Measure The Surface Before You Plan The Pour
Use this Concrete Square Foot Calculator to measure slab, patio, driveway, garage, sidewalk and pad area in square feet, then convert that surface area into cubic feet, cubic yards, approximate bag quantities and optional ready-mix material cost using your planned concrete thickness.
Calculate rectangular, multi-section or circular concrete area, or start with a known square-foot figure and convert it into a concrete volume estimate.
Enter length and width to calculate square feet, then add slab thickness for concrete volume.
Finished concrete length
Finished concrete width
Use the specified/project thickness
Enter a local supplier rate if desired
This tool estimates material quantities from dimensions you enter. Slab thickness, reinforcement, base preparation and structural requirements should follow the project design, local requirements and applicable specifications.
Split an L-shaped or irregular project into as many as four rectangles and add the areas automatically.
Calculate the surface area and concrete volume of round pads, circular bases and other full circles.
Already know your project area? Enter square feet and thickness to estimate cubic feet and cubic yards.
A Concrete Square Foot Calculator answers two related questions that are often mixed together on a jobsite: how much surface area will be covered and how much concrete will that area require. Square feet describe surface area only. A 20 ft × 10 ft patio is 200 square feet whether it is 3 inches or 8 inches thick. The amount of concrete changes only when thickness is introduced.
For that reason, this page calculates area first and concrete volume second. In the rectangle mode, length is multiplied by width. In the circular mode, the area is based on the radius of the circle. In the multi-section mode, each rectangle is calculated independently before all section areas are added together. The known-area mode skips the geometry step when you already have a square-foot measurement from a plan, takeoff, survey or another calculator.
Once square footage is known, the calculator converts thickness from inches to feet, multiplies area by thickness, and displays cubic feet. Cubic feet are then divided by 27 to estimate cubic yards for ready-mix planning. An optional ordering allowance can be applied to the calculated volume to help with normal field variation, but the appropriate quantity to order should still be verified against actual site conditions and the concrete supplier's ordering requirements.
This determines the two-dimensional surface area of a rectangular concrete project.
Thickness must be converted to feet before multiplying it by square footage.
Measure patios, garages, driveways, walkways, equipment pads and other flatwork.
Convert surface area into concrete volume by entering the planned slab thickness.
See exact geometric cubic yards and an optional adjusted ordering quantity.
Estimate bag counts and optional material cost using a price you supply.
The most important concept when using a concrete square footage calculator is the difference between area and volume. Square feet (ft²) measure a surface. Cubic feet (ft³) measure a three-dimensional volume. Cubic yards (yd³) are commonly used when discussing ready-mix concrete quantities in the United States.
If a supplier asks how many cubic yards you need, square footage by itself is not enough. The thickness of the concrete must also be known. A 500-square-foot slab at 4 inches thick and a 500-square-foot slab at 6 inches thick cover exactly the same surface area, but the 6-inch slab contains 50% more concrete because its thickness is 50% greater.
| Measurement | What It Describes | Typical Concrete Use | Example |
|---|---|---|---|
| Square Feet (ft²) | Surface area | Slab size, sealer coverage, finishing area | 20 ft × 10 ft = 200 ft² |
| Cubic Feet (ft³) | Volume | Bag yield and small-project volume | 200 ft² × 4/12 ft = 66.67 ft³ |
| Cubic Yards (yd³) | Ready-mix volume | Truck / supplier ordering | 66.67 ÷ 27 = 2.47 yd³ |
| Square Yards (yd²) | Area in yards | Area comparison | 200 ft² ÷ 9 = 22.22 yd² |
Accurate square footage begins with the actual finished concrete dimensions. For a new slab, measure the inside dimensions of the forms or the intended finished edges. For an existing slab, measure from edge to edge. Do not automatically use excavation dimensions: excavation may be wider than the finished concrete because of formwork, drainage, aggregate base, working room or edge preparation.
Decide whether the project is a rectangle, circle or combination of simpler shapes.
Use finished dimensions in feet. Convert inches to decimal feet where necessary.
Multiply length × width for each rectangular section or use the circle formula.
Use the thickness specified for the actual project rather than choosing it from volume alone.
Review volume, site variation, supplier minimums, delivery conditions and ordering increments.
The surface area is 24 × 12 = 288 square feet. Four inches equals 0.3333 feet. Multiplying 288 ft² × 0.3333 ft gives approximately 96 cubic feet. Dividing by 27 gives approximately 3.56 cubic yards before any optional ordering allowance.
The surface area is 600 square feet. Five inches equals approximately 0.4167 feet. 600 × 0.4167 gives about 250 cubic feet. Dividing by 27 produces approximately 9.26 cubic yards of exact geometric volume.
Assume the main rectangle is 20 ft × 10 ft and the extension is 8 ft × 6 ft. The first area is 200 ft² and the second is 48 ft². The combined project is therefore 248 square feet. The important step is to add the two real rectangles rather than surrounding the entire L-shape with one oversized rectangle that includes empty space.
