How This Mulch Calculator Works
Mulch is sold by volume, so the math comes down to three numbers: the length and width of your bed, and how deep you want the mulch layer. The calculator multiplies length × width to get the area in square feet, converts your chosen depth from inches to feet, and multiplies the two to get cubic feet. Dividing by 27 gives cubic yards — the unit bulk suppliers use — and dividing cubic feet by 2 or 3 gives the number of standard bags. The formula is:
For irregular or curved beds, break the space into simple rectangles, run each through the calculator, and add the results. A circular bed can be estimated as diameter × diameter × 0.785 for the area. When in doubt, round up — a little extra mulch always finds a home, but a second delivery fee doesn't.
Worked Example: A 20 × 10 ft Bed at 3 Inches
Say you're mulching a 20-foot by 10-foot planting bed to the standard 3-inch depth. The area is 200 square feet. At 3 inches (0.25 ft) deep, that's 200 × 0.25 = 50 cubic feet, which is 50 ÷ 27 = 1.85 cubic yards. In bags, you'd need 50 ÷ 2 = 25 two-cu-ft bags (or 17 of the 3 cu ft size). At a typical bulk price of $35 per yard, the mulch itself runs about $65 — versus roughly $100–$125 for the same volume in bags, which is why bulk delivery usually wins for anything over ten bags.
How Deep Should Your Mulch Be?
| Situation | Recommended Depth | Why |
|---|---|---|
| Flower & shrub beds | 2–3 inches | Retains moisture and blocks light to weed seeds without smothering roots |
| Weed suppression | 3 inches | The minimum layer that reliably blocks annual weeds |
| Around perennials | 1–2 inches | Heavier layers hold too much moisture against crowns |
| Around trees | 2–4 inches, pulled back from trunk | Never pile mulch against bark — "mulch volcanoes" cause rot and pests |
| Playground (rubber/wood chips) | 6+ inches | Fall-protection standards require a much thicker layer |
Depth is the single biggest driver of how much mulch you need — going from 2 to 3 inches raises your order by 50%. If you're topping up an existing bed rather than starting bare, measure what's already there and only order the difference. See the full mulch coverage chart for exact coverage per yard and per bag at every depth.
Bulk or Bagged? A Quick Rule of Thumb
Bulk mulch delivered by the yard is almost always cheaper per cubic foot, but bags are easier to move, store, and spread for small jobs. The break-even point sits around 1 cubic yard (13–14 bags): below that, bags cost about the same once you factor in delivery fees, and above it bulk pulls ahead fast. Our bagged mulch calculator compares the two side by side for your exact project. Spreading topsoil or compost first? The same volume math applies — use the topsoil calculator or compost calculator.