CalcConcrete is the one building material you truly cannot buy short. Run out of mulch and you make a second trip; run out of concrete mid-pour and you get a cold joint: a visible seam, a structural weak line, and a story you will tell with regret for years. The volume math below is genuinely simple. Doing it right, with waste included, is the whole difference between a clean pour and a patched one.
The only formula you need
Concrete is sold by the cubic yard, and a cubic yard is 27 cubic feet. For any rectangular slab:
Length (ft) x Width (ft) x Depth (ft) / 27 = cubic yards
Depth is where people fumble: convert inches to feet before multiplying. Four inches is 0.333 feet, not 4. A 10 x 10 patio at 4 inches is 100 x 0.333 / 27 = 1.23 cubic yards. Mixing feet and inches in one multiplication is the single most common ordering error in DIY concrete, and it produces answers that are off by a factor of twelve. If cubic units are new territory, our short primers on cubic feet and cubic yards make the jump from area to volume painless.
The other shapes: footings, columns, stairs
Everything reduces to the same volume idea. A footing is a long skinny slab: a 12-inch wide, 12-inch deep footing is 1 square foot of cross-section, so every 10 feet of run is 10 cubic feet, or 0.37 yards. A round column or sonotube is a cylinder: 3.14 x radius squared x height, in feet. One 12-inch diameter tube filled 4 feet deep holds 3.14 x 0.25 x 4 = 3.14 cubic feet, about 0.12 yards, so a deck on eight such piers needs roughly one yard. Stairs are a stack of rectangular steps; compute each step as its own small slab and add them. The concrete calculator handles slab, footing, and column modes so you can price the whole project in one sitting.
Always add waste, then round up
Order 10 percent extra for a simple formed slab and up to 15 percent for odd shapes, sloped ground, or a first pour. The extra is not pessimism, it is physics: subgrade is never perfectly flat, forms bow a little, some mix stays in the truck chute and the wheelbarrow, and spillage exists. For the 10 x 10 patio, 1.23 plus 10 percent is 1.36; order 1.5 yards, because ready-mix sells in quarter-yard increments. Extra concrete becomes a splash pad or a stepping stone; missing concrete becomes a seam.
Enter slab, footing, or column dimensions and get cubic yards with waste included, plus the equivalent bag count for small pours.
Bags or a truck: the one-yard rule
Bag yields are the numbers to memorize:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Our patio’s 33.3 cubic feet is 56 eighty-pound bags before waste, call it 62 with it. That is two and a half tons of material to load, haul, lift, mix, and place, and hand-mixed batches poured over hours cure as a patchwork rather than one monolithic slab. The practical rule: under about half a yard, bags are sensible; approaching one yard, price the ready-mix truck. Expect a short-load fee on small deliveries, and know that somewhere around the one-yard line the truck starts winning on money and always wins on your spine and on slab quality, since the whole pour arrives wet at once.
Ordering ready-mix like you have done it before
Call with four numbers: yards (rounded up to the quarter yard), strength in PSI (3,000 is typical for patios and walks, 3,500 to 4,000 for driveways, more where codes say so), slump if you have a preference, and whether you need fiber or air entrainment (standard in freeze-thaw climates). Confirm truck access: a loaded mixer weighs enough to crack a residential driveway, so plan the chute path or budget for a pump. Concrete waits for no one, so have forms braced, base compacted, tools staged, and at least one more set of hands than you think you need before the truck leaves the plant.
Depth and base: the part under the math
Volume assumes a depth, and depth is a decision: 4 inches suits patios, walks, and shed slabs; driveways want 5 to 6 inches; anything carrying vehicles or point loads deserves a look at local code. Under the slab, 4 inches of compacted gravel is standard for drainage and a stable base, and that gravel is its own cubic-yard order, which the cubic yard calculator sizes in seconds. Reinforcement is cheap insurance: welded wire mesh or fiber for slabs on grade, rebar where spans, edges, or loads demand it.
Common pours at a glance
| Project | Typical depth | Volume math (10 x 10 example) |
|---|---|---|
| Patio or walkway | 4 in | 1.23 yd, order 1.5 |
| Driveway | 5 to 6 in | 1.54 to 1.85 yd, order 2 to 2.25 |
| Shed slab | 4 in | 1.23 yd, order 1.5 |
| Footing, 12 in x 12 in | per 10 ft of run | 0.37 yd |
| Deck pier, 12 in tube x 4 ft | per pier | 0.12 yd |
The mistakes that ruin pours
Five failures cover most disasters. Forgetting the inch-to-foot conversion and ordering twelve times wrong in either direction. Skipping the waste factor and coming up a wheelbarrow short at the far corner. Pouring on uncompacted fill that settles and cracks the slab along a lazy S. Finishing too early or too late: concrete has a working window, and troweling bleed water back in weakens the surface. And ignoring weather: fresh concrete wants to stay above roughly 40 degrees for its first days and hates a downpour on its open face.
