How the volume works
Length × width × thickness, all in feet, gives cubic feet; divide by 27 for cubic yards. 20' × 20' × 4/12' = 133.3 ft³ ÷ 27 = 4.94 yd³. Bag counts use the standard yields of 0.60 ft³ for an 80 lb bag and 0.45 ft³ for a 60 lb bag, with 10% added for spills and an uneven base.
At 245 bags, ready-mix is usually cheaper and far less work: most plants deliver from 1 yard, with a short-load fee under 3–4 yards. Driveways and slabs that carry vehicles are usually 5–6" thick. Change the size or thickness in the calculator above.
FAQ
How many 80 lb bags of concrete for a 20x20 slab?
245 bags for a 4" thick 20' × 20' slab with 10% extra (146.7 ft³ ÷ 0.60 ft³ per bag). At 6" thick it's 367 bags.
How much does a 20x20 concrete slab weigh?
About 19,333 lb at 4" thick, using 145 lb per cubic foot for normal concrete. That's why a slab this size needs a firm, compacted base.