Concrete Slab Calculator: Yards, Bags & Size Chart

Concrete Slab size chart @ 4"

10% waste included. Tap a size for depth options, cost and the full material list.

Size	Area	Cubic yards	80 lb bags	60 lb bags	Tons
8x8	64 sq ft	0.87 yd³	40	53	1.76
10x10	100 sq ft	1.36 yd³	62	82	2.75
10x12	120 sq ft	1.63 yd³	74	98	3.30
12x12	144 sq ft	1.96 yd³	88	118	3.96
12x16	192 sq ft	2.61 yd³	118	157	5.28
12x20	240 sq ft	3.26 yd³	147	196	6.60
16x20	320 sq ft	4.35 yd³	196	261	8.80
20x20	400 sq ft	5.43 yd³	245	326	11.00
20x24	480 sq ft	6.52 yd³	294	392	13.20
24x24	576 sq ft	7.82 yd³	352	470	15.84
24x30	720 sq ft	9.78 yd³	440	587	19.80
30x40	1200 sq ft	16.30 yd³	734	978	33.00
40x60	2400 sq ft	32.59 yd³	1,467	1,956	66.00

How the math works

Area = length (ft) × width (ft)
Volume (cu ft) = area × thickness (in) ÷ 12
Add waste: × 1.10
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ bag yield (80 lb bag = 0.6 cu ft, 60 lb bag = 0.45 cu ft, 40 lb bag = 0.3 cu ft) — always round up
Weight = cubic yards × 4,050 lb/yd³

Trade pro tips

Order 5–10% extra. Subgrade is never perfectly flat, and running short mid-pour leaves a cold joint.
Compact 4" of gravel base and slope the forms ¼" per foot away from any structure.
Chair the rebar so it sits in the middle third of the slab — rebar lying on the dirt does nothing.
Cut control joints ¼ of the slab depth, no more than 2–3 × the thickness (in feet) apart, within 6–18 hours.

Frequently asked questions

How do I calculate how much concrete I need for a slab?

Multiply length × width (ft) × thickness (in) ÷ 12 to get cubic feet, add 5–10% waste, then divide by 27 for cubic yards. Example: 10 × 10 × 4" = 33.3 cu ft × 1.10 = 36.7 cu ft = 1.36 yd³.

How many 80 lb bags of concrete make a yard?

An 80 lb bag yields about 0.60 cu ft, so 1 yd³ (27 cu ft) takes 45 bags. With 60 lb bags (0.45 cu ft) it takes 60; with 40 lb bags (0.30 cu ft) it takes 90.

How thick should a concrete slab be?

4" for patios, walkways and shed floors; 5" for driveways and hot tub pads; 6" for garages, RV pads and anything carrying heavy vehicles. Always over 4" of compacted gravel.

Should I use bagged concrete or ready-mix?

Under about 1 yd³ (45 × 80 lb bags), bags are practical. Above that, ready-mix is cheaper and more consistent. Many plants charge a short-load fee under 3–4 yd³.

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