Concrete Calculator: How to Calculate the Exact Amount of Concrete You Need for Slabs, Posts, and Patios

Ordering too little concrete leaves you short on pour day. Ordering too much wastes money on material you cannot return. A concrete calculator takes the guesswork out of your next project by telling you exactly how many cubic yards or cubic meters you need based on your project dimensions.

Common Concrete Projects and Their Calculation Methods

Concrete Slabs (Patios, Driveways, Floors)

For a flat slab, the formula is straightforward: Length × Width × Thickness. Measure all three dimensions in feet, multiply them together, then divide by 27 to get cubic yards (since there are 27 cubic feet in a cubic yard).

Project	Dimensions	Thickness	Cubic Yards Needed
Small patio	10′ × 12′	4″ (0.33′)	1.48 cu yd
Driveway	20′ × 24′	5″ (0.42′)	7.47 cu yd
Walkway	4′ × 30′	4″ (0.33′)	1.48 cu yd
Garage floor	22′ × 24′	4″ (0.33′)	6.51 cu yd

Concrete Posts and Footings

For cylindrical posts or footings, use the formula for the volume of a cylinder: π × r² × h. Measure the radius (half the diameter) and height in feet, square the radius, multiply by π (3.14), then multiply by the height. Divide by 27 for cubic yards.

Post Type	Diameter	Depth	Concrete per Post
Fence post	8″	2′	0.026 cu yd
Deck support	12″	3′	0.087 cu yd
Porch column	16″	3′	0.155 cu yd

How Many Concrete Bags Do You Need?

If you are using bagged concrete mix (like Quikrete or Sakrete), here is how many bags you need per cubic yard:

Bag Size	Bags per Cubic Yard	Best For
40 lb (18 kg)	90 bags	Small repairs, stepping stones
60 lb (27 kg)	60 bags	Walkways, small slabs
80 lb (36 kg)	45 bags	Driveways, patios, large projects

Tips for Ordering Concrete

• Order 10% extra to account for spillage, uneven ground, and variation in depth
• For ready-mix: Minimum delivery is usually 3-4 cubic yards. For smaller projects, use bagged mix
• Check access: Can the concrete truck reach your pour site? For tight spaces, consider a concrete pump truck
• Weather matters: Do not pour concrete in freezing temperatures or heavy rain

Before you head to the hardware store or call the ready-mix company, use the free concrete calculator to get precise measurements for any project shape and size.