Concrete Calculator — Cubic Yards & Bags
Calculate concrete volume for a slab or footing in cubic yards, metres and feet, plus how many 40/60/80 lb bags you need. Free, in your browser.
🔒 Calculated in your browser. Bag yields are typical for pre-mix — check your product. Add a waste % for spillage and uneven subgrade.
Volume and bag count
Enter the length and width in feet and the thickness in inches for a slab or footing. The tool returns the volume in cubic feet, cubic yards and cubic metres, and estimates the number of pre-mix bags for the size you pick (40, 60 or 80 lb). Add a waste percentage for spillage and uneven ground.
Order a little extra
Bag yields are typical values — always check your product, and remember that sub-base variation and spillage mean you usually want a small buffer. For large pours, ready-mix by the cubic yard is often cheaper than bags. Everything runs in your browser.
Bags or a truck — the number that decides is the volume
Volume is length × width × thickness, and the thickness is the term people get wrong: it is entered in inches or centimetres while the other two are feet or metres, because that is how slabs are specified. A 10 × 12 foot slab at 4 inches is 40 cubic feet, or about 1.5 cubic yards. Ready-mixed concrete is ordered by the cubic yard or cubic metre and almost every supplier has a minimum load plus a short-load charge below it, so there is a crossover: below roughly a cubic yard, bags win on cost and convenience; above two, a truck wins on both, and on the fact that a large slab poured in bag-sized batches develops cold joints.
Waste, and why the bag count is always a little short
The ground is never as flat as the drawing. Sub-base that dips, formwork that bows outward under the weight, and spillage all take concrete that the arithmetic does not know about, which is why a waste allowance of 5 to 10% is normal for a slab and more for footings dug in soft ground. Bag yields are the manufacturers' figures for set concrete and assume you mix to the stated water ratio; mixing wetter makes the concrete go further and makes it weaker, which is a trade nobody should take. Buy one bag more than the count says.
Frequently asked questions
How many bags of concrete do I need?
Divide the volume in cubic feet by the bag yield (about 0.6 ft³ for an 80 lb bag, 0.45 for 60 lb, 0.3 for 40 lb) and round up. The tool does this for you.
How do I convert to cubic yards?
Cubic yards = cubic feet ÷ 27. The tool shows cubic feet, yards and metres at once so you can order whichever way your supplier prices it.
Bags or ready-mix?
Below about a cubic yard, bags. Above about two, a truck — it is usually cheaper per yard once you count your time, and a big slab poured from a mixer in batches risks cold joints where one batch sets before the next arrives. Between the two, it depends on the minimum load charge your supplier applies.
How much waste should I add?
5 to 10% for a slab on a prepared sub-base, more for footings in soft or uneven ground where the trench takes whatever it takes. The allowance is for the ground, not for the arithmetic.
Why is the thickness in inches when the slab is in feet?
Because that is how slabs are specified and sold — a "4-inch slab" is a standard phrase. The tool converts internally, so you enter each dimension in the unit you already have it in.