four inches deep.” GardenNotes #658 (2024, reviewed 2025), for newly planted trees and shrubs: “mulch the backfill area and beyond, to a 4-to-5-inch depth.”
• Utah State University Extension, Forestry: “Mulch bed depth should be maintained at 4 to 6 inches, though fine mulches may tend to mat and should not be piled as deep.”
• K-State Research and Extension news (Aug. 25, 2026): “The mulch should be 2-4 inches deep and re-applied annually.”

Why six of them, and not one

A single default would pick a side. The ISA ranges are offered because the brochure covers fine and coarse mulch and well-drained sites in one place, and other sources land on the same figures (Penn State’s and K-State’s 2 to 4 in; the Morton Arboretum’s and Iowa State’s 3 to 4 in for coarse or woody mulch). Penn State’s 1 to 2 in and Iowa State’s 2 to 3 in are offered because each is tied to something the ISA ranges do not single out: finely textured or double-shredded mulch, and heavy soils. WSU’s 4 to 6 in is offered because it disagrees with ISA on the same material: ISA gives arborist wood chips 3 to 4 in, WSU 4 to 6 in. Over 4 in, a notice quotes Penn State and WSU side by side.

Wet and heavy ground is where they differ most: ISA says more than 2 in of organic mulch on wet soils can reduce soil drying, Penn State says to use less on poorly drained soil, and Iowa State gives 2 to 3 in on heavy soils.

Bags: what the label says

Enter the volume printed on your bag, in cubic feet, litres or cubic yards, or choose one of three sizes from the makers’ pages recorded below: 1.5 and 2 cu ft (Scotts Nature Scapes Color Enhanced Mulch) and 3 cu ft (Cowart pine bark nuggets and mulch, which Cowart also bags in 2 cu ft). NIST Handbook 133 sizes the test measures for mulch at 1, 2 and 3 cu ft, and says those capacities are “based on the most common package sizes of mulch in the marketplace”.

The count assumes each bag holds the volume on its label. Handbook 133 applies a maximum allowable variation of “5 % of the declared quantity” to individual bags of mulch sold by volume, allows one bag in every 12 sampled to exceed it, and requires the sample as a whole to meet the average requirement.

Bulk mulch needs no bag size: the cubic yards are the order.

Topping up old mulch

Enter the depth of mulch already on the bed under Add detail, and the calculator orders only the difference between it and the depth you want; if the bed already has that depth or more, nothing is added. ISA: “If mulch is already present, check the depth.” Penn State: “Before replenishing the mulch each season, check the depth.” and “Applying new mulch over old mulch in successive years is the same as applying too deep a layer all at once.” UF/IFAS: “When the depth falls below two inches, it's time to replenish the mulch.”

Allowance and price: yours

The allowance for waste or settling starts blank. No percentage to add when ordering was found in the extension and arborist sources reviewed. UF/IFAS says organic mulches settle as they decompose, and that one UF/IFAS study found pine bark kept “two-thirds of its original depth after two years”: a loss over two years, not an amount to buy at the start. With your allowance, the volume to order is the volume × (1 + allowance ÷ 100), and the bags are counted from it.

Prices are yours too: a price per bag gives the cost in bags, and a delivered price per cubic yard (or cubic foot, or cubic metre) the cost in bulk. No market price is assumed.

The 3-3-3 rule

It was found in two places in the sources reviewed, neither of them a fact sheet. A UNH Cooperative Extension blog post (April 22, 2020), for older, well-established trees: “Spread a 3-inch layer of mulch in a 3-foot donut around the tree making sure to leave at least 3-inches between the mulch and tree stem.” And a City of Maple Grove, Minnesota handout, which names it on its second page as “3 inches deep in a 3-foot radius from the trunk”, while its first page says to spread mulch to “a diameter of at least 3 feet”. It was not found in the ISA, Penn State, Clemson, Iowa State, Maryland, Morton Arboretum, Colorado State, Utah State, WSU, UF/IFAS or K-State pages quoted here.

A 3 ft radius is a 6 ft diameter and four times the area of a 3 ft circle, so the reading decides the order. The FAQ works both with the ring mode.

What it does not decide

Which mulch to buy, how deep to spread it, how far from a trunk to stop it or how big to make a ring: those are yours, and the sources quoted here differ on each. It does not assess a playground surface; the U.S. Consumer Product Safety Commission’s handbook is quoted in the FAQ for that. It gives no tons: for stone or rock bought by weight, the gravel calculator converts a volume with your supplier’s density.

Sources quoted on this page

Each was read on 2026-10-01; quotations are exact, and each is given with the situation it applies to.

• International Society of Arboriculture, “Proper Mulching Techniques”, ©2021, v02.2021 (treesaregood.org, PDF).
• Penn State Extension, “Mulching Landscape Trees”, by David R. Jackson, updated April 12, 2018 (extension.psu.edu).
• Iowa State University Extension and Outreach, “How deep should the mulch layer be around trees and shrubs?”, last updated March 3, 2022 (yardandgarden.extension.iastate.edu).
• Washington State University Extension, FS160E, “Using Arborist Wood Chips as Landscape Mulch”, by Linda Chalker-Scott, January 2015 (pubs.extension.wsu.edu, PDF).
• Clemson Cooperative Extension, HGIC 1604, “Mulch”, revised June 20, 2016 (hgic.clemson.edu).
• University of Maryland Extension, “Mulching Trees and Shrubs”, updated December 6, 2023 (extension.umd.edu).
• The Morton Arboretum, “How to Mulch Trees and Shrubs”, undated (mortonarb.org).
• UF/IFAS Gardening Solutions, “Choosing and Installing Mulches”, undated (gardeningsolutions.ifas.ufl.edu).
• Colorado State University Extension, GardenNotes #245, “Mulching”, December 2015, reviewed April 2023, and GardenNotes #658, “Mulching Trees and Shrubs”, July 2024, reviewed August 2025 (extension.colostate.edu).
• Utah State University Extension, “Mulching is Important for Tree Health”, by Michael Kuhns, undated (extension.usu.edu).
• K-State Research and Extension news, “Mulch rings help to promote healthy trees”, Aug. 25, 2026 (extension.k-state.edu).
• UNH Cooperative Extension blog, “Volcanoes Kill Trees!”, by Gregory Jordan, April 22, 2020 (extension.unh.edu).
• City of Maple Grove, Minnesota, “How to Properly Apply Mulch”, handout, undated (maplegrovemn.gov, PDF).
• NIST Handbook 130 (2026), IV.B, section 2.18, Mulch, and NIST Handbook 133 (2026), Chapter 3, section 3.10 and Table 3-4 (nist.gov).
• U.S. Consumer Product Safety Commission, Public Playground Safety Handbook, Publication 325, July 2025, Table 2 (cpsc.gov, PDF).
• The Scotts Company, “Scotts® Nature Scapes® Color Enhanced Mulch” product page, and Cowart, Inc., “Pine Products” page, for bag sizes (scottsmiraclegro.com, cowartmulch.com).