Question 1

How much concrete do I need for a slab?

Accepted Answer

For a rectangular slab, multiply length x width x thickness (in feet) to get cubic feet, then divide by 27 to convert to cubic yards. A 10x10 foot slab that is 4 inches thick = 100 sq ft x 0.333 ft = 33.3 cubic feet = 1.23 cubic yards. With a 10% waste factor, order 1.36 cubic yards (about 1.5 yards for safety). For a 4-inch slab, plan on 1 cubic yard per 81 square feet; for 6-inch slab, 1 yard per 54 sq ft.

Question 2

How many bags of concrete make a yard?

Accepted Answer

It depends on the bag size. A 60-pound bag of concrete mix yields about 0.45 cubic feet, so you need about 60 bags per cubic yard. An 80-pound bag yields about 0.60 cubic feet, so you need 45 bags per cubic yard. A 50-pound bag yields 0.375 cubic feet, so you need 72 bags per cubic yard. For small projects under 1 cubic yard, bags are usually cheaper and easier than ordering ready-mix delivery.

Question 3

What is the formula for cubic yards of concrete?

Accepted Answer

The general formula is: cubic yards = volume in cubic feet / 27. For a slab: cubic yards = (length ft x width ft x thickness ft) / 27. For a column or post hole: cubic yards = (pi x radius ft squared x height ft) / 27. For a wall: cubic yards = (length ft x thickness ft x height ft) / 27. Always add 10% extra to account for spillage, over-excavation, and uneven subgrade.

Question 4

How thick should a concrete slab be?

Accepted Answer

Standard residential slabs are 4 inches thick for patios, walkways, and shed floors. Driveways and garage floors should be 5-6 inches thick to handle vehicle weight. Commercial or heavy-duty applications may need 6-8 inches. For footings, the standard is 8-12 inches deep and 16-24 inches wide, depending on the load and frost line in your area. Always check local building codes for required thickness.

Question 5

Should I order ready-mix concrete or use bags?

Accepted Answer

Use bags for small projects under 1 cubic yard (about 45-72 bags of 80lb mix). They're cheaper for tiny jobs and you can work at your own pace. Order ready-mix delivery for anything over 1 cubic yard. Ready-mix costs $125-200 per cubic yard delivered, plus a short-load fee of $40-100 if you order less than 3-4 yards. A 1-cubic-yard minimum delivery is standard in most areas. Bags require you to mix each one (a rented mixer helps) but offer zero waste from over-ordering.

Question 6

How do I calculate concrete for a round post hole?

Accepted Answer

For a cylindrical post hole, use the formula: cubic feet = pi x radius squared x depth. For example, a hole 12 inches in diameter (6-inch radius = 0.5 ft) and 24 inches deep (2 ft) = 3.14 x 0.25 x 2 = 1.57 cubic feet. Multiply by the number of holes for total volume. Each hole this size needs about 0.06 cubic yards, or one 80-pound bag of fast-setting concrete (like Quikrete Fast-Setting) which is pre-mixed for post-setting applications.

Question 7

What is the waste factor for concrete?

Accepted Answer

Industry standard is to add 10% to your calculated volume to account for waste, spillage, over-excavation, uneven subgrade, and concrete that stays in the chute or mixer. For difficult sites with poor access or unusual shapes, bump to 15%. Never order exactly what you calculate - short loads are expensive (a second delivery often costs more than the extra concrete would have). This calculator automatically adds 10% to every estimate.

Concrete Calculator

📝 How to use this concrete calculator

📌 5 tips for ordering concrete

Related Tools

🛒 Resources for Your Concrete Project