Concrete Calculator

Ordering the right amount of concrete is one of the most important steps in any pour.

Reviewed by: CalcMojo Editorial Team

Too little means a second delivery, cold joints, and delays. Too much means wasted material and money sitting in your driveway. This concrete calculator takes the guesswork out of the process by converting your slab, footing, or column dimensions into the exact volume you need, expressed in both cubic yards and the number of pre-mixed bags for smaller projects.

Enter the length, width, and thickness of your project area, and the calculator returns the concrete volume instantly. It supports rectangular slabs, circular pads, footings, columns, and custom shapes. For projects with multiple sections at different thicknesses, you can add each section separately and get a combined total. The tool also adds a configurable waste factor, typically 5% to 10%, to account for uneven subgrade, form irregularities, and spillage during the pour.

Whether you are a homeowner pouring a patio, a contractor bidding a driveway, or a builder estimating a foundation, accurate volume calculations prevent the two worst outcomes on a concrete job: running short mid-pour and paying for material you do not need.

How Concrete Volume Is Calculated

Concrete is measured and sold by the cubic yard in the United States. One cubic yard equals 27 cubic feet. The basic volume formula for a rectangular slab is:

Volume (cubic feet) = Length (ft) x Width (ft) x Thickness (ft)

Then divide by 27 to convert to cubic yards:

Volume (cubic yards) = Volume (cubic feet) / 27

For example, a patio slab measuring 20 feet long, 12 feet wide, and 4 inches thick (0.333 feet) requires:

20 x 12 x 0.333 = 79.92 cubic feet, which is 79.92 / 27 = 2.96 cubic yards.

With a standard 10% waste factor, you would order approximately 3.26 cubic yards, which most suppliers would round to 3.25 or 3.5 yards depending on their minimum load and delivery increments.

For circular slabs such as round patios or fire pit pads, the formula uses the radius:

Volume = pi x r^2 x Thickness

A circular pad with an 8-foot diameter (4-foot radius) and 4-inch thickness requires approximately 0.62 cubic yards of concrete.

For footings and columns, the calculator uses the same volume principles but adjusts for the specific dimensions of these structural elements.

Concrete for Common Project Types

Driveways. A standard residential driveway is typically 4 inches thick for passenger vehicles and 6 inches thick if heavy vehicles like RVs or trucks will use it regularly. The most common driveway size is approximately 12 feet wide and 20 to 40 feet long. A 12 x 30 foot driveway at 4 inches thick requires roughly 4.4 cubic yards. Most contractors recommend 5 to 6 inches for durability and to accommodate unknown future loads. Thicker concrete costs more material but significantly extends the lifespan of the slab.

Patios. Residential patios are usually 4 inches thick. Sizes vary widely, but a common backyard patio is 12 x 16 feet, which requires approximately 2.4 cubic yards. If you plan to place heavy furniture, a hot tub, or an outdoor kitchen on the patio, consider increasing thickness to 5 or 6 inches in those areas.

Sidewalks. Standard sidewalks are 4 inches thick and 3 to 5 feet wide. A 4-foot-wide, 50-foot-long sidewalk at 4 inches thick requires approximately 2.5 cubic yards. Public sidewalks must meet local code requirements, which typically specify 4-inch minimum thickness over a compacted gravel base.

Footings. Foundation footings support the weight of the entire structure, so they are significantly thicker and deeper than slabs. A typical residential wall footing is 8 inches thick and 16 to 24 inches wide, placed below the frost line. Column footings (also called pier footings) are square or rectangular pads, commonly 24 x 24 inches and 12 inches thick. The dimensions of footings are determined by soil bearing capacity and structural loads, so always follow your engineer’s specifications.

Garage Floors. Standard garage slabs are 4 to 6 inches thick. A two-car garage measuring 20 x 22 feet at 4 inches thick requires approximately 5.4 cubic yards. Many builders use 6-inch thickness and add fiber mesh or rebar for reinforcement since garage floors must support vehicle weight and resist cracking from thermal expansion.

