Concrete Calculator

Calculate cubic yards, bag count, and cost for slabs, footings, and columns.

ft
ft
in
Cubic Yards Needed
1.23
33.33 cubic feet
80 lb Bags
56 bags
60 lb Bags
75 bags
Ready-Mix Cost Estimate
$160.49
At $130 per cubic yard

How to Use This Concrete Calculator

  1. Select the shape — choose rectangular slab for patios, driveways, and sidewalks; circular slab for round pads; column for posts and piers; or footing for wall foundations.
  2. Enter dimensions — for rectangular shapes, enter length and width in feet and thickness in inches. For circular shapes, enter the diameter. For columns, enter the diameter and height.
  3. Read your results — the calculator shows cubic yards (for ready-mix ordering), bag counts for both 80lb and 60lb bags, and a cost estimate based on current average pricing.

Understanding Concrete Volume

Concrete is ordered by the cubic yard (27 cubic feet). Getting the volume right is critical — too little means an incomplete pour and a cold joint, while too much wastes money. This calculator helps you determine the exact amount needed for any shape.

Concrete Volume Formulas

The volume calculations depend on the shape of your project:

Rectangular: Volume = Length × Width × (Depth ÷ 12)
Circular: Volume = π × (Diameter/2)² × (Depth ÷ 12)
Column: Volume = π × (Diameter/2)² × Height
Cubic Yards = Cubic Feet ÷ 27

Bags vs Ready-Mix Concrete

For small projects under one cubic yard, bags of premixed concrete are convenient. An 80lb bag yields about 0.6 cubic feet of concrete, while a 60lb bag yields about 0.45 cubic feet. For larger projects, ready-mix delivery is more practical. A concrete truck typically delivers a minimum of one cubic yard and charges approximately $130 per yard plus a delivery fee. For projects between 1-3 yards, ready-mix is almost always more cost-effective and produces a better, more consistent result.

Concrete Thickness Guidelines

The required thickness depends on the load the concrete must bear. Sidewalks and patios need a minimum of 4 inches. Residential driveways should be 4-6 inches thick (use 6 inches if trucks or heavy vehicles will drive on it). Garage floors require 4-6 inches with reinforcement. Foundation footings are typically 8-12 inches thick and extend below the frost line. Always check your local building codes for specific requirements.

Reinforcement and Curing

Rebar or wire mesh reinforcement prevents cracking under load and temperature changes. For slabs over 4 inches thick, #4 rebar in a grid pattern with 18-24 inch spacing is standard. Fiber mesh can be mixed directly into the concrete for added strength. After pouring, curing is critical — keep the surface moist for at least 7 days by misting with water or covering with plastic sheeting. Proper curing increases final strength by up to 50% compared to letting concrete dry too quickly.

Common Mistakes to Avoid

The most frequent concrete mistakes include under-ordering material (always order 10% extra), inadequate subbase preparation (4-6 inches of compacted gravel), adding too much water to the mix (weakens the final product), pouring in extreme temperatures without precautions, and removing forms too early. Taking the time to prepare properly saves money and prevents costly do-overs.

Frequently Asked Questions

An 80lb bag of concrete yields approximately 0.6 cubic feet when mixed. A 60lb bag yields about 0.45 cubic feet. To find how many bags you need, calculate your total volume in cubic feet and divide by the bag yield. For example, a 4-inch thick 10x10 foot slab needs 33.3 cubic feet, which requires 56 bags of 80lb concrete.
For small projects under 1 cubic yard (about 45 bags of 80lb), mixing bags yourself is typically more cost-effective. For projects over 1-2 cubic yards, ready-mix delivery is faster, provides more consistent results, and is often cheaper per yard. Ready-mix is essential for large pours like driveways and foundations where continuous pouring is critical.
Standard thickness depends on the application: sidewalks and patios need 4 inches, driveways require 4-6 inches (6 inches for heavy vehicles), garage floors should be 4-6 inches, and foundation slabs typically require 4-8 inches depending on local building codes and soil conditions.
Ready-mix concrete typically costs $120-160 per cubic yard delivered, with an average of about $130. Prices vary by region, concrete mix type, and delivery distance. Bag concrete costs about $4-6 per 80lb bag, which works out to roughly $180-270 per cubic yard — more expensive but suitable for small projects.
Concrete reaches about 70% of its full strength in 7 days and 99% in 28 days. You can walk on it after 24-48 hours and drive on it after 7 days. Full curing takes 28 days. Keep concrete moist during the first week for optimal strength. Avoid heavy loads for at least 7 days after pouring.