Concrete Calculator
Calculate exactly how much concrete you need for slabs, footings, columns, and stairs. Get results in cubic yards, number of bags, and estimated cost so you can order the right amount for your project.
How to Calculate Concrete for a Slab
Pouring a concrete slab is one of the most common DIY and professional construction projects. Whether it's a patio, garage floor, sidewalk, or shed pad, the calculation starts the same way: measure length, width, and thickness, then convert everything to the same unit before multiplying.
Most residential slabs are 4 inches thick. That's the standard for patios, walkways, and garage floors that only support foot traffic and passenger vehicles. Driveways that handle heavier loads should be 5 to 6 inches thick. If you're parking an RV or heavy equipment, go with 6 inches minimum. Commercial and industrial slabs run 6 to 8 inches or more, often with rebar reinforcement.
Here's where most people get tripped up: thickness is usually measured in inches, but the volume formula needs everything in feet. Four inches is one-third of a foot, or 0.333 feet. A 10-by-10-foot patio that's 4 inches thick needs 10 times 10 times 0.333, which equals 33.3 cubic feet. Divide by 27 to get cubic yards: about 1.23 cubic yards.
Always add a waste factor. The standard recommendation is 10 percent for straightforward rectangular pours. If the subgrade is uneven, the forms aren't perfectly level, or the shape has curves and angles, bump it to 15 percent. Running short mid-pour is far worse than having a little left over. That leftover concrete can fill a small form for a stepping stone or go into a wheelbarrow for disposal.
For ready-mix delivery, most concrete companies have a minimum order of one cubic yard, and many charge a short-load fee for orders under five or six yards. A standard concrete truck carries about 10 cubic yards. If your project needs less than one yard, bagged premix from the hardware store might be more practical. One 80-pound bag yields about 0.6 cubic feet of concrete, so a one-yard project would require roughly 45 bags. That's a lot of mixing by hand, but it avoids the short-load surcharge.
Concrete for Footings and Foundations
Footings are the below-grade concrete structures that support walls, posts, and entire buildings. They distribute the weight of the structure over a larger area of soil, preventing settling and structural movement. Getting the volume right matters because under-ordering means a cold joint, and over-ordering means paying for concrete you don't need.
A typical residential wall footing is 16 to 24 inches wide and 8 to 12 inches deep, running the full perimeter of the structure. Post footings, also called pier footings, are square or round pads that support individual posts, like those for a deck. They're usually 16 to 24 inches in diameter and 8 to 12 inches thick, though building codes in your area may specify exact dimensions based on soil type and frost depth.
To calculate a continuous footing, think of it as a long, narrow slab. Measure the total linear length of the footing in feet, the width in feet, and the depth in feet. For an L-shaped foundation, add up all the straight segments. A house with a 40-by-30-foot footprint has a perimeter of 140 linear feet. If the footing is 20 inches wide and 10 inches deep, that's 1.667 feet by 0.833 feet. Volume equals 140 times 1.667 times 0.833, which is about 194 cubic feet or 7.2 cubic yards.
Frost depth determines how deep footings must go in cold climates. In Minnesota, footings need to be 42 inches below grade. In Georgia, 12 inches is often sufficient. Your local building department will tell you the required frost depth for your area. The concrete calculator handles the volume, but you're responsible for meeting code requirements on depth and reinforcement.
For stepped footings on sloped sites, calculate each level separately and add them together. The transitions between levels usually add 10 to 15 percent to the basic volume, so increase your waste factor accordingly.
Round Columns and Tube Forms
Round concrete columns are formed using cylindrical cardboard tubes, commonly sold under brand names like Sonotube. They're used for deck footings, fence posts, sign foundations, and any application where a cylindrical pier is needed. The tubes come in standard diameters from 6 inches to 24 inches for residential use, with larger sizes available for commercial work.
The volume of a cylinder is pi times the radius squared times the height. A 12-inch diameter tube has a 6-inch radius, which is 0.5 feet. If the tube is 4 feet tall, the volume per column is 3.14159 times 0.25 times 4, which equals 3.14 cubic feet. For four columns, that's 12.57 cubic feet, or about 0.47 cubic yards.
A common mistake with tube forms is forgetting to account for the bell at the bottom. Many building codes require that the base of the column flare out into a wider footing pad to distribute the load. If your pier sits on a 24-inch-wide by 8-inch-thick pad, add the pad's volume to the cylinder's volume for each column.
Smaller projects with just a few columns are often better served by bagged premix. Four 12-inch columns that are 4 feet deep need about 12.6 cubic feet of concrete, which is roughly 21 bags of 80-pound mix. That's manageable for a weekend project with a wheelbarrow and a helper. But if you're pouring eight or more columns, or if each one is 18 inches or wider, ordering ready-mix becomes worth the delivery fee.
Pro tip: when filling tube forms, pour the concrete in lifts of about 12 inches and use a piece of rebar or a stick to rod each lift, pushing it up and down to remove air pockets. This consolidation step prevents honeycombing, which are voids in the finished concrete that weaken the column. After filling, strike the top level with a trowel and cover it with plastic to cure slowly.
Ordering and Estimating Concrete Costs
Getting the order quantity right saves money and headaches. Too little concrete means you're scrambling for bags to finish the pour before the first batch sets up, leaving a weak cold joint. Too much means you're paying for excess material and figuring out where to dump it. The sweet spot is 5 to 10 percent more than the calculated volume for most projects.
