Volume is length × width × depth. Keep all measurements in consistent units and check the specified finished thickness across the slab.
How to calculate it
The dependable approach is to calculate the physical quantity first rather than starting with pack counts. Measure the project in consistent units, calculate area or volume, and then convert that result using the product’s stated coverage or yield. For concrete, the important variables are slab dimensions, bag yield and waste.
Where a manufacturer provides a figure, use it in preference to a generic rule of thumb. A useful planning assumption for this category is 25 kg bag yield varies by product, but products and site conditions vary.
Quick reference
| Area | 50 mm | 75 mm | 100 mm |
|---|---|---|---|
| 5 m² | 0.25 m³ | 0.38 m³ | 0.50 m³ |
| 10 m² | 0.50 m³ | 0.75 m³ | 1.00 m³ |
| 20 m² | 1.00 m³ | 1.50 m³ | 2.00 m³ |
| 30 m² | 1.50 m³ | 2.25 m³ | 3.00 m³ |
| 50 m² | 2.50 m³ | 3.75 m³ | 5.00 m³ |
Worked planning method
- Measure the relevant length, width, height, area or depth accurately.
- Convert all measurements to compatible units before multiplying.
- Apply the product-specific coverage, density, pack coverage or yield.
- Add a sensible allowance for cuts, waste, settlement or surface variation only where needed.
- Round up when the material is sold only in complete packs, bags, rolls, boards or tiles.
Common causes of under-ordering
Under-ordering usually comes from mixing units, forgetting a second coat or layer, using nominal dimensions instead of measured dimensions, ignoring cuts, or relying on a generic coverage figure that does not match the selected product. Irregular surfaces and complex layouts also make simple area calculations less precise.
Common causes of over-ordering
Large blanket waste percentages can create unnecessary surplus. Avoid adding the same allowance twice—for example once to the measured area and again to the pack count. Also avoid subtracting every tiny opening or adding every tiny return unless it materially changes the result.
Use the interactive calculator
For a project-specific estimate, open the Concrete Calculator. You can replace its defaults with the figures from your chosen product.
Frequently asked questions
Is this figure exact?
No. It is a planning estimate. Product formulation, site conditions, installation method and measured dimensions can change the final requirement.
Should I always add 10%?
No. Ten percent is a common planning allowance for some materials, not a universal rule. Simple projects may need less; complex layouts or fragile materials may need more.
Which figure should I trust when a product label differs?
Use the manufacturer’s stated coverage, yield or pack information for that product, provided it applies to your intended substrate and installation method.