Concrete Calculator
Calculate the amount of concrete needed in cubic yards based on slab, footing, or column dimensions.
Typical: 5-15%
Results
For 1.36 cubic yards (including 10% waste), ordering ready-mix concrete is recommended. For smaller quantities, you would need 62 80 lb bags or 82 60 lb bags.
How to Use This Calculator
Enter the dimensions of the area you need to fill with concrete. For slabs and footings, enter the length in feet, width in feet, and thickness in inches. The calculator converts thickness to feet and computes volume in cubic feet, then divides by 27 to convert to cubic yards, the standard ordering unit. For cylindrical forms such as piers or sonotubes, enter the diameter in inches and the height in feet. Add 10% to the calculated volume to account for waste, spillage, and slight over excavation. For example, a 20 foot by 12 foot patio slab at 4 inches thick requires 2.96 cubic yards, rounded up to 3 yards plus 10% waste equals 3.3 yards. Order 3.5 cubic yards from the ready mix plant.
Understanding the Concept
Accurate concrete estimation prevents costly mistakes on the jobsite. Ordering too little concrete means a second truck delivery at premium short load charges and a cold joint in the pour. Ordering too much wastes material and creates disposal problems. Ready mix concrete is sold by the cubic yard in the United States, with each yard weighing approximately 4,000 pounds. A standard delivery truck carries 8 to 10 cubic yards. Most plants charge a short load fee for orders under 5 yards, so combining small pours can save money. Concrete thickness varies by application: residential slabs typically require 4 inches, garage floors need 5 to 6 inches, driveways need 4 to 6 inches depending on traffic, and footings must meet local code requirements, often 12 inches deep and 20 inches wide for residential work. Always verify the required thickness and reinforcement with local building codes before ordering.
The Formula Explained
For rectangular areas, volume in cubic yards equals length in feet multiplied by width in feet multiplied by thickness in feet, divided by 27. Since thickness is often specified in inches, convert by dividing by 12 first. The complete formula is V = (L multiplied by W multiplied by T divided by 12) divided by 27, where L is length in feet, W is width in feet, and T is thickness in inches. For cylindrical forms, volume equals pi multiplied by radius squared multiplied by height, divided by 27. Convert the diameter from inches to feet by dividing by 12, then divide by 2 to get the radius. The formula becomes V = (3.1416 multiplied by (D divided by 24) squared multiplied by H) divided by 27, where D is diameter in inches and H is height in feet. Add a 10% waste factor to the final result.
Frequently Asked Questions
How many bags of concrete are in a cubic yard?
An 80 pound bag of premixed concrete yields approximately 0.6 cubic feet. One cubic yard equals 27 cubic feet, so you need about 45 bags of 80 pound mix per cubic yard. For areas larger than 1 cubic yard, ready mix delivery is more practical and cost effective.
How thick should a concrete slab be?
Residential patios and walkways require a minimum of 4 inches. Garage floors and driveways should be 5 to 6 inches for passenger vehicles. Commercial and industrial slabs range from 6 to 8 inches or more depending on loading requirements. Always check local building codes.
How much extra concrete should I order?
Order 10% more than the calculated volume to account for waste, spillage, uneven subgrade, and form irregularities. For complex shapes or sloped sites, increase the overage to 15%. It is always better to have a small amount left over than to run short during a pour.
What is the difference between concrete and cement?
Cement is a powder ingredient, typically Portland cement, that acts as the binder. Concrete is the finished product made from cement, water, sand, and gravel or crushed stone. Ready mix concrete is delivered wet and ready to pour. The terms are not interchangeable.
How do I calculate concrete for steps or irregular shapes?
Break the area into simple rectangular or cylindrical sections, calculate each section separately, and add the volumes together. For steps, calculate each tread and riser as a rectangular block. Add 10 to 15% waste factor for complex shapes.