How Much Concrete for a 20 × 20 Slab?

A 20 × 20 ft slab is a common two-car parking pad or compact two-car garage floor. Garage floors are often poured thicker than patios, so compare the 4 and 6-inch rows.

20 × 20 slab at other thicknesses

How it's worked out

For bags, the 146.7 ft³ (with extra) is divided by each bag's yield: 0.60 ft³ for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb, then rounded up. At 4 inches the slab itself weighs about 18,700 lb, using the 140 lb/ft³ unit weight QUIKRETE lists for its Concrete Mix.

Bags or a truck?

At 5.43 yd³, bags are not realistic: it would take 245 × 80-lb bags. This is a ready-mix pour, and at this size often a job for a concrete contractor with a crew, and sometimes a pump.

For a slab this size, the number to give a ready-mix supplier is the yardage: 5.43 yd³ at 4 inches or 8.15 yd³ at 6 inches, with 10% extra already included. Some people order a little less than the full allowance and keep a few bags on hand to top off.

Don't forget the base: most slabs sit on a compacted layer of gravel, and thicker edges or footings around the perimeter add concrete. Use the footing calculator for a thickened edge.