How to Calculate Concrete: A Complete Guide

Ordering the right amount of concrete is one of the most important steps in any pour. Too little means a costly second delivery and a cold joint in your slab. Too much means paying for material that ends up in the waste pile. This guide covers the formula, common project types, and tips for getting your order right.

The Basic Concrete Formula

Concrete is sold by the cubic yard in the United States. One cubic yard equals 27 cubic feet. The formula for any rectangular pour is:

Volume (cubic yards) = (Length x Width x Depth in feet) / 27

For example, a 20 ft x 10 ft patio slab poured 4 inches thick:

1. Convert 4 inches to feet: 4 / 12 = 0.333 ft
2. Volume in cubic feet: 20 x 10 x 0.333 = 66.6 cu ft
3. Volume in cubic yards: 66.6 / 27 = 2.47 cu yd

You would order at least 2.5 cubic yards, plus a waste allowance.

Slabs, Footings, and Columns

The formula stays the same, but the shape changes. Slabs (patios, driveways) are typically 4 inches thick for foot traffic or 6 inches for vehicles. Footings are continuous strips usually 12 to 24 inches wide and 8 to 12 inches deep. Columns use the cylinder formula: pi x radius squared x height. For complex shapes like steps, break them into rectangles and add the volumes together.

Bags vs. Ready-Mix

For jobs under 1 cubic yard, pre-mixed bags are practical. An 80-pound bag yields about 0.6 cubic feet, meaning you need roughly 45 bags per cubic yard. For anything larger, ready-mix from a batch plant is faster and cheaper per yard. Most plants have a minimum order of 1 yard and may charge a short-load fee for orders under 3 to 5 yards.

When You Need This Calculator

• Homeowners pouring a patio, sidewalk, or shed pad who need to know how many bags to buy or how large a truck to order.
• Contractors estimating material costs who need accurate yardage across multiple footings and slabs.
• DIYers building fence post footings who want to calculate total concrete for dozens of post holes at once. Pair this with the Fence Calculator to get a full materials list.

Common Mistakes to Avoid

• Forgetting the waste factor. Add 5 to 10 percent to cover spillage, uneven subgrade, and over-excavation.
• Mixing up inches and feet. A 4-inch slab entered as 4 feet gives a result 12 times too large. Always verify your units.
• Ignoring the subgrade. Uneven gravel or over-excavated soil increases actual pour depth. Use our Gravel Calculator to estimate the base layer, then measure in several spots and use the average.
• Ordering exact quantities. Running short mid-pour creates a cold joint — a structural weak point. Always round up.

Pro Tips

• Schedule early. During peak season, ready-mix trucks book up fast. Reserve your pour at least a few days in advance.
• Plan for leftovers. Set up forms for a stepping stone or splash block so extra concrete does not go to waste.
• Check the weather. Avoid pouring below 40 degrees or above 90 degrees Fahrenheit without special precautions.
• Know your PSI. Residential slabs use 3,000 to 4,000 PSI. Driveways benefit from 4,000 PSI.

Frequently Asked Questions

How many bags of concrete do I need for a 10x10 slab?

A 10 ft x 10 ft slab at 4 inches thick requires about 1.23 cubic yards. In 80-pound bags, that is approximately 56 bags. For a project this size, ordering ready-mix is usually easier and more economical.

How much does a yard of concrete cost?

Ready-mix typically costs $125 to $175 per cubic yard for standard residential mixes, plus delivery and pump fees. Pre-mixed bags run $5 to $7 each, making a full yard of bagged concrete $250 to $300.

What is a waste factor and how much should I add?

A waste factor accounts for spillage, uneven ground, and imperfect forms. Add 5 to 10 percent for most residential projects. For irregular shapes or sloped ground, increase to 15 percent.