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Concrete Calculator

Concrete volume for a rectangular slab is calculated as Length x Width x Depth, with the result converted to cubic yards (divide cubic feet by 27). A standard 10 x 10 foot slab at 4 inches thick requires 1.23 cubic yards of concrete, or approximately 56 80-lb bags of premix. This calculator supports three shapes: slabs, cylindrical columns/posts, and stairs. Select the shape and enter dimensions for an instant volume and bag count estimate.

Quick Answer

A 10 x 10 foot slab at 4 inches thick requires approximately 1.23 cubic yards of concrete, or about 56 bags of 80 lb premix.

Common Examples

Input Result
Slab: 10 ft x 10 ft x 4 in 33.33 cu ft, 1.23 cu yd, 75 bags (60 lb), 56 bags (80 lb)
Slab: 20 ft x 20 ft x 6 in 200 cu ft, 7.41 cu yd, 445 bags (60 lb), 334 bags (80 lb)
Column: 12 in diameter x 48 in tall 3.14 cu ft, 0.12 cu yd, 7 bags (60 lb), 6 bags (80 lb)
Stairs: 5 steps, 7.5 in rise, 10 in run, 36 in wide 6.51 cu ft, 0.24 cu yd, 15 bags (60 lb), 11 bags (80 lb)

How It Works

The Formulas

Rectangular Slab:

Volume (cu ft) = Length (ft) x Width (ft) x Depth (in) / 12

This is the most common concrete calculation. The depth is divided by 12 to convert inches to feet so all dimensions share the same unit.

Cylindrical Column:

Volume (cu ft) = pi x (Diameter in inches / 24)^2 x (Height in inches / 12)

The diameter is divided by 24 (converting inches to feet and halving to get the radius). This formula covers round post holes, sonotube forms, and cylindrical footings.

Stairs:

Stair volume is calculated by summing the volume of each progressive step. Each step has a volume equal to the cumulative rise up to that step times the run times the width. The total is the sum of all individual step volumes.

Unit Conversions

  • Cubic Yards = Cubic Feet / 27 (ready-mix concrete is sold by the cubic yard)
  • 60 lb bags each yield approximately 0.45 cubic feet of mixed concrete
  • 80 lb bags each yield approximately 0.60 cubic feet of mixed concrete

Ordering Tips

For small projects (under 1 cubic yard), premixed bags from a home improvement store are practical. For larger pours, ordering ready-mix concrete by the truck is more economical. Most ready-mix companies have a minimum order of 1 cubic yard. Adding 5% to 10% extra to the calculated volume accounts for spillage, uneven subgrade, and slight form variations.

Worked Example

For a patio slab measuring 10 feet by 10 feet at 4 inches thick: Volume = 10 x 10 x (4 / 12) = 10 x 10 x 0.333 = 33.33 cubic feet. In cubic yards: 33.33 / 27 = 1.23 cubic yards. In 80 lb bags: 33.33 / 0.60 = 55.56, rounded up to 56 bags. In 60 lb bags: 33.33 / 0.45 = 74.07, rounded up to 75 bags. Adding 10% for waste brings the estimate to approximately 1.36 cubic yards or 61 bags of 80 lb premix.

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Frequently Asked Questions

How many bags of concrete do I need for a 10 x 10 slab?
A 10 x 10 foot slab at 4 inches thick requires approximately 33.33 cubic feet or 1.23 cubic yards of concrete. That equals about 56 bags of 80 lb premix or 75 bags of 60 lb premix. At 6 inches thick, the same slab requires approximately 50 cubic feet, 1.85 cubic yards, or 84 bags of 80 lb premix.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27. One cubic yard equals 27 cubic feet (3 ft x 3 ft x 3 ft). For example, 54 cubic feet equals 54 / 27 = 2.0 cubic yards. Ready-mix concrete trucks deliver concrete measured in cubic yards.
What thickness should a concrete slab be?
A standard residential patio or walkway slab is 4 inches thick. Driveways typically require 4 to 6 inches. Garage floors are usually 4 to 6 inches. For heavy loads like RVs or commercial vehicles, 6 to 8 inches may be needed. Local building codes may specify minimum thickness requirements for certain applications.
How much extra concrete should I order?
Adding 5% to 10% extra to the calculated amount is a common practice. This accounts for spillage, uneven ground beneath the slab, and slight variations in formwork dimensions. For smaller projects using bags, rounding up to the next full bag provides a sufficient buffer.
Can I use this calculator for post holes?
Yes. Select the Column shape and enter the hole diameter and depth. A standard fence post hole is 10 to 12 inches in diameter and 24 to 36 inches deep. For a deck post, 12 inches in diameter and 42 to 48 inches deep is common. Multiply the single column result by the number of post holes for the total.

Learn More

How to calculate concrete for any project

Concrete is sold by the cubic yard. Convert your slab, footing, or sonotube dimensions to cubic yards with the formula and a worked example for each.

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