How Much Concrete for a Slab?

Concrete volume depends on the slab's length, width, and thickness. At 4 inches thick, 1 cubic yard covers 81 square feet. Here's the formula and full reference table for all common slab sizes.

The formula

Volume (cu yd) = Length (ft) × Width (ft) × Thickness (in) ÷ 324

Bags (80 lb) = Volume (cu yd) × 45

Always add 5–10% for waste.

Concrete needed by slab size — 4 inch thick

Slab size	Square feet	Cubic yards	80-lb bags
4×4 ft	16	0.20	9
6×6 ft	36	0.44	20
8×8 ft	64	0.79	36
8×10 ft	80	0.99	44
10×10 ft	100	1.23	56
10×12 ft	120	1.48	67
12×12 ft	144	1.78	80
12×16 ft	192	2.37	107
14×20 ft	280	3.46	156
16×20 ft	320	3.95	178
20×20 ft	400	4.94	222

Add 10% to all quantities. Bags rounded up to nearest whole bag.

Concrete by thickness (100 sq ft area)

Thickness	Cubic yards	80-lb bags
3”	0.93	42
3.5”	1.08	49
4”	1.23	56
5”	1.54	69
6”	1.85	83

Bags vs. ready-mix: which to choose

Project size	Best choice	Why
Under 0.5 cu yd (40 sq ft @ 4”)	Bagged	No truck minimum order
0.5–1.0 cu yd	Either	Bag cost ≈ delivery cost
Over 1.0 cu yd	Ready-mix truck	Faster, cheaper, better mix

Ready-mix pricing: $130–$175 per cubic yard (includes delivery)
80 lb bag pricing: $5–$7 each ($225–$315 per cu yd equivalent)

For any slab larger than a sidewalk section, order ready-mix truck delivery — it’s faster and cheaper.

Step-by-step for a DIY patio slab

Excavate 8 inches (4” gravel + 4” concrete)
Compact gravel base to 4 inches
Set forms at desired height with ¼”/ft slope
Lay rebar or wire mesh if reinforcing
Order concrete — always add 10% to calculated volume
Pour and finish — screed, float, and edge while workable
Cure — keep moist with plastic sheeting for 7 days

Frequently Asked Questions

How much concrete do I need for a 10x10 slab at 4 inches thick?

A 10×10 ft slab at 4 inches thick needs 1.23 cubic yards, or about 56 bags of 80 lb ready-mix concrete. Always order 5–10% extra for waste and spillage.

Should I use ready-mix bags or order truck concrete for a slab?

For slabs under 1 cubic yard (about 80 sq ft at 4"), bagged concrete is feasible. For larger slabs, order ready-mix truck delivery — it is cheaper per cubic yard and faster to pour.

How thick should a concrete slab be?

Most residential slabs are 4 inches thick. Driveways need 5 inches minimum for passenger cars. See our concrete thickness guide for more detail by application.

How long does a concrete slab take to dry?

Concrete reaches walking strength in 24–48 hours. Light vehicle traffic in 7 days. Full cure (design strength) in 28 days. Keep the slab moist for the first 7 days for best results.

Do I need gravel under a concrete slab?

Yes — a 4-inch compacted gravel base is required under all slabs to improve drainage and prevent settling. Use ¾" crusher run or #57 stone, compacted in 2-inch lifts.