OptiDrop

Concrete Calculator — Volume & Bags Needed

Calculate concrete volume for slabs or cylinders in cubic feet, cubic yards, and the number of bags (40 lb, 60 lb, 80 lb). 100% free, no sign-up.

Free Concrete Calculator Online — Volume and Bag Estimator

OptiDrop's Concrete Calculator helps you determine the exact volume of concrete you need for any project. Whether you are pouring a rectangular slab (like a patio, driveway, or foundation) or a circular column/footing, this tool calculates the volume in both cubic feet and cubic yards, plus the number of pre-mixed bags required.

How Does the Concrete Calculator Work?

For rectangular slabs, it multiplies length by width by thickness (converted to feet). For cylinders, it uses the formula pi times radius squared times height. The resulting volume is displayed in cubic feet and cubic yards (dividing by 27). It then calculates the number of 40 lb, 60 lb, and 80 lb bags needed based on their respective yields.

When to Use Bags vs. Ready-Mix Delivery

For small projects (under 1 cubic yard), pre-mixed bags are usually more economical. For larger pours, ordering ready-mix concrete delivered by truck is more practical and cost-effective. As a rule of thumb, if you need more than about 40-50 bags of 80 lb concrete, consider ordering a ready-mix delivery instead.

Frequently Asked Questions

Measure the length, width, and thickness of the area you want to pour. Multiply these three dimensions to get the volume in cubic feet. Divide by 27 to convert to cubic yards. This calculator handles both slab and cylinder shapes automatically.
A 60 lb bag yields about 0.45 cubic feet of concrete, an 80 lb bag yields about 0.60 cubic feet, and a 40 lb bag yields about 0.30 cubic feet. Divide your total volume in cubic feet by the yield per bag to get the number of bags needed.
One cubic yard equals 27 cubic feet. Concrete is often sold by the cubic yard for large projects (ordered as ready-mix delivery) and by the bag for small projects. This calculator shows both measurements so you can order accordingly.
Yes, it is recommended to order 5-10% extra concrete to account for spillage, uneven ground, and measurement errors. Running short during a pour can cause cold joints and weaken the structure. This calculator does not include extra by default, so consider adding a buffer.

Last updated: June 2026