Mulch, Gravel & Topsoil Calculator
How much material you need to cover beds, borders, paths, or a driveway at the right depth — in bags or bulk.
How this calculator works
Loose materials are sold by volume (litres, cubic metres, cubic feet or cubic yards). The estimate is:
Volume = area length × area width × depth — with the depth converted from centimetres or inches first.
We then convert to common purchase sizes: small bags (typically 50–100 L for mulch and topsoil, 20–25 kg for gravel) and bulk bags (roughly 0.7–0.8 m³ for mulch, ~0.5 m³ for a tonne bag of gravel). Densities and bag sizes vary between suppliers, so treat the bag counts as a close guide and check the listed volume of the product you buy.
What depth do I need?
| Use | Recommended depth |
|---|---|
| Mulch for weed suppression on beds | 5–7.5 cm (2–3″) |
| Decorative mulch top-up (existing layer) | 2.5 cm (1″) |
| Gravel path (on membrane) | 4–5 cm (1.5–2″) |
| Gravel driveway (on sub-base) | 5 cm (2″) top layer |
| Topsoil for new turf | 10–15 cm (4–6″) |
| Topsoil for beds/borders | 20–30 cm (8–12″) |
Going too shallow is the classic mistake with mulch: below about 5 cm it won't suppress weeds, so you end up doing the job twice. Going too deep against plant stems or tree trunks causes rot — keep mulch a few centimetres clear of stems.
Bags or bulk?
The crossover point is around 0.5 m³ (roughly 10 small bags). Below that, bags are convenient and easy to move. Above it, a bulk bag or loose load is much cheaper per cubic metre — often half the price — and most merchants deliver. The trade-off is that a bulk bag lands on your drive and you barrow it from there.
Frequently asked questions
How many bags of mulch do I need per square metre?
At the recommended 5 cm depth, each square metre needs 50 litres — so one standard 50 L bag covers 1 m², and a 100 L bag covers 2 m².
How much area does a bulk bag of gravel cover?
A typical "tonne" bulk bag of gravel is about 0.5 m³, which covers roughly 10 m² at 5 cm deep.
Should I put membrane down under gravel or mulch?
Under gravel paths and drives, yes — it stops the gravel sinking into the soil and slows weeds. Under organic mulch on planted beds, generally no: mulch works best rotting down into the soil, and membrane blocks that.
How often does mulch need topping up?
Organic mulches (bark, wood chip) break down and typically need a 2–3 cm top-up every year or two. Gravel and stone are effectively permanent, just raked level occasionally.
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