This works out how much ready-mix concrete a slab or footing needs, in cubic yards — the unit concrete is conventionally ordered and priced by — with a waste margin included.
How it works
Concrete volume
volume = length × width × depth
Converted to cubic yards: 1 yd³ = 27 ft³ exactly, since a yard is exactly 3 feet and a cubic yard is 3³ = 27 cubic feet.
Why the waste margin exists, and why it’s editable
Concrete cannot be un-poured once mixed — ordering exactly the calculated volume risks coming up short mid-pour, a genuinely bad outcome that overshooting by a small margin avoids. The 10% default is a commonly used contractor rule of thumb for a straightforward slab, not a code requirement or a guarantee. A more irregular pour — uneven subgrade, complex formwork — reasonably calls for more, which is why the field is adjustable rather than fixed.
How to use this calculator
- Enter the length and width of the area to pour.
- Enter the depth in inches.
- Adjust the waste margin if your pour is more complex than a simple flat slab.
Frequently asked questions
Why does concrete get ordered by the cubic yard specifically?
It’s the ready-mix industry’s own standard unit in the US — trucks, batching and pricing are all built around it, which is why this calculator converts to it rather than leaving the answer in cubic feet.
Is the 80 lb bag figure accurate for my project?
It’s an estimate based on roughly 0.6 cubic feet of mixed concrete per 80 lb bag, a commonly cited figure — check your specific bag’s own coverage, since it can vary by product and by how much water is used in the mix.
Should I round up to the nearest full truck load?
For a ready-mix delivery, yes — check with your supplier on minimum order sizes and truck capacities, since ordering slightly more than this calculator’s figure to hit a round truck load is often standard practice.
Why does a small increase in depth matter so much?
Because depth is a direct linear multiplier on volume — doubling the depth exactly doubles the concrete needed for the same footprint, which is easy to underestimate when eyeballing “just a bit deeper.”
Does this calculator account for rebar or formwork?
No — this is volume of concrete only. Reinforcement and formwork are separate materials with their own estimating methods.
