Calculation Disclaimer
What these calculators are for, and where their usefulness stops.
Calculations are estimates. Always check product packaging, manufacturer instructions, local requirements and professional guidance before ordering materials or beginning work.
Estimates, not specifications
These tools calculate volume and quantity from the dimensions you type in. They do not assess whether those dimensions are appropriate. A calculator cannot see your ground, your loads, your climate or your local rules, so it cannot tell you whether a slab is thick enough or a post hole deep enough — only how much concrete the dimensions you chose would take.
Why a real pour rarely matches the arithmetic
- Excavations are almost never the exact size intended.
- Sub-grade is uneven, and hollows swallow concrete.
- Formwork can bulge under the weight of a wet pour.
- Material is lost at the chute, in the barrow and on the boards.
- Stated bag yields are nominal and vary with water content and compaction.
- Measurements taken with a tape carry their own small errors.
The waste allowance exists to absorb some of this, but it is a judgement you make about your own site, not a guarantee.
What we never assume for you
- Slab thicknesses, footing depths or post-hole dimensions.
- Mix specification, strength class, reinforcement or admixtures.
- Bag yields — these come from your product's packaging.
- Material prices, delivery costs or exchange rates.
- Building regulations or code requirements in your area.
The thickness quick-picks on the slab calculator are convenient starting numbers only. They are not a recommendation, and nothing on this site should be read as confirming that any dimension is structurally adequate.
When to involve a professional
Speak to a structural engineer, qualified contractor or your local building authority when work is load-bearing or supports a structure; when it is near or within the influence of a building's foundations; when ground conditions are poor, made-up, sloping or prone to movement; when trees, drains or services are close by; when frost depth or seismic requirements apply; or when the work needs building-control approval. That list is not exhaustive. If you are unsure whether the work is structural, treat it as if it is.
Check before you order
Confirm quantities and units with your supplier before placing an order, particularly when a price is quoted per cubic yard and your figure is in cubic metres, or vice versa. Confirm minimum loads, delivery windows and site access with the plant directly.
Reporting a problem
The full workings behind every result are published on the methodology page so you can check them. If you find an error, please tell us with the inputs and the result you expected.
Liability
Use of this site is subject to the terms of use, which limit our liability for any loss arising from reliance on these estimates.