How Much Soil, Mulch Or Gravel Do I Need? A Simple Bulk Materials Guide

Every cubic metre calculation starts in the same place, regardless of whether you're ordering soil, mulch or gravel. Multiply the length of your area (in metres) by the width (in metres), then multiply that result by your target depth (also in metres). If you're measuring depth in centimetres, convert it first: 10 cm becomes 0.10 m, 20 cm becomes 0.20 m, and so on. The final figure is your volume in cubic metres, which is exactly what suppliers need when you place a bulk order. Keep a notepad handy because you'll use this same formula across every material.

Here's what each measurement refers to in practice:

• Length: the longest dimension of the area you're filling, measured in metres
• Width: the dimension running perpendicular to length, measured in metres
• Depth: the vertical thickness of material you want once it's spread, converted to metres
  

Underestimating depth is the single most common reason a bulk order comes up short. A 5 cm mulch layer looks reasonable on paper but spreads surprisingly thin over a large garden bed. For mulch to actually suppress weeds and retain moisture, 7–10 cm is the practical minimum. Garden soil needs at least 20–30 cm for planting beds where roots need to establish. People tend to round down on depth to save money upfront, then end up paying more for a second delivery anyway.

These depth targets give you a solid starting point before you run the formula:

• Garden beds (new): 20–30 cm of soil for planting or raised beds
• Lawn top-dressing: 10 cm is usually sufficient for existing turf
• Mulch coverage: 7–10 cm for moisture retention and weed suppression
• Gravel paths: 10–15 cm over a prepared and levelled base
• Driveways: 15 cm minimum, more if vehicles are using the area regularly
  

Soil calculation follows the standard formula with one extra step factored in at the end. For a typical garden bed, say 4 metres long by 3 metres wide with a target depth of 25 cm (0.25 m), the calculation is: 4 x 3 x 0.25 = 3 cubic metres. That's your base volume. From there, add 10–15% to account for compaction and settling after the first few waterings. Garden soil, especially blends with compost, can reduce noticeably in volume within the first month. Rounding up to the nearest half cubic metre when placing your order is industry standard practice and almost always the right call.

Run through these steps to land on your soil order quantity:

• Measure length and width of the area in metres
• Set a realistic depth target (not an optimistic one) and convert to metres
• Multiply all three: length x width x depth = cubic metres
• Add 10–15% for compaction, then round up to the nearest 0.5 m³
  

Mulch is lightweight and easy to spread, which makes people underestimate how much they actually need to cover a large area properly. If you're working out how much mulch do I need, start with the same formula, but factor in that organic mulch breaks down over time. A 10 cm layer today will reduce to closer to 6–7 cm within six months in a humid coastal climate, as moisture and microbial activity accelerate decomposition. Laying a slightly deeper initial layer means you won't need to top up as frequently, and the moisture retention benefit will last longer through dry periods.

These variables affect how much mulch you should add above your base cubic metre calculation:

• Organic mulch (wood chip, bark): add 15% buffer for breakdown over the first season
• Fine mulches compact faster than coarser grades and may need topping up sooner
• Areas against fences or footings: lay thinner to maintain airflow and avoid moisture build-up
• Sloped areas: a slightly deeper layer compensates for runoff redistributing the material
  

Gravel doesn't break down like organic material, but it does shift, compact under load and migrate at edges over time. For a decorative garden path with light foot traffic, 10 cm of gravel over a stable base is generally enough. Driveways used by standard passenger vehicles need at least 15 cm, and anything with regular heavy vehicle access should be closer to 20 cm. The formula is the same as always (length x width x depth), but gravel is typically denser than soil or mulch, which affects weight per cubic metre if you're factoring in load limits for delivery.

These depth guides cover the most common residential gravel applications:

• Decorative borders and garden edges: 5–8 cm
• Walking paths and entertaining areas: 10–12 cm
• Standard residential driveways: 15 cm minimum
• Driveways with regular heavy vehicle use: 20 cm or more

Most people know their square metre area but aren't sure how to turn that into a cubic metre figure to give a supplier. Square metres tells you how much flat surface you're covering. Cubic metres tells you how much material you need to fill that surface to a specific depth. To go from one to the other, just multiply your square metre area by your depth in metres. A 20 m² courtyard at 10 cm (0.10 m) deep needs 2 cubic metres of gravel. That's it. The confusion usually comes from forgetting to convert centimetres to metres before multiplying.

Keep this reference handy when working between the two measurements:

• Square metres (m²): flat area only, no depth component
• Cubic metres (m³): flat area multiplied by depth in metres
• 1 m³ = approximately 1,000 litres of material
• 1 m³ of soil weighs roughly 1–1.4 tonnes depending on moisture content and blend
• Online soil calculator tools use the same formula, but always check what depth they assume as a default

Raw cubic metre calculations are a starting point, not the final number to order. Every bulk landscaping material settles or compacts once it's placed and subjected to traffic, rain or watering. Garden soil can reduce by 10–20% in volume within the first few weeks. Compost-heavy mixes compact even more. Gravel shifts under load and migrates toward edges without proper containment. Organic mulch decomposes steadily. The standard approach is to add 10–15% to your calculated volume and round up to the nearest half cubic metre. It costs marginally more upfront but eliminates the disruption of a second partial delivery.

These compaction rates give you a realistic buffer to build into each material type:

• Garden soil and blended mixes: 10–20% volume reduction after settling
• Gravel and aggregate: 5–10% compaction under traffic
• Organic mulch: 10–15% reduction over the first three to six months
• Sandy soils: compact less than heavy clay-based blends, so a smaller buffer may apply

There's a crossover point where buying individual bags stops making financial or practical sense, and for most landscaping jobs it arrives quickly. For larger jobs, ordering bulk soil, mulch and gravel by the cubic metre works out more cost-effective than bagged products. One cubic metre of garden soil replaces roughly 30–40 standard 30-litre bags, and a delivered bulk load means no trailer hire, no hardware store trips and no stacking bags in the back of an SUV. Anyone sourcing landscape supplies in Port Macquarie for a project larger than a couple of garden beds will almost always find bulk delivery the better option on both cost and convenience.

Bulk delivery makes practical sense in all of these situations:

• Any job requiring more than half a cubic metre of a single material
• Projects covering multiple areas (beds, lawn, paths) in the same session
• Jobs where you need a consistent grade or blend across a large area
• Situations where multiple trips to pick up bags would add up in time and fuel

We at Ian Brown Sand Supplies and Tipper Hire help homeowners, tradies and landscapers get their quantities right before they place an order, not after. Whether you're filling new garden beds, spreading mulch across a large yard or gravelling a driveway, we'll help you work through the maths and recommend the right material for the job. Once you've worked out your volume, our team can help you order the right load of landscape supplies in Port Macquarie

and arrange delivery straight to your site. We supply soil, mulch, gravel, sand and more, with delivery available across the Mid North Coast for both residential and commercial projects. Not sure where to start? Get in touch with our team and we'll help you estimate your quantities, including compaction buffers and the right depth for your specific application. Getting it right the first time saves a second trip and keeps your project moving.