Concrete Calculator

How poured concrete volume is worked out

A concrete pour starts as one shape: length times width times thickness. Measure the length and width in feet and the thickness in inches, then divide the thickness by 12 to put it in feet too. Multiply the three and the answer is cubic feet. Divide by 27 and it is cubic yards, the unit ready-mix trucks sell by.

For a 10 by 10 foot patio at 4 inches thick, the math is 10 x 10 x 4 / 12 = 33.33 cubic feet. In yards that is 33.33 / 27 = 1.23 cubic yards. The same slab at 6 inches thick would be 10 x 10 x 6 / 12 = 50 cubic feet, and at 3 inches it would be 25 cubic feet, so every added inch of thickness on this footprint costs 8.33 cubic feet of concrete.

Thickness drives the numbers more than people expect, because it multiplies everything else. A 4 inch patio needs 33.33 cubic feet per 100 square feet, and the same footprint at 6 inches needs 50 cubic feet, half as much concrete again. Do not shave thickness to save bags: 4 inches is the working minimum for a patio that carries foot traffic and furniture, and driveways commonly go to 5 inches where vehicles park. Quikrete's concrete mix is made for pours 2 inches thick or more, so patches thinner than that call for a different product.

Ready-mix plants also quote by the yard and add a short-load fee for small orders. One cubic yard equals 27 cubic feet, which is 45 eighty-pound bags, 60 sixty-pound bags or 90 forty-pound bags, so the bag count tells you when to switch: a patio at 62 bags is a heavy weekend with a mixer, and a garage slab at 440 bags belongs on a truck.

Worked example: a 10 by 10 foot patio slab

Take that 10 by 10 patio again. The volume is 33.33 cubic feet, or 1.23 cubic yards, too small for a ready-mix order and a natural fit for bags. An 80 lb bag of standard concrete mix yields about 0.60 cubic feet once water is added, so the raw count is 33.33 / 0.60 = 55.56, call it 56 bags.

Nobody buys 0.56 of a bag, and no pour uses exactly what the geometry promises. Add a waste allowance and the count moves up: with 10 percent extra, the patio needs 33.33 x 1.1 = 36.67 cubic feet, and 36.67 / 0.60 = 61.11, so 62 bags go home from the store. At a typical big-box price of $5.50 a bag, 62 x 5.50 = 341.00 dollars for the patio mix alone.

The weight adds up faster than the dollars. Sixty-two 80 lb bags weigh 62 x 80 = 4960 lb of dry mix, about 2.48 tons, which is two loads in a half-ton pickup and a hard hour with a mixer. Renting a small mixer and doing it in sessions is the normal path for a pour this size, and each 80 lb bag wants about 6 pt of water to start, per Quikrete's data sheet.

A sidewalk shows the same steps on a longer shape. Four feet wide, forty feet long, four inches thick: 4 x 40 x 4 / 12 = 53.33 cubic feet, 1.98 cubic yards. With 10 percent waste that is 58.67 cubic feet, so 58.67 / 0.60 = 97.78, meaning 98 eighty-pound bags. In sixty-pound bags the same 58.67 cubic feet needs 58.67 / 0.45 = 130.38, so 131 bags, and the smaller bags cost more per cubic foot every time.

Bag sizes and yields

Bags of concrete mix come in three common sizes, and each yields a fixed volume once mixed. Quikrete's spec data for its standard Concrete Mix 1101 lists the yields: a 40 lb bag about 0.30 cubic feet, a 60 lb bag about 0.45 cubic feet, and an 80 lb bag about 0.60 cubic feet. Those three numbers are all the bag math you need. Quikrete publishes the full spec data sheet for Concrete Mix 1101.

One cubic yard is 27 cubic feet, so 27 / 0.30 = 90 forty-pound bags, 27 / 0.45 = 60 sixty-pound bags, and 27 / 0.60 = 45 eighty-pound bags. The bigger the bag, the fewer trips with the wheelbarrow, and the price per cubic foot usually drops too. Water per bag scales the same way: an 80 lb bag takes about 6 pt of water to start, with a final range of 6 to 9 pt, and the spec sheet's table scales that down for the smaller sizes.

The yields above come from the manufacturer's published data for one standard mix, so treat them as the planning numbers they are. High-strength mixes, crack-resistant blends and fast-setting products all yield slightly different volumes, and every brand prints its own sheet. If your bag states a different yield, divide your volume by that number and round up.