Cost math: bags vs truck on the same patio
Run the 10 x 10, 4-inch patio both ways, using illustrative prices that vary by region. Bags: 62 eighty-pound bags at $6 apiece is about $372, plus a mixer rental around $60, plus a full day of heavy labor for two people, plus the quality risk of a patchwork pour. Truck: 1.5 yards at $170 per yard is $255, plus a short-load fee near $75, call it $330 delivered wet and ready in twenty minutes. The truck costs less before you assign any value to your Saturday or your slab’s consistency, and the gap widens with size: at 2 yards and beyond, bags stop being a real option and become a cardio program with a concrete problem.
The pour-day run sheet
The day before: forms built, braced, and oiled; base compacted and pinned to grade; rebar or mesh placed on chairs; tools staged (screed board, bull float, edger, groover, broom, gloves, boots); helpers confirmed. Truck morning: dampen the base so it does not steal water from the mix, then place concrete starting at the far corner and work toward the chute. Screed flat immediately, bull float once to push aggregate down, and then wait: bleed water must evaporate before finishing, and troweling it back in makes a weak, dusty surface. When the sheen leaves, edge the perimeter, cut or tool the control joints, broom for traction, and start curing: keep the slab damp (cure compound, mist, or plastic) for the first week. The window from truck to broom is a few hours; the schedule is the concrete’s, not yours.
Control joints: telling concrete where to crack
Concrete shrinks as it cures, and shrinking slabs crack somewhere. Control joints are you choosing where: straight, tooled or saw-cut lines at least one quarter of the slab depth, spaced roughly 24 to 30 times the thickness in inches, which for a 4-inch slab means panels every 8 to 10 feet. Cut them into the wet surface or saw within the first day; wait longer and the slab publishes its own opinion as a wandering diagonal. Keep panels close to square, avoid interior corners without a joint radiating from them, and the “cracks” your patio develops will all be the tidy ones you drew.
Weather windows: hot, cold, and wet
Concrete is chemistry on a clock, and weather changes the clock speed. Hot, dry, or windy days accelerate surface drying faster than the slab beneath, inviting plastic shrinkage cracks: pour at dawn, dampen the base, use windbreaks or misting, and finish promptly. Cold is the opposite problem: below roughly 40 degrees the reaction crawls, and freezing before the concrete gains strength ruins it, so use blankets, consider an accelerator, and never pour on frozen ground. Rain on a fresh, open surface washes cement paste out of the top layer; if the sky is arguing, have plastic sheeting staged and be ready to cover after finishing. None of this is exotic, it is just deciding the date with the forecast instead of the calendar.
Rent, buy, or borrow: the tool list
Rent the expensive, occasional things: the mixer if you insist on bags, a plate compactor for the base, a power screed on big slabs. Buy the cheap hand tools you will keep: a magnesium float, an edger, a groover, a stiff broom, gloves, and rubber boots, together well under the cost of one delivery fee. Borrow the extra wheelbarrow and the extra friend; both are load-bearing on pour day, and only one accepts pizza as payment.
Edge cases: posts, pads, and thick slabs
A few jobs bend the standard math. Fence posts use bagged concrete by the hole, not the yard: a typical 8-inch hole at 2 feet deep holds about 0.7 cubic feet, one generous 80-pound bag per post, two for gate posts or loose soil, and many builders pour it dry and let ground moisture cure it. Equipment pads (AC units, hot tubs, generators) run 4 inches for light gear and 5 to 6 with reinforcement under real weight; a filled hot tub can exceed 5,000 pounds, which is footing territory, not patio territory. Garage slabs want 6 inches with mesh and thickened edges where the car actually rolls. The volume formula never changes across any of these, only the depth decision and the reinforcement, which is exactly why the calculator asks for depth first.
Frequently asked questions
How long until I can walk and drive on it?
Foot traffic in 24 to 48 hours, vehicles after about 7 days, full design strength near 28 days. Keeping it damp for the first week meaningfully improves the final product.
How heavy is a cubic yard?
About 4,000 pounds wet, two tons per yard. This is why trucks, chutes, and pumps exist and why “we’ll just wheelbarrow it” deserves a long pause.
Do I need rebar in a patio?
A 4-inch patio on well-compacted base is commonly poured with mesh or fiber rather than rebar. Rebar earns its cost at edges, spans, driveways, and anywhere real loads live.
Can I pour half today and half tomorrow?
Not within one slab without creating a cold joint. If the pour must be split, split it at a planned control joint so the seam is a design feature instead of a flaw.
Bags say “yields 0.60 cu ft” but my slab came up short. Why?
Yield assumes full mixing and no spillage; real-world yield runs a few percent lower, which the waste factor exists to absorb.
What PSI mix should I order?
3,000 PSI covers patios, walkways, and shed slabs; 3,500 to 4,000 suits driveways and anything freeze-thaw exposed; garage and structural work follows local code. Higher PSI costs a little more per yard and buys durability where tires, loads, and winters live.
Size the pour with the concrete calculator, the base with the cubic yard calculator, and when the project moves above grade, the roof pitch guide and lumber math guide pick up where the slab ends. The rest of the construction calculators cover the materials in between.