How Much Concrete Do I Need? Calculator & Formula Guide

Whether you're pouring a new patio, setting fence posts, or laying a foundation, getting the right amount of concrete is critical. Order too little and you'll have an incomplete pour; order too much and you're wasting money on material you can't use. Our concrete calculator takes the guesswork out of the process, but understanding the math behind it helps you plan better and avoid costly mistakes.

Concrete is sold by the cubic yard for ready-mix delivery or by the bag for smaller DIY projects. Most homeowners underestimate their needs by 10-20%, leading to frustrating delays when they run short mid-pour. This guide will teach you the exact formulas professionals use and show you how to account for waste, subgrade irregularities, and form variations.

The Basic Concrete Formula

The fundamental formula for calculating concrete volume is simple:

Volume (cubic yards) = (Length × Width × Depth) ÷ 27

All measurements must be in feet. The division by 27 converts cubic feet to cubic yards (since 3 × 3 × 3 = 27 cubic feet per cubic yard).

Calculating Concrete for Slabs

Slabs are the most common concrete project. For a standard 4-inch thick patio or sidewalk:

• 10×10 foot slab (4" thick): 10 × 10 × 0.33 ÷ 27 = 1.22 cubic yards
• 12×12 foot patio (4" thick): 12 × 12 × 0.33 ÷ 27 = 1.76 cubic yards
• 20×24 foot driveway (6" thick): 20 × 24 × 0.5 ÷ 27 = 8.89 cubic yards

Note: Convert inches to feet by dividing by 12. So 4 inches = 0.33 feet, and 6 inches = 0.5 feet.

Calculating Concrete for Footings

Footings support walls, posts, and foundations. They're typically deeper than slabs:

• Continuous footing (40 linear feet × 16" wide × 8" deep): 40 × 1.33 × 0.67 ÷ 27 = 1.32 cubic yards
• Pier footing (24" diameter × 36" deep): π × 1² × 3 ÷ 27 = 0.35 cubic yards each

Calculating Concrete for Columns and Posts

For round columns, use the cylinder formula:

Volume = π × radius² × height

A 12-inch diameter column that's 8 feet tall: π × 0.5² × 8 ÷ 27 = 0.23 cubic yards

Adding Waste Factor

Always order 5-10% extra concrete to account for:

• Uneven subgrade or forms
• Spillage during pouring
• Thicker spots where the ground dips
• Material left in the truck or mixer

For a project requiring 5 cubic yards, order 5.5 cubic yards (10% extra). Running short mid-pour is far worse than having a little extra.

Bags vs. Ready-Mix: When to Use Each

Use bagged concrete (60 lb or 80 lb bags) when:

• Project requires less than 1 cubic yard
• You have time to mix multiple batches
• Access to the site is limited for trucks

Use ready-mix delivery when:

• Project requires more than 1 cubic yard
• You need to pour continuously
• Labor for mixing isn't available

One cubic yard equals approximately 45 bags of 80-lb concrete or 60 bags of 60-lb concrete.

Frequently Asked Questions

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

A 10×10 slab at 4 inches thick requires about 1.22 cubic yards, which equals approximately 55 bags of 80-lb concrete or 74 bags of 60-lb concrete. Add 10% extra for waste.

What thickness should my concrete slab be?

Standard thickness is 4 inches for patios, sidewalks, and shed floors. Use 5-6 inches for driveways that support vehicles. Garage floors should be 4-6 inches depending on expected loads.

How do I convert cubic feet to cubic yards?

Divide cubic feet by 27. There are 27 cubic feet in one cubic yard (3 feet × 3 feet × 3 feet = 27).

Should I order extra concrete?

Yes, always order 5-10% more than your calculated amount. Subgrade variations, form imperfections, and spillage during pouring can easily consume extra material.

How much does concrete cost per cubic yard?

Ready-mix concrete typically costs $125-$150 per cubic yard delivered in 2026, depending on your location, mix specifications, and delivery distance. Bagged concrete costs more per cubic yard but makes sense for small projects.