How Many Bags of Concrete Do I Need? (10×10 Slab and More)

Running out of concrete halfway through a pour is one of the most common and most costly DIY mistakes. Concrete starts to set quickly, so a second trip to the store can leave a weak joint in the middle of your slab. The good news is that working out how many bags you need takes only a few minutes once you know the formula.

Step 1: Work Out the Volume

Concrete is measured by volume. For any rectangular slab, footing or pad, multiply the three dimensions together:

Volume (cu ft) = Length (ft) × Width (ft) × Thickness (ft)

The thickness is usually given in inches, so divide it by 12 first. A 4 inch slab is 0.333 ft thick, a 5 inch slab is 0.417 ft and a 6 inch slab is 0.5 ft.

Step 2: Divide by the Bag Yield

Every bag of pre-mixed concrete makes a set volume once mixed with water. These yields are typical for standard concrete mix:

• 80 lb bag: about 0.60 cubic feet
• 60 lb bag: about 0.45 cubic feet
• 40 lb bag: about 0.30 cubic feet

Bags = Volume (cu ft) ÷ Bag yield, rounded up

For the 10 × 10 ft slab: 10 × 10 × 0.333 = 33.3 cu ft. Then 33.3 ÷ 0.60 = 55.6, which rounds up to 56 bags of 80 lb concrete.

Concrete Bags Chart for 4 Inch Slabs

This chart shows the bags needed for common slab sizes at a standard 4 inch thickness, without any waste allowance:

Slab size	Volume	Cubic yards	80 lb bags	60 lb bags	40 lb bags
4 × 8 ft	10.7 cu ft	0.4	18	24	36
5 × 5 ft	8.3 cu ft	0.31	14	19	28
10 × 10 ft	33.3 cu ft	1.23	56	75	112
12 × 12 ft	48 cu ft	1.78	80	107	160
10 × 20 ft	66.7 cu ft	2.47	112	149	223
20 × 20 ft	133.3 cu ft	4.94	223	297	445

For a 6 inch slab, multiply these numbers by 1.5. For a 5 inch slab, multiply by 1.25.

Add Extra for Waste

Always buy a little more than the exact figure. Sub-grades are rarely perfectly level, forms can bow outwards, and some concrete is always left in the mixer or wheelbarrow. Most contractors add 5–10%. For the 10 × 10 slab, 10% extra brings the total to 62 bags of 80 lb concrete.

Bags or Ready-Mix?

Mixing bags by hand or in a small mixer is practical for small jobs such as fence posts, steps, shed bases and small patios. Once you need more than about one cubic yard, which is around 45 bags of 80 lb concrete, ready-mix delivered by truck is usually cheaper and much less work. It also gives a more consistent slab, because the whole pour comes from the same batch.

Concrete for Post Holes

Round holes use the cylinder formula: π × radius² × depth. A 10 inch wide hole that is 2 ft deep holds about 1.09 cubic feet. With a 4×4 post in the middle, you need about 0.92 cu ft, or roughly two 80 lb bags per post. Our fence calculator works this out for a whole fence line.

Tips Before You Pour

• Compact a 4 inch gravel base under slabs to reduce cracking. Our gravel calculator sizes the base.
• Check local codes for slab thickness, reinforcement and footing depth.
• Have enough helpers and tools ready; concrete sets faster in hot weather.
• Keep the bags dry until you use them. Damp bags start to harden.

Frequently Asked Questions

How many 80 lb bags of concrete make a yard?

About 45. A cubic yard is 27 cubic feet and each 80 lb bag yields about 0.6 cubic feet.

How many bags of concrete do I need for a 12×12 slab?

A 12 × 12 ft slab at 4 inches thick needs 48 cubic feet, which is 80 bags of 80 lb concrete or 107 bags of 60 lb concrete before waste.

Is it cheaper to mix concrete myself?

For small jobs under about one cubic yard, bags are usually cheaper. For larger slabs, ready-mix is normally cheaper per yard and saves a lot of labor.

Related Guides

• How Many Concrete Blocks Do I Need for a Wall?
• How Much Gravel Do I Need for a Driveway?
• How Much Mulch Do I Need? Bags, Yards and Coverage

Try the free Concrete Calculator or browse all home improvement calculators.