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 (or 20/25/40 kg) bags you need.
Bags of each size: 80 lb: 62 · 60 lb: 82 · 40 lb: 123
Mixing it yourself: cement, sand and aggregate
| Cement | 725 lb · 8 bags of 94 lb |
|---|---|
| Sand | 0.6 yd³ |
| Aggregate (gravel) | 1.2 yd³ |
A nominal volume mix, for the volume above with its waste: the dry materials are taken as 1.54 times the concrete’s volume, and cement at 1,440 kg/m³. Anything structural wants a designed mix from an engineer or a supplier.
🔒 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 — or switch Units to metric, which converts the measurements already typed into metres and centimetres rather than leaving the numbers as they were. 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. Mixing on site instead? The folded “Mixing it yourself: cement, sand and aggregate” section turns that volume, waste included, into cement by weight and in sacks, and sand and aggregate by the loose cubic metre or yard, for a nominal mix from C15 to C30 — not the designed mix structural work needs.
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.
What about round columns, tubes, curbs or steps?
Choose the shape — a slab, footing or wall, a column or round footing, a circular slab or tube, a curb and gutter, or stairs — and give a quantity when several are the same; bags are rounded once, on the total.
Can it price the pour, or say what the concrete will weigh?
Yes. Type a “Price per bag” and a ready-mix price per cubic yard (per cubic metre in metric) and “In bags” and “As ready-mix” appear side by side — the comparison that decides bags against a truck; a supplier's short-load fee is not included. A weight figure names in its label the density it assumes.
Can it tell me how much cement, sand and gravel to mix myself?
Yes. Open “Mixing it yourself: cement, sand and aggregate” and pick a mix, from C15 (1:2:5) to C30 (1:1.5:3). For the volume above, waste included, it gives the cement by weight and as a count of sacks in each common size, and the sand and aggregate as loose cubic metres or cubic yards. The dry-volume rule and the cement density it uses are printed under the table; anything structural wants a designed mix from an engineer or a supplier.