Concrete Calculator

How to Use the Concrete Calculator

Whether you are pouring a patio slab, setting fence posts, or building a foundation footing, knowing the right amount of concrete is essential. Enter the length and width of your project in feet and the depth in inches. The calculator converts the depth to feet, computes the volume in cubic feet, then converts to cubic yards, which is the standard ordering unit for ready-mix concrete.

For DIY projects using premixed bags, the calculator also shows how many 60-pound and 80-pound bags you need. A 60-pound bag yields approximately 0.45 cubic feet of mixed concrete, while an 80-pound bag yields about 0.6 cubic feet. Both figures are rounded up so you always have enough material to complete the pour without interruption.

Concrete Volume Formula

The basic volume formula is straightforward: Volume = Length x Width x Depth. Since depth is commonly measured in inches, you must first convert it to feet by dividing by 12. The result is in cubic feet. To convert cubic feet to cubic yards, divide by 27 (since 3 ft x 3 ft x 3 ft = 27 cubic feet per cubic yard). For bag calculations, divide the total cubic feet by the yield per bag (0.45 for 60 lb, 0.6 for 80 lb) and round up.

Recommended Slab Thickness

Choosing the right thickness depends on the intended use. Sidewalks and garden paths can be as thin as 3.5 to 4 inches. Patios and standard slabs should be a minimum of 4 inches. Driveways need at least 4 inches for cars and 5 to 6 inches for heavier vehicles like trucks and RVs. Garage floors are typically 4 to 6 inches. Always check your local building codes, as structural applications such as foundations may require thicker pours and reinforcement with rebar or wire mesh.

Bags vs. Ready-Mix Delivery

Premixed bags are economical for small projects under one cubic yard. For larger pours, ordering ready-mix concrete from a batch plant is more practical and cost-effective. A standard ready-mix truck delivers 8 to 10 cubic yards. Many suppliers also offer short-load trucks for quantities between 1 and 4 cubic yards, though these may carry a small-load surcharge. For very large projects, always schedule your pour and have enough labor on hand to place and finish the concrete before it begins to set.

Frequently Asked Questions

How do I calculate how much concrete I need?

Multiply the length by the width to get the area in square feet. Convert the depth from inches to feet by dividing by 12. Multiply the area by the depth in feet to get cubic feet. Divide cubic feet by 27 to convert to cubic yards. One cubic yard of concrete covers about 81 square feet at 4 inches thick.

How many bags of concrete do I need?

A 60-pound bag of premixed concrete yields approximately 0.45 cubic feet, while an 80-pound bag yields about 0.6 cubic feet. Divide your total cubic feet by the yield per bag and round up. For example, 27 cubic feet (1 cubic yard) requires about 60 of the 60-lb bags or 45 of the 80-lb bags.

How thick should a concrete slab be?

A standard concrete slab for a patio or walkway is 4 inches thick. Driveways should be at least 4 to 6 inches thick depending on vehicle weight. Garage floors are typically 4 to 6 inches. Foundation footings and structural slabs may require 8 to 12 inches or more, as specified by local building codes.

What is the difference between concrete and cement?

Cement is a powder ingredient that acts as a binder. Concrete is the finished material made by mixing cement with water, sand, and gravel (aggregate). Premixed concrete bags contain all the dry ingredients and only require the addition of water to create ready-to-pour concrete.

Should I order extra concrete?

Yes, it is wise to order 5-10% more concrete than calculated to account for spillage, uneven subgrade, and slight variations in form dimensions. Running short during a pour is much more costly than having a small amount left over, as cold joints from a delayed pour weaken the finished slab.