Working out how much fill dirt you need is straightforward once you break your section into simple shapes and measure carefully. The basic formula is length × width × depth = volume in cubic metres (m³), and you'll want to add extra to account for compaction. Get this right upfront and you'll avoid costly over-ordering or frustrating shortfalls.
The Basic Formula
All fill calculations come down to volume. Work in metres throughout to get your answer in cubic metres (m³), which is the standard unit suppliers use.
Volume (m³) = Length (m) × Width (m) × Depth (m)
For example, if you need to fill an area 10 m long, 5 m wide, and 0.3 m deep:
10 × 5 × 0.3 = 15 m³
If your section is an irregular shape, divide it into rectangles or triangles, calculate each section separately, then add them together.
Triangular Areas
For a triangular section: (Base × Height ÷ 2) × Depth
Sloped or Uneven Ground
If the ground isn't flat, measure the average depth — take several depth readings across the area and average them. For significantly sloped sites, a qualified earthmover or surveyor can produce a cut-and-fill calculation using site levels, which is far more accurate.
Allow for Compaction
This is the step many homeowners miss. Fill dirt compacts when it's placed and compressed, meaning the settled volume will be less than what you ordered. As a general guide:
- Add 10–15% for lightly compacted fill (garden beds, landscaping)
- Add 15–25% for engineered fill that will be mechanically compacted (under driveways, building platforms)
So for our 15 m³ example, order around 17–19 m³ to be safe.
A Quick Reference Table
| Area Size | Depth 100 mm | Depth 200 mm | Depth 300 mm |
|---|---|---|---|
| 5 m × 5 m | 2.5 m³ | 5 m³ | 7.5 m³ |
| 10 m × 5 m | 5 m³ | 10 m³ | 15 m³ |
| 10 m × 10 m | 10 m³ | 20 m³ | 30 m³ |
| 20 m × 10 m | 20 m³ | 40 m³ | 60 m³ |
Remember to add your compaction allowance on top of these figures.
Choosing the Right Type of Fill
Not all fill dirt is the same. Using the wrong material can cause settlement, drainage problems, or even Building Code issues.
- General fill / bulk fill: Suitable for raising ground levels in non-structural areas like gardens.
- Engineered fill (AP40, AP65, or similar compacted aggregate): Required under building platforms, driveways, and foundations. Must meet specific compaction standards.
- Topsoil: Used as the final layer for lawns and gardens — not a structural fill.
If you're filling under or near a building platform, the fill specification must comply with NZS 3604 (Timber-framed buildings) or your engineer's design. MBIE Building Performance has guidance on when fill under a building requires specific engineering input.
Do You Need a Building Consent?
In New Zealand, significant earthworks may require a building consent or resource consent from your local council. Schedule 1 of the Building Act lists some exemptions for minor work, but these have limits — for example, rules around how much material can be moved and how close to boundaries or structures. Always check with your local council before starting major earthworks. Requirements vary by district, and some councils have specific rules around fill near waterways or on sloped sites.
BRANZ is a useful resource for understanding how fill and ground conditions affect building performance in NZ conditions.
Practical Tips for Ordering Fill
- Order in m³ — this is how NZ suppliers price and deliver fill.
- Ask your supplier whether the quoted volume is loose (truck) volume or compacted volume — there's a difference.
- For large jobs, get two or three quotes and confirm the material type and source.
- Consider truck access to your site — large tip trucks need adequate clearance and a firm surface to tip safely.
- If you're in a wet part of NZ (think Northland, West Coast, or Fiordland), factor in that fill placed in wet conditions may need more compaction effort or drying time.
When to Call a Professional
For anything beyond simple garden levelling, it's well worth bringing in a qualified earthmover or civil contractor. You should definitely get professional help when:
- You're filling a building platform or area under a future structure
- The site is steeply sloped or has unstable ground
- You need cut-and-fill calculations based on surveyed levels
- Fill is going near retaining walls, boundaries, or drainage lines
- The volume is large (generally over 50–100 m³)
- Your council requires a geotechnical report or engineering sign-off
An experienced earthmover will have the machinery to place and compact fill correctly, and can advise on material specifications. You can find qualified local earthmoving contractors on tradedirectory.co.nz.
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