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.
Newer calculators
Our newer calculators each have their own formula. Here’s exactly what each one does with your measurements:
Paver Calculator
Pavers = ceil(area ÷ paver area × 1.10)Bedding sand (m³) = area (m²) × 0.03We divide your paving area by the area of a single paver, round up to a whole paver and add a 10% cutting allowance; the bedding sand figure assumes a standard 30 mm sand bed.
As with our other calculators, these are estimates — the exact paver count depends on your laying pattern and how many cuts your edges need.
Concrete Calculator
Volume (m³) = Length (m) × Width (m) × (Depth (cm) ÷ 100)20 kg bags = ceil(volume ÷ 0.01 × 1.10)We convert your depth from centimetres to metres for the volume, then divide by the yield of a 20 kg bag (about 0.01 m³), round up and add 10% wastage.
This is an estimate — ground that isn’t perfectly level, and formwork that flexes, can change how much concrete you actually need.
Rainwater Tank Calculator
Annual harvest (L) = Roof area (m²) × Rainfall (mm) × 0.9Suggested tank (L) = ceil(annual ÷ 12 × 3 ÷ 1000) × 1000We multiply your roof area by your annual rainfall and keep 90% to allow for evaporation and runoff losses, then suggest a tank sized to hold roughly three months of that harvest.
These are estimates — actual harvest varies with rainfall patterns, roof material and how often you use the stored water.
Solar Panel Calculator
System size (kW) = Daily usage (kWh) ÷ Peak sun hours ÷ 0.8Panels = ceil(kW × 1000 ÷ Panel watts)We divide your daily electricity use by your peak sun hours, allow 20% for system losses, then convert the system size into a whole number of panels based on the panel wattage.
This is an estimate — actual output depends on panel orientation, shading, roof angle and your inverter.
Pool Chemical Calculator
Volume (m³) = Length × Width × Depth (or π × (diameter ÷ 2)² × Depth for round pools)Chlorine (grams) = Litres ÷ 10000 × 15 × ppmWe work out your pool’s volume in litres, then calculate how many grams of chlorine will raise it by your target parts per million.
This is an estimate — always follow the dosing instructions on your chemical packaging and retest your water afterwards.
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.