Online Concrete Calculator

Picture filling a perfectly rectangular box with water. The amount of water you need is simply the three dimensions multiplied together. Concrete volume works exactly the same way — length times width times thickness gives you the raw space that needs to be filled. The only complication is that concrete comes measured in cubic yards, while most people think in feet and inches, which forces a unit conversion that trips up a lot of DIYers.

The formula adds one practical layer on top of pure geometry: a waste allowance. No real pour goes into the forms with zero loss. Concrete spills off trowels, settles into low spots in the subgrade that your tape measure did not capture, and partially fills honeycombs in the form faces. A standard 10 percent waste buffer means you order a volume 10 percent larger than the theoretical box — so if your slab needs 1.48 yd³ yd³ of concrete, you actually order 1.63 yd³ yd³.

Bag counts are derived from the manufacturer-rated yield of premix bags. A 60 lb bag is rated at 0.45 cubic feet of mixed concrete; an 80 lb bag at 0.6 cubic feet. The tool divides your total adjusted volume by each yield and rounds up to the nearest whole bag — you cannot buy a fraction of a bag, and coming up one short on pour day is a serious problem. Always round up.

Use this calculator any time you are pouring a uniform slab, pad, footing, or column and need to know how much material to buy or order before the truck arrives. It is most reliable for simple rectangular geometry: driveways, sidewalks, garage floors, patio slabs, and post footings. The result is an estimate, not a guarantee — on-site conditions always introduce variation.

This tool is less appropriate for complex pours with tapers, multiple lifts, or varying cross-sections. A stepped footing, for example, needs to be broken into individual rectangular sections and summed. Large commercial pours with engineered drawings should always be quantity-taken from those drawings, with this calculator used only as a rough sanity check.

If your result exceeds roughly 10 cubic yards, step away from the bags-of-concrete aisle and call a batch plant. Ready-mix is priced by the yard and the per-unit cost drops significantly at volume, while the labor cost of mixing and placing hundreds of bags by hand becomes prohibitive. The boundary warning in this tool flags that threshold automatically.

Measuring thickness in feet instead of inches. This is the most frequent input error and it produces a result roughly twelve times too large. If the tool shows a volume that seems unreasonably high, check whether you entered thickness as 4 inches or accidentally typed 4 feet. A 4-foot slab is a dam, not a patio.

Skipping the waste allowance and assuming zero loss. Experienced finishers know that concrete volume is never exact in practice. The subgrade compacts unevenly, forms bow under pressure, and some material always stays in the mixer drum. Ordering exactly the calculated net volume leaves no margin — the most likely outcome is a short pour that either leaves a gap in the slab or forces a cold joint repair that costs more than a few extra bags would have.

Using bag count as a proxy for ready-mix quantity. Ready-mix trucks deliver in cubic yards, not bags. The bag count shown here is for purchasing premix at a hardware store. If you are getting a truck delivery, give the batch plant your cubic-yard number — which is 1.63 yd³ yd³ for the example project — not the bag count. Mixing up these numbers can result in a truck delivering the wrong load.

The core geometry for a rectangular slab converts all dimensions to feet first. Length and width are already in feet; thickness in inches is divided by 12 (the number of inches per foot) to get feet. Volume in cubic feet is then length (ft) × width (ft) × thickness (ft). For the example slab of 12 ft × 10 ft × 4 in, the net volume works out to 1.48 yd³ yd³ after the cubic-yard conversion.

Converting cubic feet to cubic yards requires dividing by 27 — the number of cubic feet in one cubic yard. This single conversion is where most hand-calculation errors occur: people divide by 9 (the square-foot conversion) instead of 27 (the cubic conversion). The adjusted volume with waste is: net volume × (1 + waste percent ÷ 100). For the example with 10 percent waste, that gives 1.63 yd³ yd³.

Circular columns use the cylinder formula: volume = π × radius² × height. The radius is half the diameter. All dimensions are converted to a common base unit in metres first, then the result is converted back to cubic feet and cubic yards for display. Bag counts are always computed on the waste-adjusted volume and always rounded up: bags needed = ceiling(total cubic feet ÷ yield per bag). For 60 lb bags yielding 0.45 ft³ each, the example slab requires 98 bags bags.

The bag-yield figures used here (0.45 ft³ for 60 lb, 0.6 ft³ for 80 lb) are manufacturer-rated yields at the water-to-cement ratio specified on the bag. In practice, contractors often add slightly more water for workability, which reduces compressive strength and increases actual yield marginally — meaning real-world bag counts can run 2 to 5 percent higher than this tool predicts. For structural applications, use the manufacturer spec and accept the conservative count.

The 10 percent waste default is a rule-of-thumb for flat, well-prepared subgrades. Geotechnical variation, form deflection under hydraulic pressure, and pump or chute losses on larger pours can push actual waste to 15 percent or more. Experienced site supervisors calibrate their waste factor to the specific subgrade condition and form system, not a fixed percentage — treat this tool as the starting point, not the final answer.