Concrete Calculator: How to Estimate Yards for Your Project

Understanding Cubic Yards

Concrete is sold by the cubic yard. One cubic yard equals 27 cubic feet and weighs approximately 4,000 pounds when mixed. Ready-mix concrete trucks typically carry 8 to 10 cubic yards, and most companies have a minimum order of 1 cubic yard with a short-load fee for orders under 4 to 5 yards.

For smaller projects, you can buy pre-mixed concrete in 40, 60, or 80-pound bags. An 80-pound bag yields about 0.6 cubic feet of mixed concrete, meaning you need 45 bags to make one cubic yard. At $5 to $7 per bag, bagged concrete costs significantly more per yard than ready-mix but makes sense for projects under one cubic yard.

The Cubic Yards Formula

Every concrete calculation starts with the same basic formula. Measure your project dimensions in feet, then convert cubic feet to cubic yards.

For a 20 x 20 foot patio slab that is 4 inches thick: 20 x 20 x 0.333 = 133.2 cubic feet. Divide by 27 to get 4.93 cubic yards. Add 10 percent for waste and you need about 5.4 cubic yards, so order 5.5 yards.

Our concrete calculator handles all of these conversions automatically. Just enter your measurements and get your answer in cubic yards and bags.

Calculating Concrete for Slabs

Slabs are the most common concrete project for homeowners. Whether you are pouring a patio, driveway, garage floor, or sidewalk, the calculation is the same: length times width times thickness.

For irregular-shaped slabs, break the area into rectangles, calculate each section separately, and add the results together. For curved areas, use the closest rectangular approximation and add 15 percent waste instead of the usual 10 percent.

Footings and Foundation Walls

Footings are the widened base sections that sit below foundation walls or support posts. They distribute the weight of the structure over a larger area of soil. Footing dimensions are determined by local building codes and soil conditions.

A typical residential continuous footing is 16 to 24 inches wide and 8 to 12 inches deep. Calculate the volume as length times width times depth, remembering to convert everything to feet first. For a 100-foot perimeter footing that is 20 inches wide and 10 inches deep: 100 x 1.67 x 0.83 = 138.6 cubic feet, or about 5.1 cubic yards.

Foundation walls are calculated similarly: total linear feet times wall thickness times wall height. An 8-inch thick foundation wall that is 4 feet tall around a 100-foot perimeter requires: 100 x 0.67 x 4 = 268 cubic feet, or about 9.9 cubic yards.

Columns and Piers

Round columns and piers use the cylinder formula: pi times radius squared times height. A common Sonotube form is 12 inches in diameter and 4 feet deep. The radius is 6 inches or 0.5 feet.

For rectangular piers, use length times width times depth. A 16 x 16 inch pier that is 3 feet deep contains 1.33 x 1.33 x 3 = 5.3 cubic feet, or about 0.2 cubic yards each. Multiply by the number of piers in your project.

Waste Factors and Ordering Tips

Always order more concrete than your exact calculation. Concrete is not a material you want to run short on because once a pour starts, it needs to be completed continuously. Adding concrete to a partially set slab creates cold joints that are structurally weak.

When ordering ready-mix, call the batch plant at least one day in advance. Tell them your project type, total cubic yards needed, any special mix requirements (such as fiber reinforcement or high-early strength), and the desired slump (typically 4 inches for most residential work). Ask about short-load fees if you need less than their minimum.

Concrete Mix Strength and Types

Concrete strength is measured in PSI (pounds per square inch) at 28 days after pouring. Different projects require different strengths.

Bags vs. Ready-Mix: When to Use Each

The crossover point between bagged concrete and ready-mix depends on your project size and local pricing. As a general rule, ready-mix becomes more cost-effective at about 1 to 1.5 cubic yards (roughly 45 to 68 eighty-pound bags).

Bagged concrete makes sense for post holes, small pad footings, repair work, and any project under 0.5 cubic yards. You control the pace, can mix small batches, and have no minimum order concerns. The downside is the labor of mixing, which becomes exhausting after 20 or 30 bags.

Ready-mix is the clear choice for slabs, driveways, foundations, and any pour over 1.5 cubic yards. The concrete arrives already mixed to specification, is poured directly into your forms, and eliminates hours of manual mixing. The cost per cubic yard is typically $120 to $185 for the concrete itself, plus delivery fees of $50 to $100.

Common Mistakes to Avoid

• Not accounting for waste: Always add 5 to 15 percent to your calculated volume. The ground is never perfectly level, forms shift, and concrete is lost during pouring.
• Ordering too late: Ready-mix plants schedule deliveries. During busy seasons, you may need to order 2 to 3 days in advance. Saturday deliveries often cost extra.
• Wrong thickness: Driveways need at least 4 inches, preferably 6 inches. Sidewalks can be 4 inches. Cutting thickness to save money risks cracks and failure.
• Skipping reinforcement: Wire mesh or rebar significantly reduces cracking. Fiber mesh is an easy addition that helps but does not replace steel reinforcement for structural applications.
• Adding water on site: Asking the driver to add water makes concrete easier to pour but weakens it. Specify the correct slump when ordering instead.
• Ignoring the weather: Do not pour concrete when temperatures are below 40 degrees F or above 90 degrees F without taking special precautions. Cold delays curing; heat causes rapid drying and cracking.

Concrete Steps and Curbs

Steps and curbs require separate calculations because of their complex shapes. For concrete steps, calculate each step as a rectangular block: step width times step depth (tread) times step height (riser). A typical step is 36 inches wide, 11 inches deep, and 7 inches tall, yielding about 0.16 cubic feet per step. A four-step stairway with a 4-foot wide landing on top might need 0.5 to 1 cubic yard total.

Concrete curbs are calculated as length times width times height. A standard 6-inch by 18-inch curb that runs 50 feet requires: 50 x 0.5 x 1.5 = 37.5 cubic feet, or about 1.4 cubic yards. Add the gutter section if applicable. Many contractors pour curbs and gutters together as a single monolithic piece for better strength and fewer joints.

For complex shapes like curved walkways, stamped patios with borders, or decorative elements, the easiest approach is to break the project into simple geometric shapes, calculate each one separately, and add them together. Add 15 to 20 percent waste for irregular shapes since more concrete is lost to forming and finishing curves.

Frequently Asked Questions