How Much Concrete Do I Need?

Concrete calculator

Enter your project length and width in feet and thickness in inches. The calculator converts your dimensions into cubic feet and cubic yards, then shows a recommended quantity with a 10% planning allowance plus approximate 80-lb and 60-lb bag counts.

How the concrete calculation works

For a rectangular project, cubic feet are calculated as length × width × thickness in feet. Cubic yards are calculated by dividing cubic feet by 27 because one cubic yard contains 27 cubic feet.

Cubic yards = length × width × thickness in feet ÷ 27

The calculator accepts thickness in inches so the input is practical for common slab measurements. Four inches equals one-third of a foot and six inches equals one-half of a foot.

Concrete estimating guide

1. Measure the finished area

Measure the actual length and width of the area receiving concrete. If the project contains several rectangular sections, calculate each section separately and add the volumes together. This is more reliable than using rough overall dimensions when the project has cutouts, steps, curves, or changes in thickness.

2. Confirm the thickness

Use the thickness specified for the project rather than choosing a thickness simply because it is common. Slab thickness can depend on intended use, site conditions, loads, reinforcement, plans, and local requirements.

3. Convert the volume to cubic yards

Multiply the three compatible dimensions to get cubic feet, then divide by 27 to get cubic yards. The exact calculated volume is useful when checking the math or discussing a ready-mix order with a supplier.

4. Consider the planning allowance

ATHENA displays a 10% planning allowance as a starting point for normal measuring and placement uncertainty. It is not a universal rule. Uneven subgrade, irregular forms, spillage, difficult access, and changes in thickness can affect the final quantity.

Example: 30 × 10 ft at 4 inches

A 30 × 10 ft area is 300 square feet. At 4 inches thick, the volume is about 100 cubic feet, or about 3.7 cubic yards before an allowance. With a 10% planning allowance, the suggested planning quantity is about 4.1 cubic yards.

This example is intended to show the calculation method. It is not a recommendation for the structural thickness of a particular project.

Ready-mix concrete vs. bagged concrete

Ready-mix is commonly ordered in cubic yards, so the cubic-yard result is the most useful number when requesting a supplier quote. Smaller repairs and projects may use bagged concrete instead.

Bag counts are approximate because products have different yields. ATHENA uses planning yields of about 45 80-lb bags or 60 60-lb bags per cubic yard. Always check the exact yield printed on the product you plan to purchase.

If a manufacturer gives a cubic-foot yield per bag, a product-specific conversion is: bags = recommended cubic yards × 27 ÷ cubic feet of yield per bag. Round up to a whole bag.

What this calculator does—and does not—estimate

This calculator estimates rectangular concrete volume and provides planning quantities for cubic yards and common bag sizes. It does not determine structural slab thickness, reinforcement design, footing dimensions, soil bearing requirements, drainage, control joints, finishing specifications, permits, delivery fees, labor, or local code requirements.

For structural work or unusual site conditions, use project-specific plans and confirm requirements with the appropriate qualified professional. For ready-mix deliveries, confirm the final quantity, minimum load, delivery charge, short-load charge, and mix requirements with the supplier.

Concrete estimating checklist

• Confirm finished length and width.
• Confirm the specified concrete thickness.
• Break irregular areas into measurable sections.
• Check for changes in elevation or thickness.
• Choose an allowance appropriate to the actual site conditions.
• Verify exact product yield when using bagged concrete.
• Ask the ready-mix supplier about minimum loads, delivery, and short-load charges.
• Keep reinforcement, forms, base preparation, finishing, curing, and cleanup in the project scope.

Concrete guides and related calculators

Use these guides to understand the estimate and check common project scenarios:

About ATHENA Concrete Calculator

ATHENA Calculators is a practical construction-tool collection designed to make common estimating math easier to check and reuse. The calculation is intentionally transparent so you can see the volume, planning allowance, and approximate bag quantities instead of receiving one unexplained number.

The results are intended for preliminary material planning. They are not a substitute for project drawings, engineering, supplier specifications, inspections, permits, or local requirements.