🏗️ Concrete Calculator

This concrete calculator works out the volume of a concrete slab — in cubic feet, cubic yards, or cubic meters — and how many bags of pre-mixed concrete you need to pour it, based on your slab's length, width, and thickness. It's designed for DIYers and small contractors pouring patios, walkways, footings, and small slabs who want an accurate material estimate before buying bags.

How It Works

The calculator multiplies slab length by width by thickness (after converting thickness from inches or centimeters into feet or meters) to get the total volume. In US mode, it converts cubic feet to cubic yards by dividing by 27, since ready-mix concrete is typically ordered by the cubic yard. It then divides the total volume by the yield of your selected bag size — for example, a 60 lb bag yields about 0.45 cubic feet, or a 25 kg bag yields about 0.0125 cubic meters — and rounds up to the number of whole bags needed.

Why It Matters

Concrete volume scales directly with thickness, so a small miscalculation in slab depth can significantly change how many bags you need — running short partway through a pour can create a visible seam or "cold joint" where the concrete doesn't fully bond. Getting an accurate bag count up front helps you buy the right amount and pour the whole slab in one continuous session.

Tips for Accurate Results

• Add 5-10% extra to your calculated volume to account for uneven subgrade, spillage, and measurement imprecision.
• Match your slab's thickness to its intended use — walkways are often 4 inches, while driveways and structural slabs may need 5-6 inches or more.
• Check the exact yield printed on your specific bag brand and mix type, since yields can vary slightly from the standard figures used here.
• For large pours (more than a cubic yard or so), ready-mix delivery is often more practical and cost-effective than bagged concrete.