How this works
Mulch, gravel and topsoil are sold by volume — cubic yards in the US, cubic metres in most of the rest of the world, plus by-the-bag for small jobs. The math is simple: volume = area × depth. The trap is unit consistency — depth is usually quoted in inches (or centimetres) but the result wants to be cubic yards (or cubic metres), so you need to convert depth to feet (or metres) before multiplying. The calculator above handles the unit conversion automatically; you just enter dimensions and depth, and it returns the volume in both unit systems plus a bag count.
Depth is the value most people get wrong. Standard recommendations: 2-3 inches (5-8 cm) of mulch for established garden beds — enough to suppress weeds and conserve moisture without smothering shallow roots; 3-4 inches (8-10 cm) for new beds where you also want soil-amendment effect as the mulch breaks down; 1-2 inches (2.5-5 cm) for fine decorative gravel; 4-6 inches (10-15 cm) for drainage gravel under a paver patio or French drain. Going much thicker than the recommended depth doesn't add benefit, just wastes material — and over-mulching against tree trunks ("mulch volcanoes") actively damages bark and roots.
Three practical points. (1) Bagged mulch in the US comes in 2 cubic feet bags; one cubic yard = 27 cubic feet = 13.5 bags. Buying in bulk (yard or half-yard delivered) is dramatically cheaper than bags above ~5 cu yd, but you need somewhere to dump the pile and a way to move it. Below ~3 cu yd, bags often win once you factor in delivery fees. (2) Different materials have different weights per volume — pea gravel is about 1.4 tons per cubic yard, mulch is 0.4-0.5 tons per cubic yard, topsoil is 1.0-1.3 tons per cubic yard. If you're buying by weight rather than by volume, ask the supplier their conversion factor for the specific material. (3) Volume needed scales linearly with depth, so doubling the depth doubles the cost. For mulch on existing beds, you usually only need to top up by 1 inch (2.5 cm) per year as the previous layer breaks down — don't recalculate the full depth every season.
The formula
Area is the footprint of the bed (length × width for a rectangle, or paste in directly if you have an irregular shape already calculated). Depth is in inches in imperial mode, centimetres in metric mode. The calculator returns volume in both systems plus the bag count for whichever bag size you select. For non-rectangular beds, break into rectangles, calculate each separately, and sum the volumes.
Example calculation
- Mulching a 4 ft × 8 ft bed at 3 inches deep.
- Volume = 4 × 8 × (3/12) = 8 cu ft = 0.296 cu yd ≈ 0.226 m³.
- Bags: 8 / 2 = 4 US 2-cu-ft bags, or ceil(226 / 50) = 5 × 50 L EU bags.
Frequently asked questions
How deep should I lay it?
For mulch on garden beds: 2-3 inches (5-8 cm) on established beds with mature plants — enough to suppress weeds and hold moisture without smothering shallow roots. 3-4 inches (8-10 cm) on new beds where you want longer-lasting weed control and the slow soil-amendment benefit as it breaks down. For decorative gravel: 1-2 inches (2.5-5 cm) for fine gravel paths, 2-3 inches (5-8 cm) for coarser stones; deeper than 3 inches looks heavy and walks badly. For drainage gravel under a paver patio or dry well: 4-6 inches (10-15 cm) compacted with a plate compactor. Don't pile mulch against tree trunks or perennial crowns — keep a 3-4 inch (8-10 cm) bare ring around the base of woody stems to prevent rot and rodent damage.
Bags or bulk delivery?
Crossover is around 3-4 cubic yards (2.5-3 m³). Below that, bagged product is usually cheaper once you factor in delivery fees on a small bulk order — and you can move bags one at a time, which matters if you don't have a wheelbarrow-friendly path to the bed. Above ~5 cubic yards, bulk delivery is dramatically cheaper per cubic yard (often half the per-yard cost) and means one delivery visit instead of dozens of trips to the garden centre. Practical considerations: bulk needs a flat dumping spot the truck can access, you need a way to move it (wheelbarrow + plywood ramp into the bed), and you generally want to spread it within a few weeks before it breaks down or grows mushrooms. For one-time hardscape jobs (gravel under a patio), bulk wins almost regardless of size because the volume is usually large.
My bed is irregular — how do I estimate area?
Two practical methods: (1) Break the shape into rectangles and triangles, calculate each, and sum. A "kidney-shaped" bed is usually two big rectangles plus a triangle. Use the largest enclosing rectangle for awkward curved edges and round up from there. (2) Pace it out: walk the perimeter heel-to-toe and count steps to estimate length and width, then multiply. Either method gets you within ±15% — close enough given that mulch over-orders rarely matter (excess goes on next year's top-up) and bag-counts round up anyway. For very large irregular beds, draw it on graph paper to scale (1 square = 1 sq ft) and count squares. Don't overthink it; depth matters far more than the area precision.