Pre-Mixed Bags vs Ready-Mix Delivery

For small projects, pre-mixed concrete bags from the hardware store are practical. For larger jobs, ready-mix delivery from a batch plant is more efficient and usually less expensive per yard.

Pre-mixed bags. A standard 80-pound bag of concrete mix yields approximately 0.6 cubic feet of mixed concrete. An 60-pound bag yields approximately 0.45 cubic feet. To cover one cubic yard (27 cubic feet), you need approximately 45 bags at 80 pounds or 60 bags at 60 pounds. At roughly $5 to $7 per 80-pound bag, one cubic yard of bagged concrete costs $225 to $315 in materials alone, plus significant labor for mixing. Bags are practical for projects under 0.5 cubic yards, such as setting fence posts, repairing small sections, or pouring individual pier footings.

Ready-mix delivery. Ready-mix concrete is delivered by truck in precise quantities and can be poured directly into your forms. Typical pricing ranges from $125 to $175 per cubic yard, depending on the mix design and your location. Most suppliers require a minimum order, commonly 1 to 3 cubic yards, and charge a short-load fee for orders below the minimum. Delivery fees range from $50 to $150 depending on distance. For any project over 1 cubic yard, ready-mix delivery is almost always more cost-effective and produces a more consistent result than hand-mixing bags.

Understanding Concrete Mix Strength

Concrete strength is measured in pounds per square inch (PSI), which indicates how much compressive force it can withstand after curing. Common residential mix strengths include:

• 2,500 PSI: General-purpose mix suitable for footings, low-traffic slabs, and non-structural applications.
• 3,000 PSI: Standard mix for driveways, patios, sidewalks, and garage floors. This is the most common residential strength.
• 3,500 PSI: Higher-strength mix used for heavy-traffic areas, commercial applications, and situations where additional durability is needed.
• 4,000+ PSI: Structural and commercial applications, including building foundations, parking garages, and industrial floors.

The PSI rating does not change the volume calculation, but it affects the price per yard. Higher-strength mixes cost more because they use a higher ratio of cement to aggregate. When ordering ready-mix, specify the PSI you need and the supplier will adjust the mix design accordingly.

How to Account for Waste

No concrete pour uses 100% of the ordered material. Some concrete always remains in the truck chute, overfills low spots in the subgrade, or fills form irregularities. The standard waste factors are:

• 5% waste: Well-prepared site with level subgrade and sturdy forms. Appropriate for experienced crews on straightforward rectangular pours.
• 10% waste: Standard recommendation for most residential projects. Accounts for typical subgrade variation, minor form movement, and normal spillage.
• 15% waste: Sites with uneven subgrade, irregular shapes, or first-time pourcasters who may encounter unexpected issues.

This calculator includes a configurable waste field that defaults to 10%. Adjust it based on your site conditions. It is always better to have a small amount of extra concrete than to run short during a pour, because a cold joint (where fresh concrete meets partially cured concrete from a second delivery) is a structural weak point.

Preparing for a Concrete Pour

Accurate volume calculation is only one part of a successful pour. Proper preparation prevents the most common failures.

Subgrade preparation. Remove all topsoil, organic material, and loose fill. Compact the remaining soil, then add 4 to 6 inches of compacted gravel base for drainage and uniform support. A poorly prepared subgrade is the leading cause of slab cracking and settling.

Formwork. Build forms from straight 2×4 or 2×6 lumber, staked every 3 to 4 feet. The top of the form should be at the desired finished concrete height. Use a string line or laser level to ensure the forms are at the correct grade and slope. For drainage, most exterior slabs should slope at least 1/8 inch per foot away from buildings.

Reinforcement. Rebar or welded wire mesh provides tensile strength that plain concrete lacks. For most residential slabs, number 3 or number 4 rebar on 18 to 24 inch centers, or 6×6-W1.4xW1.4 welded wire mesh, is standard. Place reinforcement at the mid-height of the slab on rebar chairs.

