No black boxes. This page explains exactly how every WriteFlux calculator turns your measurements into a materials estimate.
The formulas
Our calculators use two simple steps:
Step 1 — Volume
Volume (m³) = Length (m) × Width (m) × Depth (m)Step 2 — Weight
Tonnes = Volume (m³) × Bulk density (t/m³)Bulk densities we use
Bulk density is how much one cubic metre of a loose material weighs. Because densities vary with moisture and compaction, we use typical Australian values — and we publish them here so you can check our workings:
| Material | Bulk density (t/m³) |
|---|---|
| Blue Metal | 1.6 |
| Pea Gravel | 1.7 |
| Crushed Rock | 1.8 |
| Decorative Pebbles | 1.5 |
| River Pebbles | 1.6 |
| Crusher Dust | 1.8 |
| Topsoil | 1.2 |
| Compost | 0.8 |
The 10% wastage allowance
By default, our calculators add a 10% wastage allowance on top of the calculated volume. Here’s why:
- A little material is always lost in transport, spreading and compaction.
- Depths are rarely perfectly even across a whole area.
- Running short mid-job costs more in time and delivery fees than ordering slightly too much.
If you’d prefer an exact figure with no allowance, simply subtract 10% from the result — or check whether the calculator you’re using lets you adjust it.
Important disclaimer
Our calculators provide estimates only. Actual quantities can vary with moisture content, compaction, particle size and how evenly you spread the material. Always confirm your order with your supplier before purchasing, especially for large or structural jobs.