How to calculate mulch
- Measure each bed — length × width for rectangles, diameter for circles, base × height ÷ 2 for triangles. Odd beds: measure them as several sections and add each in the calculator.
- Choose a depth — 2 in tops up an old layer, 3–4 in is typical for bare soil, 6 in only for specific suppressive uses. Enter any custom depth.
- Set the waste allowance — 10% is the default for spillage and settling; set 0 if you don't want it.
- Check the bag size on the shelf — 1.5, 2, 2.5 and 3 cubic feet bags are all common, and the count changes with the box.
- Compare bagged vs bulk — enter your prices per bag and per yard (and delivery) in the prices card and both estimates appear side by side.
The formula
Area (sq ft) = length x width (circles: 3.1416 x (diameter/2)^2; triangles: base x height / 2)
Volume (cu ft) = Area x depth (in) / 12
Volume (cu yd) = Volume / 27
With 10% waste = Volume x 1.10
Bags = round-up(Adjusted cu ft / bag size)Unit conversions are exact: 12 inches per foot, 27 cubic feet per cubic yard. See the calculation methodology for the full derivation and assumptions.
Worked example
A 10 ft × 10 ft bed at 3 inches: 100 sq ft × 3 ÷ 12 = 25 cu ft = 0.9259 cu yd. With a 10% allowance that's 27.5 cu ft — 14 two-cubic-foot bags (at $4.50 = $63) or, at $45/yard plus $25 delivery, about $70.85 bulk. Type those prices in and the calculator compares them for your plot.
Measuring tips
- Measure along the middle of long borders, not the curved edge.
- For a tree ring, measure the whole circle's diameter — the mulch circle, not the trunk.
- Sketch each bed with its dimensions; label sections in the calculator so subtotals make sense.
Common mulch ordering mistakes
- Reading the delivery ticket in cubic yards while the store bags are cubic feet — the two aren't interchangeable without the ÷27 step.
- Buying exactly the calculated number with no allowance, then coming up short after settling.
- Assuming "a standard bag" — bag sizes vary by brand, and a whole bag number has to be bought.
- Skipping the depth measurement for a top-up year: many beds only need 1–2 inches.
Frequently asked questions
How many bags of mulch do I need for a cubic yard?
It depends on bag size: there are 27 cubic feet in a cubic yard, so 14 two-cubic-foot bags, 18 one-and-a-half bags, or 9 three-cubic-foot bags. The calculator uses the exact bag size you tell it.
How much mulch covers 100 square feet?
At 2 inches deep, 100 sq ft needs about 16.7 cubic feet (0.6 cubic yards). At 3 inches, about 25 cubic feet (0.9 cubic yards) — the same bed area changes the order by half.
How deep should mulch be?
Commonly 2–4 inches for ornamental beds: 2 inches refreshes an existing layer, 3–4 inches on bare soil suppresses weeds. Coarser or rubber mulches run shallower. The depth presets are options, not dictates.
Should I add 10% extra mulch?
A 10% allowance covers spillage and settling and it is the calculator default — but you can set any percent, including zero.
Is bulk mulch cheaper than bags?
Usually by the third cubic yard, but not always — the comparison panel shows both costs only after you enter the prices you were quoted.