Concrete Calculator

Volume is length × width × thickness, with the thickness converted from inches to feet. A 10 ft by 10 ft slab at 4 inches is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet. Concrete is ordered by the cubic yard, and there are exactly 27 cubic feet in a cubic yard, so that slab is about 1.23 cu yd before any allowance. That 27 is the number that trips people up. A cubic yard is a cube 3 ft on every side, and 3 × 3 × 3 = 27 — it is not three cubic feet. Ordering by the wrong factor is the difference between a full truck and a wheelbarrow. For a footing or a trench, the same formula applies with the depth in place of the thickness. For round forms such as a deck pier or a post hole, the volume is π × radius² × depth, so a 12 in diameter hole 3 ft deep holds about 2.4 cubic feet — call it four 80 lb bags per hole, which adds up fast across a deck.

An 80 lb bag of premixed concrete yields roughly 0.6 cubic feet and a 60 lb bag roughly 0.45, though the yield is printed on the bag and varies a little by product — use the number on the bag you are buying rather than the rule of thumb. Those figures mean a cubic yard takes about 45 of the 80 lb bags, which is more than 3,600 pounds of material to carry, open and mix by hand. The 10 by 10 ft slab above needs around 56 bags. That is the practical crossover: past roughly half a cubic yard, ready-mix delivered by truck is usually cheaper per yard, far faster, and much more consistent, because a plant batches it to a specified mix instead of you approximating it one bag at a time. Bags earn their place on small, scattered pours — post holes, mailbox bases, small pads, repairs — where a truck cannot reach or where a delivery minimum would dwarf the job. Ready-mix suppliers generally set a minimum load and charge extra for undersized orders, so ask before assuming a small pour is cheap to deliver. Either way, round up. A pour that runs short partway through leaves a cold joint, a plane of weakness where fresh concrete meets concrete that has already begun to set. That is not a cosmetic problem, and it cannot be fixed by adding more later.

Concrete is cement, sand, coarse aggregate and water. The cement and water react chemically — hydration — and the paste that forms binds the sand and stone together. Aggregate is not filler; it carries much of the load and reduces shrinkage. A mix ratio like 1:2:4 means one part cement to two parts sand to four parts gravel by volume. Bagged premix has already made this decision for you, which is one of its virtues. What the bag does not decide is the water. The water-to-cement ratio is the single strongest control on the strength of the finished concrete, and the relationship runs the wrong way from intuition: more water makes concrete weaker, not stronger. Only a limited amount of water is needed for hydration. Anything beyond that stays in the mix as free water, eventually leaves, and leaves behind pores. Those pores are permanent, and they reduce strength and durability. A soupy mix is easier to pour and place, and that convenience is paid for in the cured slab. So mix to the water quantity on the bag, not to the consistency that feels easiest to work with. If the mix is genuinely too stiff to place, the answer a professional reaches for is an admixture that improves workability without adding water, not a hose. Concrete also does not "dry" — it cures. Keeping the surface damp for the first several days lets hydration continue and produces stronger concrete than letting it dry out fast in sun and wind. The strength quoted for a mix, in psi, is conventionally measured at 28 days, and concrete keeps gaining strength slowly well beyond that.

Almost every pour uses more than the calculated volume, and the allowance exists because of what happens below the forms rather than above them. Subgrade is never flat. An excavation that averages 4 in deep will be 4.5 in in places, and every fraction of an inch across a large slab is real volume. On a 10 by 10 ft slab, a half inch of extra depth is about 0.15 cubic yards on its own. Forms flex and subgrade compresses. Wet concrete is heavy, and it pushes form boards outward and settles into soft ground. Spillage and the wheelbarrow that never fully empties account for the rest. Before ordering, the subgrade should be compacted and, where the design calls for it, covered with a compacted gravel base and a vapor barrier. Concrete poured on loose fill settles and cracks no matter how good the mix is. Reinforcement — rebar or welded wire — and control joints cut or tooled at planned intervals do not stop concrete from cracking; they decide where the cracks go. Every slab cracks somewhere.

These figures are for planning quantities. They tell you how much material to buy, and nothing about whether the thing you are pouring is adequate for what it will carry. Anything structural — footings supporting a building, a garage or driveway slab carrying vehicle loads, retaining walls, anything holding back soil or water, anything a deck or carport bears on — must be sized by a licensed engineer or built to a design that satisfies your local building code. Thickness, mix strength in psi, reinforcement, footing depth relative to the local frost line and bearing capacity of the soil are all determined by that design, not by a calculator. Many of these jobs also need a permit and inspection before the pour, and inspections happen before the concrete goes in, not after. Two safety points worth stating plainly. Wet concrete is strongly alkaline and causes chemical burns on prolonged skin contact, which is why waterproof gloves and boots matter and why kneeling in fresh concrete in ordinary clothes is a bad idea. And cutting, grinding or dry-sweeping cured concrete releases respirable crystalline silica, a recognized lung hazard — cut wet and use appropriate respiratory protection.