Concrete Calculator

How to Use the Concrete Calculator

The Concrete Calculator estimates the volume of concrete you need for any project, whether it is a patio slab, sidewalk, driveway, footing, column, or stairs. Enter the dimensions and the calculator provides the volume in cubic yards, cubic feet, and cubic meters, plus the number of premix bags needed.

Slab Calculations

For rectangular slabs (patios, driveways, sidewalks), enter the length, width, and thickness. A standard patio might be 12 ft × 10 ft × 4 inches. Volume = 12 × 10 × (4/12) = 40 cubic feet = 1.48 cubic yards. Add 10% for waste, bringing the order to about 1.63 cubic yards.

Footing and Column Calculations

For footings, enter the length, width, and depth. A typical residential footing might be 20 inches wide × 8 inches deep running 100 linear feet. For round columns, enter the diameter and height. The calculator uses πr² × h for cylindrical shapes. A 12-inch diameter column that is 8 feet tall needs about 6.3 cubic feet.

Premix Bag Estimates

The calculator converts the volume into bags of premixed concrete. Standard bag yields: 40-lb bag = 0.011 cubic yards (0.30 cu ft), 60-lb bag = 0.017 cubic yards (0.45 cu ft), 80-lb bag = 0.022 cubic yards (0.60 cu ft). For 1 cubic yard, you need approximately 45 bags of 80-lb mix or 90 bags of 40-lb mix.

Ordering Ready-Mix Concrete

For projects over 1 cubic yard, ordering ready-mix delivery is usually more economical and efficient than mixing bags. Ready-mix trucks typically deliver minimum 1 cubic yard. For a 2.5 cubic yard project, order 2.75 yards to account for waste (about 10% extra). Inform the dispatcher of the concrete type needed (standard, high-strength, fiber-reinforced).

Common Project Volumes

A 10×10 ft patio (4 in thick): 1.23 cu yd. A two-car driveway 20×24 ft (5 in thick): 7.41 cu yd. Sidewalk 3 ft × 50 ft (4 in thick): 1.85 cu yd. A deck pier 12 in diameter × 4 ft deep: 3.14 cu ft. These estimates help you budget before measuring your specific project.

Frequently Asked Questions

Q: How much extra concrete should I order?

A: Order 5-10% more than the calculated amount for waste, spillage, uneven subgrade, and slight over-excavation. For slabs, 5% extra is usually sufficient. For footings and irregular shapes, order 10% extra. Running short during a pour is much worse than having a small amount left over.

Q: What thickness should my concrete slab be?

A: Sidewalks and patios: 4 inches. Driveways for cars: 4-5 inches. Driveways for heavy vehicles: 5-6 inches. Garage floors: 4-6 inches. Building foundations: per engineering specifications, typically 6-12 inches. Thicker slabs are stronger but use more material and cost more.

Q: How many bags of concrete do I need for a fence post hole?

A: A typical fence post hole is 10 inches in diameter and 24 inches deep, requiring about 1.1 cubic feet of concrete. One 80-lb bag fills about 0.6 cu ft, so you need 2 bags per post. For a 100-foot fence with posts every 8 feet (13 posts), you need about 26 bags of 80-lb premix.