Ready-mix concrete is sold by the cubic yard. Prices vary significantly by region, mix design, and delivery distance. As a rough range, expect to pay $120 to $200 per cubic yard for a standard 3,000 PSI residential mix. High-strength mixes (4,000 to 5,000 PSI), fiber-reinforced mixes, or colored concrete cost more. Most companies charge a delivery fee on top of the per-yard price, and short-load fees apply to orders below their minimum, typically three to five yards.
Bagged premix is more expensive per cubic foot but eliminates delivery fees and minimums. An 80-pound bag costs $5 to $7 at most hardware stores and yields about 0.6 cubic feet. That works out to roughly $225 to $315 per cubic yard if you're mixing by hand, compared to $120 to $200 delivered. The crossover point where ready-mix becomes cheaper is usually around one to two cubic yards, depending on local pricing.
Beyond material cost, factor in these expenses: gravel base material at $30 to $50 per cubic yard, rebar or wire mesh at $0.50 to $0.75 per square foot, form lumber at $3 to $8 per linear foot, and a concrete pump if the truck can't reach the pour site at $150 to $300 per hour. Labor for professional flatwork typically runs $4 to $8 per square foot for the finishing work alone.
When calling the ready-mix plant, tell them your project type, the total cubic yards, the day and time you want delivery, and whether you need any additives. They'll recommend the right PSI rating and slump for your application. Order a day or two in advance, because concrete plants can be booked solid during busy season. And have enough people on site when the truck arrives, because ready-mix waits for no one. Most companies charge standby time after 5 to 7 minutes per yard.
Concrete Volume Formulas
Slab: V = L × W × T | Column: V = πr² × H × n | Total = V × (1 + waste%)
For slabs and footings, volume equals length times width times thickness. Convert thickness from inches to feet by dividing by 12 before multiplying. For round columns, volume equals pi times the radius squared times the height, multiplied by the number of columns. For stairs, each step is calculated as a rectangular solid and stacked: the bottom step supports the full height of all steps above it, so each step's volume includes the concrete beneath it. The waste factor adds a percentage on top to cover spillage, uneven subgrade, and form irregularities. One cubic yard equals 27 cubic feet. One 80-pound bag of premix concrete yields approximately 0.6 cubic feet when mixed.
Where:
- L = Length of the project
- W = Width of the project
- T = Thickness or depth
- r = Radius of round column (diameter / 2)
- H = Height of column
- n = Number of columns
Example Calculations
Patio Slab
A 12 ft by 10 ft patio, 4 inches thick, with 10% waste factor.
Volume = 12 ft x 10 ft x (4/12) ft = 40 cu ft = 1.48 cu yd. Adding 10% waste: 1.48 x 1.10 = 1.63 cu yd. At $150 per yard, that's about $244. You'd need approximately 74 bags of 80-lb premix if mixing by hand.
Deck Pier Footings
Six 12-inch diameter round columns, each 3.5 feet deep.
Each column: π x (0.5 ft)² x 3.5 ft = 2.75 cu ft. Six columns: 16.49 cu ft = 0.61 cu yd. Adding 10% waste: 0.67 cu yd. This is a manageable hand-mix project with about 31 bags of 80-lb premix.
Frequently Asked Questions
One 80-pound bag of premix concrete yields approximately 0.6 cubic feet when mixed. One 60-pound bag yields about 0.45 cubic feet. To find the number of bags, calculate the total volume in cubic feet (including waste), then divide by the yield per bag. For example, 30 cubic feet divided by 0.6 equals 50 bags of 80-pound mix. For larger projects over one cubic yard (27 cubic feet), ready-mix delivery is usually more economical.
Standard thickness for residential slabs is 4 inches for patios, walkways, and shed pads. Driveways should be 5 to 6 inches to handle vehicle weight. Garage floors are typically 4 inches with thickened edges at 8 to 12 inches where walls or columns bear. Heavy-duty applications like RV pads or shop floors should be at least 6 inches thick with rebar reinforcement. Always check local building codes, as requirements vary by region and soil conditions.
Ready-mix concrete typically costs $120 to $200 per cubic yard for a standard 3,000 PSI residential mix, depending on your region and the delivery distance. Delivery fees usually add $50 to $100 per load. Many plants charge a short-load fee of $50 to $100 for orders below their minimum, which is usually three to five cubic yards. Specialty mixes like colored concrete, high-strength, or fiber-reinforced cost $20 to $50 more per yard.
The waste factor accounts for concrete lost to spillage, uneven subgrade, slight over-excavation, and form movement during the pour. A standard 10 percent waste factor works for most flat slab projects with well-prepared forms. Increase to 15 percent for uneven ground, complex shapes, or remote pours where you can't easily get more material. Reduce to 5 percent for precision projects with laser-leveled forms and experienced crews.
For projects under about 0.5 cubic yards (roughly 14 cubic feet or 23 bags of 80-lb mix), bagged premix is usually more practical and often cheaper after you factor in delivery and short-load fees. Between 0.5 and 1.5 cubic yards, it depends on your budget and willingness to mix by hand. Beyond 1.5 cubic yards, ready-mix is almost always the better choice for both cost and quality. Hand-mixing that much concrete is exhausting and makes it hard to maintain consistent quality across the pour.