Concrete Calculator — Bags Needed for Slabs, Footings & Posts
Work out how much concrete a slab, footing, column pour or a batch of fence post holes needs: pick the shape, enter its real dimensions, and get the volume in cubic yards and cubic meters plus the number of pre-mixed bags to buy.
Result
Ordering concrete is a costly place to guess: too little and a fresh batch has to be mixed and poured before the first one sets, too much and the leftover hardens in the wheelbarrow. This calculator handles four common pours — a slab, a footing, a round column or tube (a sonotube form), and a batch of identical fence post holes — each with the volume formula that actually applies to its shape, so you get a real number instead of a rough guess.
Dimensions are grouped the way contractors actually measure them: length and width (or a column's height, or a post hole's depth) are entered in the larger unit — feet or meters — while a slab's thickness, a footing's depth and a diameter are entered in the smaller unit — inches or centimeters — because a 4-inch slab is far easier to specify than a 0.33-foot one. Pick imperial or metric once and every field switches together.
The result shows the volume in both cubic yards and cubic meters, since ready-mix trucks are ordered in yards in the US and Canada and in meters almost everywhere else. That volume then converts into a bag count for whichever pre-mixed bag size you're buying, using the manufacturer-published typical yields — about 0.30 ft³ for a 40 lb bag, 0.45 ft³ for a 60 lb bag, and 0.60 ft³ for an 80 lb bag. These are typical figures, not a guarantee — always check the exact yield printed on your bag, since formulas vary by brand. An optional checkbox adds 10% to the volume before the bag count, covering spillage, an uneven subgrade and general overfill.
A worked example: a 10 × 10 ft slab at 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.33 ft³, which is about 1.23 cubic yards or 0.94 cubic meters — roughly 75 bags of 60 lb pre-mix at the published yield. If you're finishing the same project, the paint calculator and tile calculator on this site use the same kind of straightforward area and volume math for what comes after the concrete cures. Everything is calculated locally in your browser — nothing you enter is uploaded or stored.