Every concrete project, from a backyard patio to a full driveway, fails the same way: the pour runs short with the forms half full, or a truck leaves with half its load still in the drum and your money in it. Both mistakes are just volume math done carelessly. Take the small patio this calculator opens with — 10 feet by 10 feet, poured 4 inches thick. The exact volume is 33.33 cubic feet, which is 1.23 cubic yards. Add the standard 10 percent for waste and uneven ground and you should order 1.36 cubic yards — or, if you are mixing it yourself, 62 bags of 80 lb premix. This page shows where every one of those numbers comes from, so you can check them against your own project before you spend anything.

The only formula you actually need.

Concrete is sold by volume, and the volume of any slab is its surface area times its thickness. For a rectangle, multiply length by width to get square feet, then multiply by the depth in feet. Because thickness is measured in inches, the working formula is length × width × (inches ÷ 12), which gives cubic feet. A 10 × 10 patio at 4 inches: 10 × 10 × (4 ÷ 12) = 33.33 cubic feet. Ready-mix trucks sell by the cubic yard, and one cubic yard is 27 cubic feet, so divide by 27: 33.33 ÷ 27 = 1.23 cubic yards. A round slab swaps the rectangle's area for a circle's: π × (diameter ÷ 2)². A 12-foot round patio has an area of about 113.1 square feet, and at 4 inches thick it needs 37.70 cubic feet, or 1.40 cubic yards. A footing is just a long, narrow rectangle — a 40-foot footing, 2 feet wide and 12 inches deep, is 40 × 2 × 1 = 80 cubic feet, or 2.96 cubic yards. Every output on this page is those three steps and nothing more.

How thick should the slab be?

Thickness is the one input people guess, and guessing wrong in either direction costs money — too thin and the slab cracks under loads it was never built for, too thick and you pay for concrete that does nothing. Standard practice for light-duty flatwork — patios, walkways, and shed floors — is 4 inches, which is what this calculator starts with. Driveways carry vehicles, so 4 inches is the floor, not the target: 5 to 6 inches is the common choice for cars and pickup trucks, and the worked example below uses 6. Garage floors land in the same 5-to-6-inch range. Footings are a different animal: their depth is set by your local frost line and building code, not by preference, because a footing that heaves in a freeze takes the wall with it. Twelve inches is a common minimum depth, but check your local code before you dig — this calculator does the volume math for whatever depth your code requires. Whatever the project, measure the depth of your actual forms, not the depth you planned; a form that sits half an inch proud pours half an inch deeper, everywhere.

Bags or a ready-mix truck?

If the job is small, bagged premix is the flexible route: buy bags, mix what you need, return the rest. Each bag covers a fixed volume once mixed — an 80 lb bag yields about 0.6 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.3, the yields the manufacturers print on the bag. Turned around, one cubic yard takes 45 of the 80 lb bags, 60 of the 60 lb bags, or 90 of the 40 lb bags. Now weigh that against the patio above: 62 eighty-pound bags is over two tons of material to lift, open, mix, and place before the first batch starts to set. As a working rule, hand-mixing stays reasonable up to roughly a quarter to a half cubic yard; beyond that, most people rent a mixer or order the truck. Ready-mix flips the problem — the concrete arrives mixed and keeps coming — but trucks carry minimum loads, and a 1.36-yard order on a truck built for 8 or 9 yards usually attracts a short-load fee. Neither route is wrong; this calculator prices the truck route and counts the bags so you can compare them honestly.

The worked examples, run honestly.

The driveway is where the numbers get serious. A 20 × 12 foot driveway at 6 inches thick is 240 square feet × 0.5 feet = 120 cubic feet, exactly 4.44 cubic yards. With 10 percent waste the order becomes 132 cubic feet, or 4.89 cubic yards — at the calculator's default price of $165 a yard, about $806.67 of concrete. Mix that same driveway from bags and you are looking at 220 of the 80 lb bags, which is why driveways are truck work. The round patio, 12 feet across and 4 inches thick, needs 1.40 cubic yards exactly and 1.54 with waste: 70 eighty-pound bags, about $253.42 by truck. The footing example — 40 feet long, 2 feet wide, 12 inches deep — comes to 2.96 cubic yards, 3.26 with waste, or 147 eighty-pound bags. Notice the pattern: the waste allowance adds roughly a quarter to a half yard on real jobs, and it is the difference between finishing the pour and finishing it tomorrow with a visible cold joint.

Why the waste allowance is not optional.

Ten percent sounds like a fudge factor, and it is — a deliberate one, bought cheap. Real ground is never perfectly level, so a slab formed at exactly 4 inches pours 4 inches in some spots and 4.5 in others, and that extra half inch across 100 square feet is 4 cubic feet you did not order. Add spillage, the concrete that coats the wheelbarrow and the chute, and forms that bow slightly under the load, and a zero-waste order runs short on almost every job. Running short is the expensive outcome: a second delivery for a fraction of a yard carries the same fees as the first, and a slab poured in two sessions shows the seam forever. Leftover concrete, by contrast, costs only its purchase price and can usually be spread on a gravel drive or a small pad. That asymmetry is why this calculator builds 10 percent in by default — drop it to 5 percent for a small, carefully leveled job on a hard base, and raise it toward 15 percent for rough, hand-dug ground.

What this estimate leaves out.

The cost figure here is the concrete itself at the per-yard price you enter — $165 is the calculator's default, and in recent years US ready-mix has commonly landed somewhere in the $150 to $200 per cubic yard range before fees, but prices are local, so get a quote before you treat the estimate as a budget. Delivery charges, short-load fees, pumping, and weekend scheduling all sit outside it. The calculator also does not size reinforcement: most slabs still need rebar or welded wire mesh, gravel base underneath (4 inches of compacted base is typical under flatwork), and forms — all real costs, none of them concrete. And a final honest limit: this page computes volume for single rectangles, circles, and footings. An L-shaped patio is two rectangles — run each part separately with the quantity field, or add the two results; a slab with a step or a thickened edge is a slab plus a footing. Break the project into the shapes the calculator knows, and the totals will take care of themselves.

With the pre-filled defaults — Project Shape Rectangular Slab, Length 10 ft, Width 10 ft, Diameter 10 ft, Depth (thickness) 4 in, Quantity 1, Extra for Waste & Spillage 10%, Ready-Mix Price (per cubic yard) $165 — the Concrete Calculator returns Volume (cubic feet) 33.33 ft³, Volume (cubic yards) 1.23 yd³, Volume with Waste (cubic feet) 36.67 ft³, Concrete to Order (cubic yards) 1.36 yd³, Volume with Waste (cubic meters) 1.04 m³, 80 lb Bags Needed 62, 60 lb Bags Needed 82, 40 lb Bags Needed 123, Estimated Ready-Mix Cost $224.07. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.