ES

How Much Concrete for a 12 × 20 Slab?

A 12 × 20 ft slab is 240 sq ft. It needs 3.26 cubic yards of concrete at 4 inches thick or 4.89 yd³ at 6 inches, including 10% extra.

12 × 20 ft slab, 10% extra included
Order4 in thick6 in thick
Cubic yards3.264.89
80-lb bags147220
60-lb bags196294
50-lb bags235352
40-lb bags294440
Cubic meters2.493.74
What are you pouring?
Units

Slab, patio or driveway

4 in is typical for patios and walks; 5–6 in for driveways and garage floors.

Bag size
Add your prices (optional)

Use your supplier's prices. We don't show average prices because they vary too much by area.

Concrete needed

3.26 cubic yards

88.0 ft³ · 2.49 m³ · includes 10% extra

You need about 3.26 cubic yards of concrete, or 147 bags of 80 lb mix.

Bags of premix

  • 29440 lb bags
  • 23550 lb bags
  • 19660 lb bags
  • 14780 lb bags
  • Before extra2.96 yd³ (80.0 ft³)
  • Weight of bags11,760 lb
  • Ready-mix costAdd a price per yd³
  • Bagged costAdd a price per bag

One 80 lb bag makes about 0.60 ft³ (manufacturer's published yield). Bags are rounded up.

At 3.26 yd³ that is 147 × 80 lb bags (11,760 lb) to mix. Most people order ready-mix at this size; ask suppliers for their minimum load and any short-load fee.

Estimate only, not a quote or engineering advice. Disclaimer

A 12 × 20 ft slab is a single carport or RV-side pad, or a long patio along the back of a house.

12 × 20 slab at other thicknesses

Concrete for a 12 × 20 ft slab
Thicknessyd³ exactyd³ +10%80-lb bags60-lb bags
3.5 in2.592.85129172
4 in2.963.26147196
5 in3.704.07184245
6 in4.444.89220294
8 in5.936.52294392

How it's worked out

12 × 20 = 240 sq ft · 240 × 4 ÷ 12 = 80.0 ft³ · ÷ 27 = 2.96 yd³ · × 1.10 = 3.26 yd³

For bags, the 88.0 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 11,200 lb, using the 140 lb/ft³ unit weight QUIKRETE lists for its Concrete Mix.

Bags or a truck?

At 3.26 yd³ you would need 147 × 80-lb bags (about 11,760 lb) and a long session with a mixer and helpers. Most people order ready-mix at this size: call suppliers with the yardage and ask for their minimum load and any short-load fee.

For a slab this size, the number to give a ready-mix supplier is the yardage: 3.26 yd³ at 4 inches or 4.89 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.

Frequently asked questions

How many yards of concrete for a 12x20 slab?

2.96 cubic yards at 4 inches thick and 4.44 yd³ at 6 inches, before waste. With 10% extra, order 3.26 or 4.89 yd³.

How many bags of concrete for a 12x20 slab?

At 4 inches with 10% extra: 147 bags of 80-lb mix, 196 of 60 lb, 235 of 50 lb or 294 of 40 lb. At 6 inches: 220 bags of 80 lb or 294 of 60 lb.

How many cubic feet of concrete for a 12x20 slab?

80.0 ft³ at 4 inches (240 sq ft × 4 ÷ 12) and 120.0 ft³ at 6 inches, before waste.

How thick should a 12x20 slab be?

Four inches is usual for patios, walkways and shed pads. Garage floors, driveways and pads for heavy loads are commonly 5 to 6 inches. Follow local code or your plans for anything structural.

Other slab sizes

Related

Sources