Gravel Calculator Australia: How Much You Need

This gravel calculator works out the volume of gravel you need for a driveway, path, or garden bed in cubic metres and cubic feet. You enter the length, width, and depth of the area, and it returns the amount of gravel to order, along with the volume in cubic feet for smaller jobs. It uses the standard formula for a rectangular prism, so you can plan your purchase with confidence before you contact a supplier.

Formula

The calculator uses the basic volume formula for a rectangular space:

Volume (yd³) = Length (ft) × Width (ft) × Depth (in) ÷ 324

Here, length and width are measured in feet, and depth is measured in inches. The number 324 converts the result from cubic feet to cubic yards (since 1 cubic yard = 27 cubic feet, and depth is in inches, so you divide by 12 first: (L × W × D/12) / 27 = L × W × D / 324). For the cubic feet output, the formula is simply:

Volume (ft³) = Length (ft) × Width (ft) × Depth (in) ÷ 12

This gives you the volume in cubic feet directly. If you prefer metric, you can measure in metres and use: Volume (m³) = Length (m) × Width (m) × Depth (m). The calculator defaults to imperial units because gravel is often sold by the cubic yard in Australia, but the maths is the same in any unit system.

Worked Example

Using the default values from the calculator: length = 40 ft, width = 25 ft, depth = 3 in.

1. Convert depth to feet: 3 in ÷ 12 = 0.25 ft
2. Calculate volume in cubic feet: 40 × 25 × 0.25 = 250 ft³
3. Convert to cubic yards: 250 ÷ 27 = 9.26 yd³ (rounded to 2 decimal places)

The calculator shows 9.26 yd³ and 250 ft³, matching this calculation. So for a 40 ft by 25 ft area with 3 inches of gravel, you would need about 9.26 cubic yards.

Understanding Gravel Volume and Coverage

Gravel is sold by volume, not weight, in most Australian landscaping supply yards. A cubic metre of gravel typically weighs between 1.4 and 1.7 tonnes depending on the type and moisture content, but you order by volume. The depth you choose depends on the purpose:

• Paths and walkways: 50 mm (2 inches) is usually enough for light foot traffic.
• Driveways: 100 mm (4 inches) is common for car traffic, sometimes more for heavy vehicles.
• Under pavers or slabs: 100–150 mm (4–6 inches) as a base layer.
• Decorative garden beds: 50–75 mm (2–3 inches) is typical.

If you are ordering from a supplier, they will often ask for the area in square metres and the depth in millimetres. The calculator gives you the volume, but you can also work out the area separately: Area (m²) = Length (m) × Width (m). Then multiply by depth in metres to get cubic metres.

How to Use the Calculator for Your Project

Measuring accurately is the key to getting the right amount. Here is a practical step-by-step approach:

1. Measure the length and width of the area in feet or metres. For irregular shapes, break them into rectangles and calculate each separately.
2. Decide on the depth. For a new driveway, 4 inches (100 mm) is a common starting point. For a path, 2 inches (50 mm) is often enough.
3. Enter the values into the calculator. If you measured in metres, convert to feet (1 m = 3.28084 ft) or use the metric formula manually.
4. Read the volume in cubic yards (or cubic metres). Add 10–15% for compaction and waste, especially if you are spreading by hand.

For example, if you have a 5 m × 3 m path at 50 mm deep, the volume is 5 × 3 × 0.05 = 0.75 m³. That is roughly 1 cubic yard (0.75 m³ × 1.30795 = 0.98 yd³). Many suppliers sell in fractions of a cubic metre, so you would order 0.8 or 1 m³ depending on what they offer.

Common Mistakes and Misconceptions

One frequent error is mixing units. If you measure length in metres but depth in millimetres, you must convert everything to the same unit before multiplying. Another is forgetting that gravel compacts. When you spread it, the volume decreases by about 10–20% depending on the type. So if you need a final depth of 100 mm, you might need to order 110–120 mm worth of loose gravel.

A common misconception is that gravel is sold by weight, so people ask for “tonnes� without knowing the density. In Australia, most landscape suppliers quote in cubic metres, but some also give a weight estimate. The density varies: crushed rock is about 1.5 t/m³, while pea gravel is around 1.4 t/m³. Always confirm with your supplier whether the price is per cubic metre or per tonne.

