How to Use the Concrete Calculator

Use the Builzee concrete calculator to estimate how much concrete you need for slabs, patios, footings, curbs, walls, round piers, and post holes.

Start by selecting the pour type that best matches your project. Enter the required dimensions and use the Qty field when you have several identical pours.

If your project includes different concrete shapes or dimensions, select Add Another Pour and enter each section separately.

Finally, choose a waste allowance. The calculator combines all pours and provides the total volume in cubic yards, cubic feet, and cubic meters, along with approximate quantities for common premixed concrete bag sizes.

How to Calculate Concrete for a Slab or Patio

For a rectangular slab, patio, sidewalk, or similar pour, concrete volume is based on:

Length × Width × Thickness

For example, a slab measuring 10 feet by 10 feet with a 4-inch thickness requires approximately:

1.23 cubic yards of concrete before waste

Be careful when entering slab thickness. In Imperial mode, length and width are entered in feet while thickness is entered in inches.

Concrete slabs are rarely perfectly uniform in real-world conditions, so adding a reasonable waste allowance can help account for variations in the subgrade and forms.

How to Calculate Concrete for Footings, Curbs, and Walls

Footings, curbs, grade beams, and concrete walls can generally be calculated as rectangular volumes.

Enter:

Length × Width × Depth or Height

For example, a continuous footing might be 20 feet long, 12 inches wide, and 8 inches deep.

If several identical footings or sections are being poured, use the Qty field rather than entering each one separately.

For irregular foundations, divide the project into individual rectangular sections and add each section as a separate pour.

How to Calculate Concrete for a Round Pier or Post Hole

Round piers and post holes use a cylindrical volume rather than a rectangular calculation.

The calculator uses the diameter and depth of the hole to determine the required concrete volume.

This can be useful for projects such as deck piers, fence posts, round foundation piers, and concrete-filled form tubes.

The calculator assumes the entire circular hole or pier is filled with concrete.

If a wood or steel post is embedded in the concrete, the calculator does not deduct the volume displaced by the post. For most general material estimates, that small displacement is usually better handled through the overall waste allowance rather than trying to subtract it precisely.

Should You Subtract Rebar From Concrete Volume?

No. For normal material estimating, you generally do not need to subtract the volume occupied by reinforcing steel.

Rebar and welded wire reinforcement occupy a relatively small portion of the total concrete volume, and actual pours also include variations from excavation, forms, subgrade conditions, and placement.

Calculate the full formed or excavated concrete volume and use an appropriate waste allowance instead.

How Much Extra Concrete Should You Order?

An exact volume calculation assumes that the dimensions of the finished pour are perfectly uniform.

Actual concrete requirements can be affected by uneven excavation, subgrade variations, form movement, spillage, over-excavation, and other jobsite conditions.

For many straightforward projects, 5% to 10% extra concrete is a useful general planning range.

More irregular excavations or projects where running short would cause a major problem may justify a larger allowance.

For ready-mix deliveries, also consider the supplier’s minimum order requirements and ordering recommendations before placing the final order.

Understanding Cubic Yards of Concrete

Ready-mix concrete is commonly ordered by the cubic yard in the United States.

One cubic yard equals:

27 cubic feet

A project requiring 54 cubic feet of concrete would therefore require:

54 ÷ 27 = 2 cubic yards

The calculator performs this conversion automatically.

Even relatively large-looking slabs can require fewer cubic yards than expected because concrete volume depends heavily on slab thickness.

Understanding Concrete Bag Quantities

Smaller projects are often completed using premixed concrete sold in bags rather than ordering ready-mix concrete.

The Builzee calculator estimates quantities for common 40 lb, 50 lb, 60 lb, and 80 lb bags.

Bag yield varies by manufacturer and product, so these quantities should be treated as estimates.

Before purchasing material, check the actual yield printed on the bag or listed by the manufacturer. If the manufacturer’s stated yield differs from the calculator’s assumed yield, use the manufacturer’s information when determining the final quantity.

For larger projects requiring dozens or hundreds of bags, ready-mix concrete may be more practical than mixing individual bags.

Calculating Multiple Concrete Pours

Many projects include more than one concrete shape.

For example, a project might include a patio slab along with several round pier footings.

Use Add Another Pour to calculate each section separately. Each pour can have its own shape, dimensions, and quantity.

The calculator combines all of the entered pours before applying the project waste allowance.

This is usually more accurate than trying to convert different shapes into one average measurement.

Before You Order Concrete

Verify all measurements before ordering concrete or purchasing bags.

For excavated footings and post holes, measure the actual excavation whenever possible rather than relying only on the planned dimensions. Soil conditions and excavation can create wider or deeper areas than originally intended.

For formed slabs and walls, verify the inside dimensions of the forms and the final concrete thickness.

The Builzee Concrete Calculator is intended as a material-planning tool. Structural dimensions, reinforcement, concrete strength, footing sizes, and other design requirements should follow your project plans, applicable requirements, and professional guidance when needed.