Concrete Calculator
Estimate the concrete you need for slabs, footings, columns, walls, steps and tube forms. Get results in cubic yards, cubic feet and cubic meters, plus optional bag and cost estimates.
How the Concrete Calculator Works
For rectangular pours such as slabs, footings and walls, the calculator multiplies length by width by thickness, all converted to feet, to get a volume in cubic feet: Volume = Length × Width × Thickness.
Once dimensions are converted to feet, cubic yards are found by dividing cubic feet by 27: Cubic Yards = Cubic Feet ÷ 27, since one cubic yard equals a 3 ft × 3 ft × 3 ft cube.
For cylindrical shapes like round columns and tube forms, the calculator uses the cylinder volume formula: Volume = π × radius² × height, where the radius is half the diameter.
A waste allowance is then applied to the raw volume to account for real-world conditions: Adjusted Volume = Raw Volume × (1 + Waste ÷ 100).
Worked Example
A homeowner needs a slab that is 20 ft long, 12 ft wide and 4 inches thick.
First, convert the thickness to feet: 4 inches ÷ 12 = 0.3333 ft.
Volume: 20 × 12 × 0.3333 ≈ 80 cubic feet.
Cubic yards: 80 ÷ 27 ≈ 2.96 cubic yards.
With a 10% waste allowance: 2.96 × 1.10 ≈ 3.26 cubic yards, which a supplier might round up to a 3.25 or 3.5 yard order depending on their minimum order policy and truck load size.
What Is a Concrete Calculator?
A concrete calculator is a planning tool that converts the physical dimensions of a construction project into an estimated concrete volume. Instead of manually working through cubic-foot and cubic-yard conversions for a slab, footing, column, wall, staircase or tube form, this concrete calculator does the math instantly, and adds a configurable waste allowance so the number you see is closer to what you should actually order.
How to Calculate Concrete Volume
The basic formula behind every rectangular concrete calculation is straightforward: length multiplied by width multiplied by thickness, with all three measurements expressed in the same unit. Because slab and wall thickness is usually described in inches while length and width are described in feet, the trickiest part of doing this by hand is remembering to convert inches to feet before multiplying. This concrete slab calculator and concrete footing calculator handle that conversion automatically.
How Many Cubic Yards of Concrete Do You Need?
Ready-mix concrete trucks are loaded and priced by the cubic yard, not the cubic foot, so most projects eventually need a cubic-yard figure regardless of how the raw volume was calculated. Since a cubic yard equals 27 cubic feet, dividing your cubic-foot total by 27 gives the cubic yardage. This concrete calculator in cubic yards performs that step for every mode, so slabs, footings, columns and walls all produce a comparable cubic-yard figure.
Why Add a Waste Factor?
Real job sites rarely match theoretical geometry exactly. Common reasons actual concrete usage runs higher than the calculated volume include:
- Uneven or over-excavated subgrade that absorbs extra material
- Minor form leakage or spillage during the pour
- Small measurement or rounding differences on site
- Irregular edges, thickened edges, or reinforcement that displaces less volume than assumed
A 5-10% waste allowance is standard for most residential slabs and footings, and this calculator lets you adjust that percentage for your own project and site conditions.
Concrete Calculator for Slabs
The concrete calculator for slabs is built for patios, sidewalks, driveways, garage floors and interior slabs. Enter the length and width in feet and the thickness in inches, and the tool returns raw cubic feet, cubic yards, cubic meters and a waste-adjusted recommended order. Typical residential slab thickness ranges from 4 inches for patios and sidewalks to 5-6 inches for driveways carrying vehicle traffic.
Concrete Calculator for Footings
The concrete calculator for footings uses the same length × width × depth formula, but footings are typically narrow and deep rather than wide and shallow, so accurate depth measurement has an outsized effect on the total. Always confirm footing depth requirements against local building code before ordering.
Concrete Calculator for Columns and Piers
Columns can be rectangular or circular. Rectangular columns use length × width × height, the same as a short wall segment. Circular columns and concrete piers use the cylinder formula, π × radius² × height, where the radius is half the diameter you measure. This calculator switches automatically between the two formulas based on the shape you select.
How Many Bags of Concrete Do You Need?
The concrete bag calculator divides your required cubic feet by the approximate yield of a single bag. As a general reference, an 80-lb bag yields roughly 0.60 cubic feet, a 60-lb bag roughly 0.45 cubic feet, a 50-lb bag roughly 0.375 cubic feet, and a 40-lb bag roughly 0.30 cubic feet. These figures vary by manufacturer and product line, so always check the coverage printed on the actual bag before finalizing a purchase, especially for larger pours where small yield differences add up quickly.
How Much Does Concrete Cost?
Ready-mix concrete pricing depends heavily on location, mix design and strength specification, delivery distance, minimum load requirements, seasonal demand and local taxes. This concrete cost calculator lets you enter your own local price per cubic yard along with delivery and additional fees to generate a rough planning estimate; it is not a quote and actual pricing should always be confirmed with a local ready-mix supplier.
Ready-Mix Concrete vs. Concrete Bags
| Factor | Ready-Mix Delivery | Bagged Concrete |
|---|---|---|
| Best for | Larger pours (roughly 1+ cubic yard) | Small repairs, posts, small slabs |
| Labor | Truck delivers mixed concrete | Mixed on-site, more labor-intensive |
| Consistency | Batched to a specific mix design | Consistent per bag, but more seams |
| Typical minimums | Supplier minimum order/truck fees | No minimum, buy by the bag |
Concrete Calculation Example
See the fully worked slab example above for a step-by-step walkthrough of converting a 20 ft × 12 ft × 4 in slab into cubic feet, cubic yards, and a waste-adjusted recommended order.
Tips for Ordering Concrete
- Round your final order up, not down; running short mid-pour is far more costly than a small surplus.
- Ask your supplier about their minimum order size, since small residential orders sometimes carry a short-load fee.
- Confirm the mix design (strength, aggregate size, air entrainment) matches your project's requirements, particularly for driveways and exterior slabs exposed to freeze-thaw cycles.
- Schedule your pour date around weather; extreme heat or cold both affect curing.
- After calculating volume here, a future Construction Cost Calculator can help estimate broader project expenses beyond just the concrete itself.