Concrete Calculator

Concrete volume is calculated by multiplying length times width times depth. Since depth is often measured in inches, convert it to feet by dividing by 12. The result in cubic feet is then divided by 27 to convert to cubic yards, which is how concrete is typically sold.

For example, a 10-foot by 12-foot patio with a 4-inch depth requires: 10 × 12 × (4/12) = 40 cubic feet, which equals 1.48 cubic yards. For irregular shapes like L-shaped patios or curved walkways, break the area into rectangles and circles, calculate each section separately, and add the results together. This calculator handles the standard rectangular calculation automatically and gives you results in cubic feet, cubic yards, and the number of bags you need to purchase.

For small projects, bags of premixed concrete (60 lb or 80 lb) are convenient. An 80-lb bag yields about 0.6 cubic feet. For larger projects over 1 cubic yard, ordering ready-mix from a concrete truck is more economical and provides a better finish.

The cost difference is significant. An 80-lb bag costs $5 to $7 at most home improvement stores, which means 1 cubic yard from bags costs approximately $225 to $315 for the concrete alone. A ready-mix truck delivers the same cubic yard for $125 to $175, already mixed to professional specifications. Ready-mix trucks typically have a minimum order of 1 cubic yard and may charge a short-load fee for orders under 10 yards. For projects between 1 and 3 cubic yards, weigh the convenience of bags against the cost savings and quality of ready-mix delivery.

Always order 5% to 10% more concrete than calculated to account for spillage, uneven ground, and form irregularities. Running short during a pour can compromise the structural integrity of the slab.

Running out of concrete mid-pour is one of the worst scenarios in a concrete project because cold joints — where new concrete meets partially cured concrete — create weak points that are prone to cracking. It is far better to have a small amount of leftover concrete than to come up short. If you do have extra, use it to fill other small projects like fence post holes, stepping stones, or equipment pads. For projects on sloped or uneven ground, increase your waste factor to 15% since the actual depth will vary across the pour area.

Standard depths are 4 inches for sidewalks and patios, 6 inches for driveways and garage floors, and 8 to 12 inches for footings and foundations. Thicker slabs provide more strength for heavier loads.

The depth you choose should match the expected load on the surface. A 4-inch slab supports foot traffic and light furniture but will crack under vehicle weight. Residential driveways should be at least 6 inches because passenger vehicles weigh 3,000 to 5,000 pounds concentrated on four tire contact patches. If you expect heavy trucks, RVs, or construction equipment to cross the slab, consider 8 inches with reinforcement. Adding wire mesh or rebar to any slab improves tensile strength and helps control cracking, which is especially important in regions with freeze-thaw cycles that cause soil expansion beneath the concrete.