Gravel Calculator NZ: How Much Do You Need?

This gravel calculator works out the volume of gravel you need for a New Zealand driveway, path, or garden area. Enter the length, width, and depth of the space, and you get the volume in cubic metres and cubic feet—the two units most NZ suppliers quote. The calculator also shows the total in cubic yards, which some bulk suppliers still use.

Formula

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

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

To get cubic yards, divide the cubic feet by 27 (since 1 yard = 3 feet, so 1 yd³ = 27 ft³). The calculator combines both steps:

Volume (yd³) = (L × W × D) / 324

Where:

• L = length in feet (enter metres and convert, or use the ft input)
• W = width in feet
• D = depth in inches (12 inches = 1 foot)
• 324 = 12 × 27, the conversion factor from (ft × ft × in) to yd³

The first output (cubic yards) is the headline answer because most NZ bulk gravel suppliers price per cubic metre or per tonne, and yards are still used in some regions. The second output gives cubic feet, which is handy for smaller projects or when comparing with bagged gravel.

Worked Example

Using the default values: 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 decimals)

The calculator gives 9.26 yd³ and 250 ft³, which matches the manual calculation. Note that the calculator uses the combined formula (40 × 25 × 3 / 324 = 9.26), so it agrees exactly.

Understanding the Result

The result is a volume, not a weight. To order gravel, you need to convert volume to tonnes because NZ suppliers sell by weight (per tonne) or by volume (per cubic metre). The density of gravel varies with the type and moisture content:

• Dry gravel: about 1.4–1.7 tonnes per cubic metre
• Wet gravel: can be 1.7–2.0 tonnes per cubic metre
• Pea gravel (dry): roughly 1.4–1.6 tonnes per cubic metre

To convert cubic yards to cubic metres, multiply by 0.7646 (since 1 yd³ = 0.7646 m³). For the example, 9.26 yd³ ≈ 7.08 m³. At 1.6 t/m³, that would be about 11.3 tonnes. Always ask your supplier for the exact density of the gravel you’re ordering—they can give you the correct conversion.

How to Choose the Right Depth

The depth you need depends on the purpose of the gravel:

• Footpaths: 50–75 mm (2–3 in) of compacted gravel is typical
• Driveways (light vehicles): 100 mm (4 in) of compacted base course plus a 20–25 mm (1 in) top layer of chip
• Driveways (heavy vehicles): 150 mm (6 in) or more of compacted base, often with a geotextile underlay
• Garden beds or decorative: 50–75 mm (2–3 in) is usually enough

If you’re unsure, ask a local landscaper or your gravel supplier. They know the local soil conditions and can recommend the right depth for your project.

Common Mistakes When Ordering Gravel

People often make these errors when calculating gravel needs:

• Mixing units: Entering length in metres but depth in inches, or vice versa. Always convert everything to the same unit system before calculating.
• Forgetting compaction: Gravel compacts by about 10–20% under rolling. If you order exactly the volume of the hole, you’ll end up short. Order 10–15% extra to account for compaction and settling.
• Ignoring the sub-base: For driveways, you need both a base course and a top layer. The calculator gives the total volume, but you may need to split it between different gravel types.
• Rounding down: Gravel is sold in whole cubic metres or tonnes. Always round up to the next whole unit, or you’ll run short.

Comparing Suppliers: Volume vs. Weight

NZ gravel suppliers quote in different ways:

• Per cubic metre (m³): Common for bulk orders. You need to know the volume.
• Per tonne: Many suppliers price by weight. You need to convert volume to tonnes using the density.
• Per bag: For small projects, gravel comes in bags (usually 20–25 kg). A 20 kg bag covers about 0.015 m³ at 1.6 t/m³, so you’d need about 67 bags per cubic metre.

When comparing prices, always check whether the quote includes delivery and GST. Some suppliers charge extra for small loads, so it’s worth asking.

Limits of the Calculator

This calculator assumes a rectangular area with a uniform depth. Real driveways and paths often have curves, slopes, or varying depths. For irregular shapes, break the area into rectangles and calculate each separately, then add them together. For slopes, the volume calculation is more complex because you need to account for the angle—a simple length × width × depth may under- or over-estimate. Also, the calculator does not account for compaction, so add 10–15% to the result for compacted gravel. Finally, the calculator gives volume, not weight—always confirm the density with your supplier.

Frequently Asked Questions

How many cubic metres of gravel do I need for a 10m × 3m driveway at 100mm deep?

First convert 100 mm to metres: 0.1 m. Then multiply: 10 × 3 × 0.1 = 3 m³. Add 10–15% for compaction, so order about 3.3–3.5 m³.

How many tonnes is 1 cubic metre of gravel?

It depends on the density. Dry gravel typically weighs 1.4–1.7 tonnes per cubic metre. Wet gravel can be heavier. Ask your supplier for the exact density of the gravel you’re ordering.

Can I use this calculator for pea gravel or river stones?

Yes, the volume calculation is the same for any loose material. The depth you choose will vary by use—pea gravel for paths is usually 50–75 mm, while river stones for drainage may need more.

Do I need to order extra gravel for compaction?

Yes, gravel compacts by about 10–20%. Order 10–15% extra to ensure you have enough after rolling. This is especially important for driveways.

Should I use a base course and a top layer for a driveway?

For a durable driveway, yes. A typical spec is 100 mm of compacted base course (AP20 or similar) topped with 20–25 mm of chip (such as AP5 or GAP5). Calculate the volume for each layer separately using the appropriate depth.

How do I convert cubic yards to cubic metres?

Multiply the cubic yards by 0.7646. For example, 9.26 yd³ × 0.7646 = 7.08 m³.

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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