Question 1

How many bags of concrete do I need for a 10x10 slab?

Accepted Answer

A 10×10 slab at 4 inches deep is 1.23 cubic yards. That's about 56 bags of 60lb mix or 41 bags of 80lb mix (each 80lb bag yields ~0.6 cu ft).

Question 2

How much concrete do I need for a 4-inch slab?

Accepted Answer

Take length × width × 0.333 ft (4 in). A 10×10 slab = 33.3 cu ft = 1.23 cu yd. Order a little extra — 5–10% for spillage and uneven forms.

Question 3

How many cubic feet are in a bag of concrete?

Accepted Answer

An 80lb bag yields about 0.60 cu ft, a 60lb bag about 0.45 cu ft, and a 40lb bag about 0.30 cu ft. There are 27 cu ft per cubic yard.

Question 4

Should I order ready-mix or bags?

Accepted Answer

Under ~1 cubic yard, bagged mix is usually cheaper and easier. Above that, ready-mix delivery saves labor. Our calculator gives you both options.

Question 5

How do I calculate concrete for a round post hole?

Accepted Answer

Use the cylinder formula: π × radius² × depth. A 12-inch diameter hole 24 inches deep needs about 1.6 cu ft — roughly three 60lb bags. (This tool covers slabs; see our gravel calculator for similar volume math.)

Slab Size	4-in Depth	6-in Depth
4×4 ft (shed pad)	0.20 yd³	0.30 yd³
8×10 ft (patio)	0.99 yd³	1.48 yd³
10×10 ft	1.23 yd³	1.85 yd³
10×20 ft (driveway)	2.47 yd³	3.70 yd³
20×30 ft (garage)	7.41 yd³	11.11 yd³

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