Concrete calculator

Multiply length × width × depth with every dimension in feet, then divide by 27 to get cubic yards, which is the unit every ready-mix plant quotes in. The trap is thickness: a 4-inch slab is 4 ÷ 12 = 0.333 feet deep, not 4, and getting that conversion wrong overstates the order twelve-fold. As a sanity check, one cubic yard covers 81 square feet at 4 inches thick, 65 at 5 inches and 54 at 6 inches. The calculator above handles six shapes, slab, post hole, footing, column, stairs and curb, and adds a 10% waste allowance to whatever it works out.

Length × width × thickness, all converted to feet, then divided by 27. A 10 × 10 slab at 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, which is 1.23 cubic yards before waste and 1.36 after the standard 10% allowance. A 20 × 20 slab at the same thickness comes to 5.43 cubic yards, or 245 eighty-pound bags. Set the shape to slab in the concrete calculator above and it returns yards, bag counts for all three bag sizes and the delivered cost side by side.

Divide the volume in cubic feet by the bag's published yield. An 80 lb bag of QUIKRETE or Sakrete concrete mix yields 0.6 cubic feet, a 60 lb bag 0.45, and a 40 lb bag 0.3. A 10 × 10 slab at 4 inches needs 62 eighty-pound bags including waste; a 20 × 20 slab needs 245. The calculator above shows all three counts at once, because what decides it at the store is which size is stocked and how many trips your car is making, 245 eighty-pound bags is 9.8 tons of material to load, carry and open.

A cubic yard is 27 cubic feet, so at published yields it takes 45 eighty-pound bags, 60 sixty-pound bags or 90 forty-pound bags. That is also the clearest way to see the price difference between supply routes: a yard bought in 80 lb bags costs about $293 against $155 delivered. Bags buy convenience and a flexible schedule, not a lower price, which is why they stop making sense above roughly 2 cubic yards.

Ready-mix concrete costs about $155 per cubic yard delivered at national-average rates, and most plants add a short-load fee near $75 on any order under 5 cubic yards. As of July 2026, ready-mix prices sit about 9% above their five-year average on the Producer Price Index, so a quote you were given a couple of years ago understates what a plant charges today. Bagged concrete is the expensive way to buy the same yard: 45 eighty-pound bags at $6.50 each works out to $293 per cubic yard, close to double the delivered rate. Delivery distance, mix strength and how busy the plant is all move the quote 10-20% either side of the average.

A 20 × 20 slab is 400 square feet and takes 5.43 cubic yards at 4 inches thick including waste. That is about $842 in delivered ready-mix plus roughly $1,400 in placing and finishing labor at national-average rates, call it $2,242, or $5.60 per square foot. On top of that, budget $200 to $600 for a compacted gravel base, plus forms, reinforcement and any excavation or grading the site needs. Labor is the half that moves with your location: select your state in the calculator above to apply the local rate.

A 30 × 40 slab is 1,200 square feet and takes 400 cubic feet of concrete, 14.81 cubic yards geometrically, or 16.30 yards with the standard 10% waste allowance. Order 16.50 yards, since plants sell in quarter-yard increments. In bags that would be 734 eighty-pound bags, 29 tons to carry, cut open and mix, which is why nobody pours a slab this size from bags. Delivered ready-mix for that volume runs about $2,526 nationally, plus roughly $4,200 in placing and finishing labor.

Ten cubic yards covers about 810 square feet at 4 inches thick: 10 × 27 = 270 cubic feet, divided by the 0.333-foot depth. The same 10 yards covers 648 square feet at 5 inches and 540 at 6 inches. One caveat worth applying: that is finished coverage with no margin, so plan a slab of about 736 square feet against a 10-yard order once the standard 10% allowance is kept back.

Yes, if the slab is small. Labor is the larger half of professional flatwork: on the 20 × 20 slab above, $1,400 of the roughly $2,242 total is placing and finishing, leaving $842 of delivered ready-mix, or $1,593 in eighty-pound bags, if you do the work yourself. The catch is that concrete sets on its own schedule rather than yours. Everything has to be placed, screeded, floated and edged before it goes off, which is genuinely difficult past about 2 cubic yards with a wheelbarrow and one helper, and a 20 × 20 slab is 5.43 yards. DIY suits a shed pad, a small patio, post holes and footings; a driveway or garage floor is worth paying a crew for.

