What this calculator does
Bulk material volume is area times depth, and the only trap is the unit mismatch. A 6 by 1.5 metre bed at 75 mm deep is 9 square metres and 0.675 cubic metres, which is 675 litres or fourteen 50 litre bags.
The bag count is usually the surprise. Bagged material looks like a lot in a shop and disappears alarmingly fast on a bed: fourteen bags to cover nine square metres at a normal depth. Above about half a cubic metre, bulk delivery is almost always cheaper than bags.
The formula
The bed area is multiplied by the depth converted from millimetres to metres, giving cubic metres. One cubic metre is 1,000 litres, and the bag count divides by the bag size and rounds up. The same calculation covers mulch, soil, compost, sand and gravel, since all are sold by volume.
| Term | Meaning |
|---|---|
| Cubic metre | The bulk unit, equal to 1,000 litres. |
| Mulch depth | 50 to 75 mm for most beds. Deeper suppresses weeds better but can starve roots of air. |
| Bulk delivery | Loose by the cubic metre, far cheaper than bags above about half a metre. |
| Settling | Loose organic mulch compacts over months, so the finished depth is less than the laid depth. |
The inputs explained
| Field | What to enter |
|---|---|
| Bed length (m) | Bed length in metres. |
| Bed width (m) | Bed width in metres. |
| Depth (mm) | Depth in millimetres. 50 to 75 mm suits most mulches; gravel paths are often deeper. |
| Price per m³ ($) | Price per cubic metre, for the cost estimate. |
When to use it
Ordering mulch for a bed
Suppliers sell by the cubic metre, and the conversion from area and depth is not intuitive.
Deciding bags against bulk
The bag count makes the crossover obvious, and it arrives sooner than most people expect.
Topping up soil in raised beds
The same calculation applies to soil and compost.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does the depth change the order?
The same bed at a range of depths.
| Depth | Volume needed | In litres | 50 L bags |
|---|---|---|---|
| 25 mm | 0.225 m³ | 225 | 5 |
| 50 mm | 0.450 m³ | 450 | 9 |
| 75 mm | 0.675 m³ | 675 | 14 |
| 100 mm | 0.900 m³ | 900 | 18 |
Questions
How deep should mulch be?
Usually 50 to 75 mm. Thinner does not suppress weeds effectively; much thicker can stop water and air reaching the roots and can cause collar rot if piled against stems. Keep mulch clear of trunks regardless of depth.
How many bags in a cubic metre?
Twenty 50 litre bags, since a cubic metre is 1,000 litres. Bagged material is typically two to three times the price of bulk per unit volume, so the crossover comes quickly, usually around half a cubic metre.
Does mulch settle?
Yes, noticeably. Loose organic mulches compact and decompose, often losing a third of their depth over a season. Laying slightly deeper than the target finished depth allows for it, and beds generally need topping up annually.
Does this work for gravel and soil?
Yes, the volume calculation is identical for anything sold by the cubic metre. What differs is the weight: gravel is around 1.5 tonnes per cubic metre against roughly 0.3 for bark mulch, which matters for delivery and barrowing rather than for the volume.
For concrete volumes, see the concrete calculator. For gravel specifically, see the gravel calculator.