How the volume is calculated
Concrete is sold by volume, not by area, which is why a slab calculator needs three numbers, not two: length, width, and thickness. Volume in cubic feet is length × width × thickness (converted from inches to feet). Divide by 27 for cubic yards.
Ready-mix bags list a poured yield in cubic feet on the packaging — divide your total cubic feet by that yield and round up, since concrete doesn't stretch to fill a shortfall.
Typical bag yields
| Bag size | Poured volume | Good for |
|---|---|---|
| 40 lb | 0.15 ft³ | small repairs, post holes |
| 60 lb | 0.225 ft³ | fence posts, footings |
| 80 lb | 0.30 ft³ | slabs, larger pours |
Related guides
Frequently asked questions
Does this include the base or gravel layer?
No — this is concrete volume only. A compacted gravel sub-base is calculated and ordered separately.
Why do contractors use cubic yards instead of bags for big pours?
Ready-mix trucks deliver by the cubic yard, and it's cheaper than bagged concrete once you're above roughly 1 cubic yard (54 bags of 60 lb mix).
Are the metric bag yields exact?
They're typical figures for premixed bags (just-add-water concrete mix) — actual yield varies a little by manufacturer, so check the bag if precision matters for a large pour.
Estimating concrete volume
Concrete quantity calculations are based on volume: length × width × depth for a rectangular slab. The calculator converts that volume into cubic yards and can also provide an approximate bag count. Because ready-mix bag yields vary by product, the bag estimate should always be checked against the manufacturer's stated coverage.
Measure the project
Measure the finished dimensions rather than the excavation if the excavation is irregular. For a slab, use the planned length, width and thickness. Keep all dimensions in the units requested by the calculator so the conversion is consistent.
Allow for real-world variation
Uneven subgrade, waste, spillage and small measurement errors can change the amount required. Contractors often include a modest contingency rather than ordering exactly the mathematical minimum. The appropriate allowance depends on the project and supplier.
Bagged mix versus ready-mix
A bag count is not universal because different products have different yields. Use the calculator's number as an estimate, then compare it with the coverage printed on your specific bag. For large pours, supplier volume in cubic yards is usually the more useful figure.