Timing and weather. Concrete should not be poured when temperatures are below 40 degrees F or expected to drop below freezing within 24 hours. Extremely hot weather (above 90 degrees F) accelerates curing and can cause surface cracking. Ideal pouring conditions are 50 to 80 degrees F with overcast skies.

Use the Square Footage Calculator to determine your slab area before entering dimensions here, especially for irregular shapes where area calculation is not straightforward.

These estimates are approximate and based on standard material calculations. Actual quantities and costs vary by region, supplier, material grade, and site conditions.

Frequently Asked Questions

How many bags of concrete do I need for a 10×10 slab?

A 10 x 10 foot slab at 4 inches thick requires approximately 1.23 cubic yards of concrete. In 80-pound bags, that is approximately 56 bags. With a 10% waste factor, purchase 62 bags. For a slab this size, consider ready-mix delivery instead, which is faster and produces a more consistent result than mixing 56 individual bags by hand.

How much does a yard of concrete cost?

Ready-mix concrete typically costs $125 to $175 per cubic yard depending on the mix strength, additives, and your location. Add delivery fees of $50 to $150. Short-load fees of $30 to $60 per yard may apply if you order below the supplier’s minimum, usually 3 to 5 yards. For bagged concrete, one cubic yard costs $225 to $315 in materials alone plus significant mixing labor.

How thick should a concrete slab be?

For most residential applications, 4 inches is the standard thickness for patios, sidewalks, and light-duty floors. Driveways and garage floors should be at least 4 inches, with 5 to 6 inches recommended for heavy vehicles or long-term durability. Foundation footings are typically 8 to 12 inches thick as specified by an engineer.

What is the difference between concrete and cement?

Cement is one ingredient in concrete. Concrete is a mixture of cement (the binding agent, typically Portland cement), aggregate (sand and gravel), and water. Cement makes up only 10% to 15% of the total concrete mix by volume. The terms are often used interchangeably in casual conversation, but they are different materials.

How long does concrete take to cure?

Concrete reaches approximately 70% of its full strength within 7 days and its rated strength (such as 3,000 PSI) at 28 days. However, concrete continues to strengthen slowly for months after that. You can walk on most residential concrete after 24 to 48 hours. Vehicles should stay off new concrete for at least 7 days, and heavy loads should wait 28 days.

Should I add a waste factor when ordering concrete?

Yes. Add 5% to 10% beyond your calculated volume to account for subgrade irregularities, spillage, and form variations. Running short during a pour creates a cold joint, which is a structural weak point. A small amount of excess concrete is far better than not having enough.

How do I calculate concrete for a circular slab?

Use the formula: Volume = pi x radius squared x thickness. Measure the diameter, divide by 2 to get the radius, square it, multiply by pi (3.14159), then multiply by the thickness in feet. A circular slab with a 10-foot diameter and 4-inch thickness requires approximately 0.97 cubic yards.

What is a short-load fee for concrete delivery?

Most ready-mix suppliers charge a short-load fee when you order below their minimum quantity, typically 3 to 5 cubic yards. The fee usually ranges from $30 to $60 per cubic yard below the minimum. For example, if the minimum is 3 yards and you order 2 yards, you might pay an extra $30 to $60. This fee covers the fixed cost of dispatching a truck regardless of the load size.

Sources & Methodology

• Volume calculations use standard geometric formulas (length x width x thickness for rectangular slabs, pi x r^2 x thickness for circular).
• Bag yield estimates based on manufacturer specifications: 80-lb bag yields 0.6 cu ft, 60-lb bag yields 0.45 cu ft (Quikrete, Sakrete published data).
• PSI strength classifications based on American Concrete Institute (ACI) 318 Building Code Requirements for Structural Concrete.
• Waste factor recommendations (5%-15%) derived from Portland Cement Association (PCA) guidelines for residential construction.
• Curing timelines based on ACI 308R Guide to External Curing of Concrete.

Default values shown are illustrative. Always verify measurements on site. Data accurate as of: March 2026