The Simple Formula
Mulch is sold by volume, so the question is really "what volume fills my beds at the right depth?" The answer takes one line of arithmetic:
Measure each bed's length and width in feet, multiply for square footage, add all beds together, then apply the formula. The 324 constant comes from a cubic yard covering 324 sq ft at a 1-inch depth. For bags, one cubic yard equals 13.5 standard 2-cu-ft bags — always round up.
What Depth Should You Use?
Depth is where most people go wrong — too thin and weeds punch through within weeks, too thick and you suffocate roots and invite fungus. The garden-center consensus: 2–3 inches for most landscape beds, a full 3 inches where weed suppression is the goal, and just 1–2 inches around shallow-rooted perennials. Around trees, keep mulch 3–6 inches away from the trunk — the "mulch volcano" look kills trees slowly. Never exceed 4 inches anywhere.
Worked Examples by Bed Size
| Bed Size | Sq Ft | At 2" Depth | At 3" Depth |
|---|---|---|---|
| 4 × 8 ft (small bed) | 32 | 0.2 yd³ / 3 bags | 0.3 yd³ / 4 bags |
| 10 × 10 ft | 100 | 0.62 yd³ / 9 bags | 0.93 yd³ / 13 bags |
| 20 × 10 ft | 200 | 1.23 yd³ / 17 bags | 1.85 yd³ / 25 bags |
| Foundation strip 40 × 4 ft | 160 | 0.99 yd³ / 14 bags | 1.48 yd³ / 20 bags |
| Whole front yard, 1,000 sq ft of beds | 1,000 | 6.2 yd³ | 9.3 yd³ |
Bags shown are the standard 2 cu ft size, rounded up. Odd-shaped beds? Break them into rectangles and triangles (half of length × width for triangles), or measure the longest and widest points and use 80% of that rectangle for a curved organic bed — close enough for ordering. The full coverage math at every depth lives on our mulch coverage chart.
Bags or Bulk?
Under one cubic yard, buy bags — no delivery fee, easy handling, storable leftovers. Over one yard, bulk delivery wins on price nearly every time: bulk runs $25–$60 per yard while the same volume in bags costs $60–$95. Run your exact numbers through the bags vs bulk calculator to see the crossover for your local prices. Topping up existing mulch rather than starting fresh? Measure the current depth first and only order the difference — refreshing 1 inch onto a faded 2-inch bed needs just a third of the original order.
Don't Forget Prep
Quantity is only half the job. Before spreading: pull or spray weeds, edge the beds, and water dry soil — mulch locks in whatever moisture state it covers. If you're rebuilding beds from scratch, order soil amendments in the same delivery: the topsoil calculator and compost calculator use the same measurements you've already taken. And for hardscaped areas like paths and driveways, gravel volumes work identically — the companion gravel calculator at GravelCalc.com handles stone by weight as well as volume.