Mulch Calculator

How to use the mulch calculator

Choose what you are spreading — mulch, soil or compost — then add each bed as a rectangle, circle or triangle. Lengths take feet and inches in separate boxes. If you have several identical beds, such as rings around three trees, enter one and set the quantity. Irregular beds can be split into a few simple shapes. Then set the depth in inches (or centimeters in metric mode).

The result shows the volume in cubic yards and cubic feet, the number of 2 and 3 cu ft bags, and — if you add a price per bag or per cubic yard — the cost. The working lists every step with your numbers. The page address keeps your beds, depth and price so you can bookmark the page or share it with whoever is doing the digging. Everything runs in your browser.

The formula

volume (cu ft) = area (sq ft) × depth (in) ÷ 12 cubic yards = cubic feet ÷ 27 bags = cubic feet ÷ bag size (round up)

Areas: a rectangle is length × width, a circle is π × (diameter ÷ 2)², and a triangle is ½ × base × height, where the height is measured at a right angle to the base. The depth must be converted to feet before multiplying — 3 inches is 0.25 ft — which is why mulch piles up faster than people expect: every inch of depth over 1,000 sq ft is 83.3 cu ft, or just over 3 cubic yards.

Worked example

The calculator opens with two beds mulched 3 inches deep:

• Bed 1: a border 20 ft × 5 ft = 100 sq ft.
• Bed 2: two tree rings 6 ft across: π × 3² = 28.2743 sq ft each, × 2 = 56.5487 sq ft.
• Volume: (100 + 56.5487) × 0.25 ft = 39.14 cu ft, and 39.14 ÷ 27 = 1.45 cubic yards.
• Bags: 39.14 ÷ 2 = 19.57, so 20 bags of 2 cu ft; or 39.14 ÷ 3 = 13.05, so 14 bags of 3 cu ft.
• Cost: at $4.00 per 2 cu ft bag (an illustration, not a typical price), 20 × $4.00 = $80.00.

How deep should you spread it?

For mulch, most extension services land on 2 to 3 inches for shrub and perennial beds. UMass Extension recommends a uniform 2–3 inch layer on perennial beds, and Illinois Extension notes 2–3 inches is common in shrub and evergreen beds but about an inch, once settled, in flower beds. Mississippi State Extension suggests 1–2 inches for annual bedding plants and 2–3 inches for shrubs and roses. More is not better: University of Maryland Extension advises not letting mulch build up beyond 3 inches and keeping it about 3 inches away from tree trunks, and Penn State Extension puts the limit at about 4 inches on well-drained soils and less on poorly drained ones. Piling mulch against trunks — the “mulch volcano” — holds moisture against the bark.

For compost, Oregon State University Extension suggests a 3–4 inch layer worked into the top 8–12 inches of soil for new beds, ¼ to 1 inch a year on established beds, and ¼ to ½ inch as a topdressing on lawns. For soil, use the depth you need to fill — the inside depth of a raised bed, for example — and remember loose soil settles once it is watered.

Tips before you buy

• Top up, don’t start over. If last year’s mulch is still there, measure how much is left and only add the difference.
• Compare bags and bulk per cubic yard. A 2 cu ft bag price × 13.5 or a 3 cu ft bag price × 9 is the equivalent price per yard. Bulk delivery usually adds a fee, so compare the total.
• Round bags up. The calculator always rounds up to whole bags; a spare bag is handy for topping up thin spots.
• Inches are not decimals of a foot. A bed 10 ft 6 in long is 10.5 ft, not 10.6 ft. Use the separate inch boxes.
• Leave space around stems. Pull mulch back from trunks, stems and the base of plants rather than burying them.

For hard landscaping, the gravel calculator gives tons of stone or sand, and the concrete calculator covers slabs and footings. To measure an irregular garden or lawn first, use the square footage calculator. For conversions, see cubic feet to cubic meters and square meters to square feet.