A 12 ft diameter circle has a 6 ft radius. The area is π × 6², or approximately 113.10 square feet. If the pad is 4 inches thick, the exact geometric volume is approximately 37.70 cubic feet or 1.40 cubic yards.
Square footage can be useful long before concrete is ordered. It helps compare slab sizes, estimate finishing labor, review formwork layouts, plan curing material coverage and communicate project scale. The table below illustrates common measurement approaches; it does not prescribe slab thickness or structural design.
| Project | Area Method | Useful Inputs | Important Note |
|---|---|---|---|
| Patio | Length × width or multiple sections | Finished edges, steps, curves | Calculate irregular additions separately |
| Driveway | Length × average / actual width | Apron, flare, parking area | Separate sections if widths vary significantly |
| Garage Floor | Inside slab dimensions | Wall line, thickened edges | Edge details may add volume beyond flat slab area |
| Shed / Equipment Pad | Length × width | Pad footprint, equipment loads | Thickness and reinforcement may be design-specific |
| Sidewalk | Total length × width | Multiple runs, curves, landings | Calculate landings separately where needed |
| Round Pad | π × radius² | Diameter or radius | Use the full finished diameter |
The same surface area can require very different concrete quantities depending on slab thickness.
A cubic yard does not cover one fixed number of square feet because coverage depends on thickness. One cubic yard contains 27 cubic feet. To find approximate coverage, divide 27 cubic feet by the slab thickness expressed in feet. The thinner the slab, the more square feet one cubic yard can cover; the thicker the slab, the fewer square feet it covers.
| Thickness | Thickness in Feet | Approx. ft² per 1 yd³ | Planning Use |
|---|---|---|---|
| 2 in | 0.1667 ft | Approx. 162 ft² | Area/overlay comparisons only; verify project specification |
| 3 in | 0.25 ft | Approx. 108 ft² | Coverage conversion |
| 4 in | 0.3333 ft | Approx. 81 ft² | Common estimating reference |
| 5 in | 0.4167 ft | Approx. 64.8 ft² | Coverage conversion |
| 6 in | 0.50 ft | Approx. 54 ft² | Coverage conversion |
| 8 in | 0.6667 ft | Approx. 40.5 ft² | Thicker slab volume comparison |
The answer depends entirely on thickness. At 4 inches, 100 square feet contains about 33.33 cubic feet, or 1.23 cubic yards before any added allowance. At the same thickness, 500 square feet contains about 166.67 cubic feet, or 6.17 cubic yards. One thousand square feet at 4 inches contains about 333.33 cubic feet, or 12.35 cubic yards.
At 6 inches, each of those quantities increases by 50% because 6 inches is 1.5 times the thickness of 4 inches. That relationship is why a square-foot calculator that ignores thickness cannot provide a complete concrete order estimate.
Bagged mixes can be practical when the total concrete volume is small, truck access is limited, or a project is being completed in stages. Bag yield varies by product, so this calculator uses approximate planning yields only. Always use the actual yield printed by the manufacturer when purchasing.
Ready-mix is commonly considered for larger slabs because concrete arrives batched and ready for placement. The quantity is typically discussed in cubic yards rather than square feet. Delivery conditions, minimum quantities, short-load charges, access, placement rate and supplier scheduling can all affect the job.
Geometric calculations assume perfectly uniform dimensions. Real slabs are rarely perfect rectangular prisms. Excavation and subgrade can vary, forms can move slightly, edges may be deeper than the field of the slab, and concrete can remain in equipment or be lost during handling. This is why the calculator offers an optional planning allowance.
An added percentage should not be treated as a universal rule. The appropriate quantity depends on measurement accuracy, slab geometry, site preparation, project size, supplier requirements and the consequences of running short during placement.
The Concrete Square Foot Calculator deliberately treats thickness as an input instead of telling the user what thickness a slab “should” be. Slab design can depend on soil support, anticipated wheel or point loads, reinforcement, jointing, moisture conditions, exposure, local requirements and the intended use of the slab. A residential patio and an industrial floor may have the same square footage but very different design requirements.
The American Concrete Institute's ACI PRC-360-10 Guide to Design of Slabs-on-Ground addresses planning, design and detailing of slabs-on-ground, including support systems, loadings, joints and several slab design approaches. ACI's Guide for Concrete Floor and Slab Construction covers construction considerations such as site preparation, materials, workmanship, joints, finishing and curing. These references illustrate why a quantity calculator should not substitute for project-specific design and construction requirements.
Some plans or supplier documents use metric measurements. The U.S. National Institute of Standards and Technology (NIST) publishes customary-to-metric conversion information. For quick planning, one square foot is approximately 0.0929 square meter. For accurate project calculations, maintain consistent units throughout each formula and use exact conversion factors appropriate to the required precision.
See the official NIST unit conversion resource for customary-to-metric references.
Square footage is often only the first step in concrete planning. Use related tools when your project needs a more specialized estimate.
This calculator is a quantity-planning tool. The formulas for rectangular area and volume are basic geometry. Technical links are provided to help users understand that slab design and construction involve requirements beyond material quantity.
Common questions about measuring concrete area and converting square feet into concrete volume.