The waste allowance: why 10 percent

The waste allowance covers what the geometry cannot see: uneven subgrade that makes one corner deeper than the plans, spillage off the screed, mix stuck to the mixer drum, and short pours that leave a wheelbarrow half full. Ten percent is the standard planning number for a slab with square corners and honest grading. Use five percent for a small, tidy rectangle and fifteen when the form has curves, notches or thickened edges.

On the patio, the allowance moved the count from 56 to 62 bags, six bags of slack for 33.33 cubic feet of geometry. Order the slack. Running out mid-pour leaves a cold joint, and a cold joint in a slab is a future crack with a schedule attached. If the extra bags sit unopened, most stores take them back.

Footings and the code minimums

Footings carry the load of a wall, pier or deck post into the soil, and the building code sets a floor under their size. The International Residential Code, R403.1.1, requires concrete footings to be at least 12 inches wide and 6 inches thick, and R403.1.4 puts the bottom of an exterior footing at least 12 inches below finished grade and below the local frost line. Solid rock is the one exception listed. Local amendments tighten these numbers in many jurisdictions, so confirm before you dig. The IRC text is public through up.codes, IRC 2021 chapter 4.

A deck footing run shows the volume math on a long, narrow shape. Forty feet of run at 24 inches wide and 10 inches thick: 40 x 2 x 10 / 12 = 66.67 cubic feet, which is 66.67 / 27 = 2.47 cubic yards. With 10 percent waste, 66.67 x 1.1 = 73.33 cubic feet, and 73.33 / 0.60 = 122.22, so 123 eighty-pound bags. That is well past the point where a ready-mix delivery pays for itself, which is exactly what the numbers are for.

The table works for planning a fence or deck line: read off the bags per 10 feet and scale to your run. Fence posts are a different shape, round holes with a post subtracted, and the post hole concrete calculator handles those. Every footing row in the table clears the IRC minimums above it, and frost depth, not bag count, is usually what decides the real dig depth in cold climates.

Common slab sizes at a glance

Common pours, common answers. Every row below uses 10 percent waste and eighty-pound bags, so read it as the shopping list for each shape. The walkway is a morning with a mixer, the patio is a full weekend, and the garage slab is a ready-mix order of nearly 9 yards.

The garage row is the one to sit with. 440 eighty-pound bags is 35,200 lb of dry mix, 17.6 tons, moved bag by bag, mixed bag by bag, inside a working window of a couple hours before concrete stiffens. Nobody does that. The crossover lands near 2 to 3 yards for most people: below it, bags win on cost and convenience; above it, the short-load fee on a small truck order is cheaper than the labor it replaces.

Read the walkway row as a units check too: 4 x 8 feet at 4 inches gives 4 x 8 x 4 / 12 = 10.67 cubic feet, and 10.67 x 1.1 = 11.74, so 11.74 / 0.60 = 19.56, rounded to 20 bags. Every other row in the table is the same arithmetic with different dimensions, which is all the calculator above automates.

A quick crossover table, in plain eighty-pound bags with no waste, makes the decision obvious:

Pouring basics: joints, water, cure

Volume is one line item of a pour. Quikrete's application notes, on the same spec sheet as the yields, describe the rest: dampen the subgrade before placing, fill the form to full depth and strike off with a straight board, then cut control joints 1 inch deep every 6 to 8 feet so cracks form where you put them. Protect the pour from freezing for the first 48 hours.

Strength develops on a schedule, not at once. The same data sheet lists 2500 psi of compressive strength at 7 days and 4000 psi at 28 days for the standard mix, tested under ASTM C387. Cure time is why the weekend of pouring turns into a month before a vehicle sits on a new slab, and why the 28-day figure is the one on drawings.

Wet concrete is caustic and it moves fast in a mixer. Wear gloves and eye protection, keep the mixer on flat ground, and never reach into it. The hour or two of open working time goes faster on a hot day, so stage tools, water and helpers before the first bag opens.

What the bag count leaves out

The count above is plain volume math. It does not include the gravel base under a slab, form lumber, stakes, wire mesh or rebar, expansion joint against an existing slab, finishing tools, or delivery. Steel is its own quantity: run the bar schedule through the rebar material calculator, and slab mesh through the concrete reinforcing mesh calculator before you order anything.

Three neighboring tools cover the shapes this page does not. This calculator is for poured concrete in rectangular forms: slabs, pads and strip footings, priced by volume and bags. The concrete block calculator instead counts CMU blocks for a wall, with mortar and openings, which is masonry rather than a pour. The post hole concrete calculator works round holes with a post in them, and the driveway cost page prices a finished driveway job, excavation to sealing, rather than raw material. For soil, mulch and gravel volumes, the cubic yard page is the general tool.