Concrete Calculator: Bags and Cubic Yards for Slabs and Post Holes

How the calculator works

For a slab, pad or rectangular footing, the volume is length times width times thickness. Length and width go in feet; thickness goes in inches because that is how slabs are usually described, so the calculator divides it by 12. A 10 by 10 foot slab 4 inches thick is 10 × 10 × 4 ÷ 12 = 33.3 cubic feet.

For round post holes, the volume of one hole is π × radius² × depth. The calculator works in inches and divides by 1,728 (the number of cubic inches in a cubic foot). If you set a post in the hole, it subtracts the post's square cross-section for the full depth. Then it multiplies by the number of holes.

It then adds your margin and divides the total by each bag's yield, rounding up, because you can't buy part of a bag.

Bag yields used

These are the yields printed on a widely sold standard concrete mix data sheet. Other brands' standard mixes list similar figures, but always check the bag.

Worked example: fence posts

You are setting four 4x4 posts in holes 10 inches across and 36 inches deep. One hole holds π × 5² × 36 = 2,827 cubic inches. The post takes up 3.5 × 3.5 × 36 = 441 cubic inches, leaving 2,386 cubic inches, or 1.38 cubic feet. Four holes need 5.52 cubic feet, or about 6.1 cubic feet with a 10% margin. That is 11 bags of 80 lb mix.

Choosing the margin

Ground is never perfectly flat, forms bow slightly, and some concrete stays in the wheelbarrow. A margin of 5–10% is common for small projects. Use the higher end when the subgrade is uneven or when you are digging holes by hand, since holes tend to end up wider at the top than planned. Running short partway through a pour is worse than having a bag left over: fresh concrete placed against concrete that has started to set forms a weak joint.

Measuring tips

• Measure the inside of the forms, not the outside of the boards.
• Check thickness at several points. A slab planned at 4 inches over a dip that is actually 5 inches deep uses 25% more concrete in that area.
• For L-shaped or irregular slabs, split the area into rectangles, work out each, and add them. Our measuring guide shows how.
• Before digging footings or post holes, call 811 to have underground utilities marked. It is free in the United States.

When to consider ready-mix

Once a job reaches about a cubic yard, you are looking at 45 bags of 80 lb mix, which is 3,600 lb to lift, mix and place. Many people switch to a ready-mix delivery or a trailer load at that point. Our bags vs ready-mix guide covers how to compare them.

What this calculator does not do

It estimates quantity only. It does not choose a slab thickness, reinforcement or footing size. Those depend on what the concrete will carry, your soil and frost depth, and your local building code. Driveways, structural footings and retaining walls may need a permit or a professional design.

Common questions

How many 80 lb bags of concrete make a cubic yard?

45. A cubic yard is 27 cubic feet, and an 80 lb bag of standard concrete mix yields about 0.60 cubic feet, so 27 ÷ 0.60 = 45 bags. Add your margin on top.

How many bags for a 10 by 10 slab 4 inches thick?

The slab is 33.3 cubic feet. With a 10% margin that is about 36.7 cubic feet: 62 bags of 80 lb, 82 bags of 60 lb or 123 bags of 40 lb mix.

Does the post subtract from the concrete in a post hole?

Yes. The calculator subtracts the post's actual cross-section (3.5 inches square for a 4x4, 5.5 inches for a 6x6) for the depth of the hole. Choose 'None' for a solid pier.

Are all concrete bags the same yield?

No. Fast-setting, high-strength and other specialty mixes can yield different volumes for the same weight. Use the 'different bag yield' option with the figure printed on your bag.