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How much concrete for a 12x20 slab?

A 12' × 20' slab 4" thick takes 2.96 cubic yards of concrete. Order about 3.26 yards with 10% extra, or buy 147 bags of 80 lb mix if you're mixing it yourself.

ThicknessCubic yardsWith 10% extra80 lb bags60 lb bags
4"2.963.26147196
5"3.704.07184245
6"4.444.89220294
8"5.936.52294392

Concrete

Pour
5–10% typical
Inches. Slab 4", driveway 5–6"
Cubic yards (ready-mix)—
Cubic feet—
Cubic meters—
Approx. weight—
80 lb bags (0.60 ft³)—
60 lb bags (0.45 ft³)—
40 lb bags (0.30 ft³)—

Bag counts include the waste % and round up. Yields are typical for standard pre-mixed concrete. Check the bag, since yields vary by brand and mix.

⚠ Check all math and verify with a licensed engineer and your local building department before cutting or building. Results are estimates.

Sources: Concrete

  1. Concrete Mix No. 1101 product data sheetQuikreteBag yields: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³.
  2. Volume formulasStandard mathSlab = L × W × D; round = π × r² × D; 27 ft³ per cubic yard.
  3. Normal-weight concrete ≈ 145 lb/ft³Typical engineering valueThe approximate weight readout.

Code tables are copied from the documents above and checked against a second copy where noted. Your local code, its edition and amendments govern. Check all math and verify with a licensed engineer and your building department. See the terms & disclaimer.

How the volume works

Length × width × thickness, all in feet, gives cubic feet; divide by 27 for cubic yards. 12' × 20' × 4/12' = 80.0 ft³ ÷ 27 = 2.96 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 147 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 12x20 slab?

147 bags for a 4" thick 12' × 20' slab with 10% extra (88.0 ft³ ÷ 0.60 ft³ per bag). At 6" thick it's 220 bags.

How much does a 12x20 concrete slab weigh?

About 11,600 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.

Other slab sizes

Related calculators

Disclaimer: Results are estimates for planning and layout only. Check all math yourself and verify with a licensed engineer and your local building department before cutting or building. FramersMath is not a substitute for engineered plans, span tables or code review. See the full disclaimer.