Yes for almost any slab that matters. A 4-inch compacted gravel base gives the slab uniform bearing so it does not crack over soft spots, breaks the capillary path that would otherwise wick groundwater up into the concrete, and gives water somewhere to go through a freeze-thaw cycle. Over clay or poorly draining subgrade, 6 inches is the usual specification. Budget $0.50 to $1.50 per square foot installed, $200 to $600 on a 20 × 20 slab, which is cheap insurance against tearing out a cracked one. The exception is a site whose subgrade is already well-draining, compacted sand or gravel; there the base is effectively in place already.

Use the same formula as the concrete: length × width × base depth in feet, divided by 27. A 20 × 20 slab with a 4-inch base needs 4.94 cubic yards of gravel; at 6 inches it takes 7.41 yards. Delivered crushed stone runs $30 to $50 per cubic yard, so that base is $148 to $247 in material before spreading and compacting. Order 10-15% over the calculated figure. That number is the compacted volume, and crushed stone loses roughly that much bulk under a plate compactor.

A 12-inch Sonotube holds 0.79 cubic feet of concrete per foot of height, so a 4-foot pier takes 3.5 cubic feet with waste, about 6 eighty-pound bags. The other common sizes: 0.35 cubic feet per foot at 8 inches, 0.55 cubic feet per foot at 10 inches, 1.40 cubic feet per foot at 16 inches. Volume rises with the square of the diameter, which is why stepping from a 10-inch form to a 12-inch one adds 44% more concrete rather than 20%. Set the calculator above to the column shape for any diameter and height, and if the piers are carrying a deck, our deck calculator prices the structure sitting on top of them.

A standard 10-inch-wide hole 30 inches deep holds about 1.2 cubic feet once the 4x4 post displaces its share, 1.9 eighty-pound bags, so 2 per post in practice. Depth is what moves the number: the same hole takes 2.3 bags at 36 inches and 3.1 at a 48-inch frost line. A 150-foot fence needs about 20 posts, so plan on 43 to 68 bags with the waste allowance in, $280 to $442 of material. Set the shape above to post hole, it subtracts the post from the cylinder, which most calculators do not, and our fence calculator prices the whole fence including the posts and concrete.

Poor man's concrete usually means one of three things: urbanite, broken-up old slab reused as paving; a lean mix with less Portland cement than a bagged concrete mix; or a dry pour, where dry mix is spread into a form and left for ground moisture and rain to hydrate. All three are cheaper than the roughly $155 per cubic yard delivered ready-mix costs, and none belongs anywhere structural. A dry pour in particular cures unevenly and reaches a fraction of its rated compressive strength, because the water never reaches the middle of the slab in a controlled way. Where these approaches genuinely work is garden paths, stepping stones, mow strips and non-load-bearing fill, not a driveway, a garage floor, a footing or anything a building will sit on.

Ten percent is the standard allowance, and the calculator above applies it automatically. It is not padding: subgrade is never perfectly level, forms bow outward slightly under the weight of wet concrete, and some is always lost to the wheelbarrow and the chute. Ordering the exact calculated volume reliably leaves you short, and being short is far more expensive than being over, because a partial second load costs the $75 short-load fee on top of a few yards, and a cold joint forms where the fresh concrete meets concrete that has already started to set.

Cement is one ingredient in concrete, not a synonym for it. Concrete is Portland cement plus sand, gravel aggregate and water; the cement is the binder that holds the aggregate together, and it is 10-15% of the mix by volume. Buying a bag labelled "cement" or "mortar mix" when you need "concrete mix" is a common and frustrating trip back to the store, for slabs, footings, columns and post holes you want concrete mix, which already has the aggregate in it.

Concrete is usually walkable in 24 to 48 hours, ready for vehicle traffic at about 7 days, and reaches roughly 90% of its design strength at 28 days. The age every strength specification is written against. Curing is a chemical reaction rather than drying out, which is why keeping the surface damp or covered for the first several days genuinely produces stronger concrete than letting it dry fast in the sun. Removing forms too early or driving on a five-day-old slab are the two most common ways a good pour gets damaged.