Mulch Calculator – How Much Mulch to Buy, in Bags or by the Yard
Standing in the garden centre wondering whether to load thirty bags or order a yard for delivery? This mulch calculator turns your beds and the depth you want into a shopping list: the volume in cubic feet, cubic yards, litres or cubic metres, the number of bags to buy, the bulk order, pine-straw bales, and how many pickup loads it takes to haul it yourself. Enter prices and it also shows whether bags or bulk is cheaper for your job.
The cubic-yard arithmetic
Mulch is sold by volume, so everything starts with area × depth. A cubic yard is 27 ft³, which spread 1 in deep covers 324 ft². Divide 324 by your depth in inches and you have a yard's coverage: 108 ft² at 3 in, 81 ft² at 4 in. The common big-box bag holds 2 ft³, so a yard is 13.5 bags. A 20 × 10 ft bed at 3 in needs 200 ft² × 0.25 ft = 50 ft³, or 1.85 yd³, which is 25 two-cubic-foot bags. The calculator adds your waste allowance, sums every bed that uses the same mulch and rounds up once, on the total, so you never buy a spare bag per bed.
How deep should mulch be?
Extension services generally recommend 2–4 in of organic mulch. Fine textures such as shredded hardwood, dyed mulch or pine-bark fines work at 2–3 in; coarse nuggets and arborist wood chips can go 3–4 in because air moves through them more easily. Compost is spread thinner, 1–2 in. Deeper is not better: more than about 4 in can starve roots of oxygen, keep the soil soggy and encourage shallow roots. The depth gauge on each bed shades the recommended range for the mulch you pick and warns when you go thinner or deeper.
Tree rings and mulch volcanoes
Use the Ring shape for a tree: the outer diameter is the edge of the mulched circle and the inner diameter is the trunk plus the gap you'll leave. Mulch piled against the bark, the "mulch volcano", holds moisture on the trunk and invites rot, insects and girdling roots. Spread a wide, flat ring instead and keep it a few inches off the bark.
Yearly top-ups
Organic mulch breaks down, and most beds lose around an inch a year. Enter your target depth and the depth still on the bed as existing mulch, and only the difference is ordered. If what's there is already deeper than about 4 in, rake some out rather than adding more.
Bags vs bulk, delivery and minimums
Bulk mulch is usually far cheaper per yard, but a delivery fee and a minimum order (often 2–3 yd³) can make bags the better buy for a small bed. Because both costs rise in steps, one more bag or one more half-yard at a time, the calculator evaluates every step exactly and reports the volume where bulk first becomes cheaper and where it stays cheaper for good, then marks your job on a cost chart.
Measuring curved and irregular beds
Split an odd-shaped bed into simple pieces: rectangles for straight runs, ovals for kidney and free-form beds (measure the longest and widest points), quarter circles for curved corner beds and triangles for straight-edged corners. If you already know the area from a plan, enter it directly. Use the "covered" percentage for shrubs, boulders or stepping stones that won't be mulched.
Weed barrier and edging
Landscape fabric under mulch helps on paths but can stop organic mulch from feeding the soil in planted beds; a thicker layer of mulch over cardboard is a common alternative. Edging keeps mulch in the bed and off the lawn. For stone, gravel or lava rock instead of mulch, use the Gravel Calculator, which works by weight as well as volume.
Playground surfacing
Loose-fill surfacing under play equipment is a safety material, not decoration. The US CPSC Public Playground Safety Handbook notes that loose fill compresses by at least 25% with use, so a 12 in fill of wood fibre settles to about 9 in, and it lists the depth each material needs for a given fall height. The playground templates estimate quantities only; check the handbook and your equipment's fall height before you install.