How Much Extra Should You Order?
Settling allowance genuinely depends on the material, and treating every material the same leads to real ordering mistakes. Organic mulch doesn't meaningfully compact when first installed — a 5% allowance for spillage and handling is enough. The commonly cited "10–20%" figure for mulch actually refers to decomposition over the first year as it breaks down into the soil, which is a topping-up consideration for next year, not this order. Gravel and crushed aggregate genuinely do compact under foot traffic and rain, warranting a larger allowance, while rounded decorative pebbles barely compact at all since they don't interlock the way angular gravel does.
- Decorative Pebbles (25mm): 5% allowance — spillage only, rounded stones don't compact.
- Gravel Paths (50mm): 10% allowance — genuine compaction under foot traffic.
- Organic Mulch (75mm): 5% allowance — doesn't compact on installation; budget to top up in 1–2 years as it decomposes.
- Heavy Bark / Drainage Aggregate (100mm): 10% allowance — denser material settles more under its own weight.
Bags or Bulk Delivery?
One cubic metre contains exactly 1,000 litres. Retail bags are convenient for small top-ups, but bulk delivery is usually far cheaper per cubic metre once you factor in how many bags a larger job actually needs — the break-even point depends on your local bag pricing versus the bulk rate plus delivery fee, which is exactly what the comparison above calculates for your real numbers.
Frequently Asked Questions
Loose organic mulch is lightweight and doesn't pack down significantly on its own after spreading. The larger percentages sometimes quoted for mulch actually describe decomposition over months, not immediate settling — a different thing entirely from what you need to order today.
Organic mulch typically breaks down and needs topping up every 1–2 years, faster in high-rainfall areas or if it's a fine, fast-composting variety. Decorative gravel and pebbles don't decompose and generally only need topping up after significant disturbance or spillage.
Yes — each bed you add can use a different material and depth, and this calculator totals the combined volume across all of them for the bags-vs-bulk comparison.
For gravel paths and decorative stone, geo-textile weed matting underneath is standard practice to stop weed growth and prevent the stone sinking into the soil over time. For organic mulch beds, matting is optional and some gardeners prefer to skip it so the mulch can break down directly into the soil.
Landscape supply yards sell and load bulk material in half cubic metre steps, not exact decimals — nobody is measuring out precisely 1.26m³ on a loader. The precise figure above shows exactly what your project needs; the rounded figure shows what you'd actually order and be quoted for.