Another mistake is ignoring the shape of the area. If your driveway is curved or has a slope, the volume calculation becomes more complex. For a slope, you may need more gravel at the bottom to maintain an even surface. In that case, measure the average depth or divide the area into sections.

Gravel Types and Their Uses

Not all gravel is the same. The type you choose affects both the look and the performance. Common options in Australia include:

• Crushed rock (blue metal, basalt): Angular, compacts well, ideal for driveways and base layers.
• Pea gravel: Rounded, smooth, good for paths and decorative areas, but it shifts underfoot.
• River pebbles: Larger, decorative, often used in garden beds.
• Recycled concrete: Cheaper, eco-friendly, good for drainage and base layers.
• Granite or quartz: More expensive, used for high-end landscaping.

The depth you need also depends on the type. Angular crushed rock locks together, so 100 mm is usually enough for driveways. Rounded gravel like pea gravel may need a deeper layer to stay in place, sometimes 150 mm, especially on slopes.

Cost Considerations and Ordering

While this calculator does not provide prices, you can estimate costs by multiplying the volume by the price per cubic metre from local suppliers. Prices vary widely by region and type, but a typical range for crushed rock is $50–$100 per cubic metre, while decorative pebbles can be $100–$300 per cubic metre. Delivery is often extra, and some suppliers have a minimum order quantity.

When ordering, it is wise to add a surplus of 10–15% to account for compaction, spillage, and uneven ground. For example, if the calculator says 9.26 yd³ (about 7.1 m³), you would order around 8 m³ to be safe. Always confirm the exact volume with the supplier, and ask if they deliver in bulk bags or loose.

Safety and Preparation Tips

Spreading gravel is physical work. Wear gloves, sturdy boots, and a dust mask if the material is dry and dusty. If you are using a wheelbarrow, do not overload it. For large areas, consider hiring a spreader or having the gravel delivered in piles and moving it with a rake.

Before laying gravel, prepare the ground properly. Remove any grass or weeds, and consider laying a weed mat to prevent growth through the gravel. For driveways, a compacted base of crushed rock is often needed to prevent rutting. If you are unsure about the right base for your soil type, consult a local landscaper.

Frequently Asked Questions

How many square metres does a cubic metre of gravel cover?
It depends on the depth. At 50 mm deep, 1 m³ covers 20 m² (since 1 ÷ 0.05 = 20). At 100 mm deep, it covers 10 m². Use the formula: coverage (m²) = 1 ÷ depth (m).

What depth of gravel do I need for a driveway?
For a residential driveway, 100 mm (4 inches) of compacted crushed rock is typical. If you have heavy vehicles, you might need 150 mm. Always check with your local supplier or a civil engineer for specific requirements.

Should I order gravel by weight or volume?
In Australia, gravel is usually sold by volume (cubic metres). Weight is only used for transport calculations. Always confirm with the supplier whether the price is per m³ or per tonne, and ask for the density if you need to convert.

How much does a cubic metre of gravel weigh?
It varies, but a typical range is 1.4 to 1.7 tonnes per cubic metre. Crushed rock is usually around 1.5–1.7 t/m³, while pea gravel is lighter at about 1.4 t/m³. Check with your supplier for the exact density of the material you are ordering.

Can I use this calculator for circular or triangular areas?
No, this calculator is for rectangular areas. For a circle, use the formula π × r² × depth. For a triangle, use (base × height ÷ 2) × depth. You can also break odd shapes into rectangles and add the volumes.

Why do I need to add extra gravel to the calculated amount?
Gravel compacts when spread and rolled, reducing its volume by about 10–20%. Also, you may spill some during handling. Adding 10–15% ensures you have enough to achieve the desired final depth.

Related

• 1 Unit to Ton Calculator for Sand
• Retaining Wall Block Calculator: Blocks per Course
• Lawn Edging Calculator: Pieces and Stakes

in l | Lawn length | ft | 40 | 0 | 100000
in w | Lawn width | ft | 25 | 0 | 100000
in d | Depth | in | 3 | 0 | 120
out yd | Volume | yd³ | l * w * d / 324 | 2
out ft3 | Volume | ft³ | l * w * d / 12 | 1