How Much Concrete Do I Need? A Complete Guide

Step-by-Step Method

1. Measure your pour's length and width in feet, and thickness in inches.
2. Convert thickness to feet by dividing by 12.
3. Multiply length × width × thickness (ft) to get cubic feet.
4. Divide cubic feet by 27 to get cubic yards.
5. Add a waste factor (5–20% depending on ground conditions) to your total.

The Formula

Worked Example

A 12 ft × 16 ft patio slab poured 4 inches thick:

• Thickness in feet: 4 ÷ 12 = 0.333 ft
• Cubic feet: 12 × 16 × 0.333 = 64 cu ft
• Cubic yards: 64 ÷ 27 = 2.37 cu yd
• With 10% waste: 2.37 × 1.10 = 2.60 cu yd

Converting to Bags

If you're hand-mixing instead of ordering a truck, divide your final cubic feet (waste included) by the bag's yield: 0.30 cu ft for a 40lb bag, 0.45 cu ft for a 60lb bag, and 0.60 cu ft for an 80lb bag, then round up.

Other Shapes: Circles and Tubes

For a round footing, column, or Sonotube, use volume = π × radius² × height. A 12 inch diameter tube (radius 0.5 ft) that is 4 feet deep holds 3.14 × 0.5 × 0.5 × 4 = about 3.1 cubic feet.

How Many Bags per Cubic Yard?

One cubic yard is 27 cubic feet, so it takes about 60 bags of 60 lb concrete, about 45 bags of 80 lb concrete, or about 90 bags of 40 lb concrete. For anything over roughly one cubic yard, ordering ready-mix is usually easier and often cheaper than buying and mixing dozens of bags.

Ordering Tips

• Add 5–10% for waste, uneven subgrade, and spillage. Add more for rough or excavated ground.
• Ready-mix trucks commonly carry about 9–10 cubic yards, and many suppliers charge a short-load fee under a minimum order.
• Have forms, reinforcement, and helpers ready before the truck arrives, because you only have a limited time to place and finish it.
• Write down your dimensions and thickness to give the supplier, along with the strength (psi) you need.

Frequently Asked Questions