How Many Bags of Concrete Do I Need? (Slabs, Footings, Posts)

Concrete math starts with volume

Concrete is sold by volume, but bagged concrete is small enough that thinking in cubic feet is usually easier than cubic yards. Start with the shape: a slab is a box, a footing is a long box, and a round post hole is a cylinder. For all three, the basic move is the same: length × width × depth, in feet. That gives cubic feet. Divide by 27 to get cubic yards.

Keep the units straight. Convert inches to feet before multiplying. A 4-inch slab is 4/12 of a foot, or 1/3 foot. A 16-inch-wide footing is 16/12 feet across. Write the depth in feet every time, and the rest of the math stays simple.

What one bag actually makes

Bag yield numbers can vary slightly by mix design, but these are the numbers the Crunch Numbers calculator uses:

That is a useful sanity check. One cubic yard is 27 cubic feet. If you hear “a cubic yard takes 45 bags,” that means 80-lb bags, not 60-lb bags. If you are working with 60-lb bags, a cubic yard takes 60 bags.

Worked example: a 10 x 12 patio slab

Picture a 10 ft by 12 ft patio slab poured 4 inches thick. Four inches is 1/3 foot. The volume is:

10 × 12 × 1/3 = 40 cubic feet

In cubic yards, that is 40 ÷ 27 = 1.48 cubic yards.

For 80-lb bags, divide 40 cubic feet by 0.60. That gives 66.7 bags, so you round up to 67 bags. For 60-lb bags, divide 40 by 0.45. That gives 88.9 bags, so you round up to 89 bags.

Worked example: a footing

Take 40 linear feet of footing, 16 inches wide and 8 inches deep. First convert the width and depth to feet:

• Width: 16 inches ÷ 12 = 1.333 ft
• Depth: 8 inches ÷ 12 = 0.667 ft

Volume = 40 × 1.333 × 0.667 = about 35.6 cubic feet.

That is about 1.32 cubic yards. Using 80-lb bags at 0.60 cubic feet each, you need 35.6 ÷ 0.60 = 59.3 bags. Round up to 60 80-lb bags.

Worked example: a 12-inch sonotube post hole

For a 12-inch diameter tube, the radius is 0.5 ft. The area of the circle is π × 0.5², or about 0.785 square feet. A 3-foot-deep hole gives:

0.785 × 3 = about 2.36 cubic feet

Divide by the 80-lb bag yield: 2.36 ÷ 0.60 = 3.93 bags. Round up to 4 80-lb bags per post.

Add the 10% waste rule

Real ground is never perfectly flat, and holes often end up a little wider than the tube. Spilling a bag or two during mixing is normal. Order about 10% extra for spillage, uneven subgrade, and holes dug a bit wide.

For the patio slab example, the base count was 67 80-lb bags. With 10% waste added, that becomes 74 80-lb bags. Always round up to whole bags.

Bag vs ready-mix

For small slabs, footings, and post holes, bags keep the job simple. You mix only what you need, and you can pause without pressure. But past roughly a cubic yard or two, a ready-mix truck starts winning on effort and time. Mixing 70 or 80 bags by hand is slow, heavy work. Small jobs are bag jobs. Bigger pours are worth a call to a redi-mix supplier.

Quick method for any job

1. Measure length and width in feet. If you need help with area first, use the Square Footage Calculator.
2. Convert depth or width from inches to feet by dividing by 12.
3. Multiply length × width × depth to get cubic feet.
4. Divide by 27 to get cubic yards, if you want the yardage.
5. Divide cubic feet by 0.60 for 80-lb bags, or by 0.45 for 60-lb bags.
6. Round up to the next whole bag, then add about 10% waste.

For tubes, use the same method but find the circular area first: radius in feet, squared, times π.

Use the Crunch Numbers concrete calculator

The Concrete Calculator does slabs, footings, tubes, steps and curbs. It gives cubic yards plus exact bag counts in both 80-lb and 60-lb sizes, and a costed shopping list so you know what to grab at the store. If the area is irregular, the Square Footage Calculator helps measure the footprint first.