Free Concrete Calculator — Cubic Yards, Bags, and Volume

How to Calculate Concrete for a Slab

Calculating the right amount of concrete for a project is critical — too little means a return trip to the store mid-pour, and too much wastes money. The basic formula for a rectangular concrete slab is straightforward: multiply the length by the width to get the area in square feet, then multiply by the thickness (converted to feet) to get the volume in cubic feet. Divide the cubic feet by 27 to convert to cubic yards, which is the standard unit for ordering ready-mix concrete.

For example, a 10-foot by 20-foot slab that is 4 inches thick: Length × Width × Depth = 10 × 20 × (4/12) = 10 × 20 × 0.333 = 66.67 cubic feet. Divide by 27 = 2.47 cubic yards. When ordering ready-mix, always round up to the nearest quarter or half yard to avoid running short during the pour.

For circular slabs (round patios, footings), the formula uses the radius: Volume = π × r² × depth. Measure the diameter and divide by 2 to get the radius. For tube-form columns (common for deck posts and fence posts), use the same formula with the height of the column instead of the slab thickness.

This calculator handles all three shapes and automatically converts between cubic feet, cubic yards, and cubic meters, so you can order confidently regardless of how your supplier quotes volume.

How Many Bags of Concrete Do I Need?

Pre-mixed concrete bags are the go-to option for small to medium DIY projects where ordering a ready-mix truck is impractical. They come in two common sizes in the US: 60 lb bags and 80 lb bags.

Coverage rates vary slightly by brand and mix type, but standard estimates are:

• 60 lb bag: Covers approximately 0.45 cubic feet of concrete when mixed.
• 80 lb bag: Covers approximately 0.60 cubic feet of concrete when mixed.

To find the number of bags needed, divide your total volume in cubic feet by the coverage rate per bag, then round up to the nearest whole number. For a 66.67 cubic foot slab: 66.67 / 0.45 = 148 bags of 60 lb, or 66.67 / 0.60 = 112 bags of 80 lb. The 80 lb bags are more economical per cubic foot but are heavier to handle — an important consideration when working alone.

For projects larger than about 1 cubic yard (27 cubic feet), ordering a ready-mix truck is often more cost-effective and results in a stronger, more consistent mix than bagged concrete. Ready-mix concrete is ordered by the cubic yard, and most trucks have a minimum order of 1 yard. Call local suppliers for pricing — ready-mix typically costs $100–$150 per cubic yard plus delivery, while bagged concrete can cost 2–3 times more for the same volume.

Cubic Yards Explained

The cubic yard is the standard unit for measuring concrete volume in the United States. One cubic yard equals a cube that is 3 feet on each side: 3 ft × 3 ft × 3 ft = 27 cubic feet. When you call a concrete supplier to order ready-mix, they quote prices by the cubic yard.

To visualise a cubic yard: it is roughly the volume of a washing machine. A standard pickup truck bed holds about 1.5–2 cubic yards of concrete (though this is far too heavy for most trucks — concrete weighs approximately 2 tons per cubic yard). For reference:

• 1 cubic yard = 27 cubic feet = 0.765 cubic meters
• A 10×10 ft slab at 4 inches thick ≈ 1.23 cubic yards
• A 20×20 ft patio at 4 inches thick ≈ 4.94 cubic yards
• A standard transit mixer (concrete truck) carries 8–10 cubic yards

When planning a large pour, coordinate with your supplier about truck access to the job site. Ready-mix trucks are heavy and require firm, flat ground to maneuver safely. For tight or restricted sites, a concrete pump may be necessary — add this cost to your project budget.

Concrete vs Gravel vs Sand — Which Calculator?

This calculator handles multiple materials beyond concrete, using the same volume formulas. The key difference between materials is density, which affects the weight calculation:

• Concrete: Approximately 2 tons (4,000 lbs) per cubic yard. Used for structural slabs, footings, driveways, steps, and countertops.
• Gravel / Crushed Stone: Approximately 1.35 tons (2,700 lbs) per cubic yard. Used for drainage layers under slabs, driveways, landscaping, and French drains.
• Sand: Approximately 1.3 tons (2,600 lbs) per cubic yard. Used as a base layer under pavers and tiles, for mixing mortar, and in children's play areas.
• Mulch: Approximately 0.4–0.6 tons per cubic yard. Used for garden beds, landscaping, and moisture retention. Sold by the cubic yard at garden centers.
• Topsoil: Approximately 0.9–1.0 tons per cubic yard. Used for lawn establishment, garden beds, and grading.

All of these materials are measured and sold by volume, so the calculation process is identical — only the weight and cost per cubic yard differ.

DIY Concrete Projects

Concrete is one of the most versatile DIY materials for homeowners. Common projects that use bagged concrete mix include:

• Fence post footings: A typical 4×4 fence post hole is 10 inches in diameter and 3 feet deep — about 0.17 cubic feet, or less than one 60 lb bag per post. Fast-setting bagged concrete (like Quikrete Fast-Setting) can be poured dry into the hole with water — no mixing required.
• Deck post footings: Deck footings need to extend below the frost line (which varies by region) and support significant loads. Calculate the tube-form diameter and depth, then use this calculator to find the number of bags.
• Small slabs and pads: Slabs for sheds, air conditioners, generators, and hot tubs typically use 4 inches of concrete. Calculate length × width × 0.333 ft for cubic feet, then divide by 0.45 for 60 lb bags.
• Concrete countertops: A growing DIY trend. Countertop mixes are formulated for higher compressive strength. Calculate area × thickness (typically 1.5–2 inches) for volume.
• Stepping stones and pavers: Cast in molds, typically 2 inches thick. Easy weekend projects requiring just a few bags of mix.

Frequently Asked Questions

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