How Many Bags of Concrete Do I Need for a 12×20 Slab?

The 12 × 20 ft slab is the standard two-car pad, RV pad, and workshop floor size, usually poured 4 inches thick over a compacted gravel base. Here is the short answer, with the full table for 4 in and 6 in below.

12×20 Slab Bag Table: 4 in and 6 in, by Bag Size

Exact counts are the mathematical minimum, rounded up. The "with waste" counts multiply the volume by 1.10 before dividing, which is the number to actually buy. Yields: 80 lb bag = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³.

Which thickness? 4 inches is the standard for car pads, patios, and workshop floors that only carry foot traffic and stored gear. Go to 6 inches for RVs, trucks, and anywhere heavy vehicles park. That jump takes the pour from 2.96 yd³ to 4.44 yd³, and from 147 bags to 220 bags of 80 lb mix with waste. Either way, pour over a compacted gravel base — figure the base with the gravel calculator, and if the pad will carry real vehicle weight, run a rebar grid with the rebar calculator.

How the Math Works

A slab is a simple box: length × width × thickness. The only trap is mixed units, so convert the thickness from inches to feet first.

1. Convert thickness: divide inches by 12. (4 in = 0.333 ft, 6 in = 0.5 ft.)
2. Multiply: 12 ft × 20 ft × thickness in feet = cubic feet.
3. Cubic yards: divide cubic feet by 27.
4. Add waste: multiply cubic feet by 1.10 for a 10% cushion.
5. Bags: divide by the yield printed on the bag and round up.

Volume (ft³) = L × W × (T ÷ 12)  ·  Bags = ft³ ÷ yield, rounded up

Worked example at 4 in: 12 × 20 × (4 ÷ 12) = 80.0 ft³, which is 80.0 ÷ 27 = 2.96 yd³. Bags of 80 lb mix: 80.0 ÷ 0.60 = 133.3, so 134 bags. With waste: 80.0 × 1.10 = 88.0 ft³, and 88.0 ÷ 0.60 = 146.7, so 147 bags.

At 6 in: 12 × 20 × 0.5 = 120.0 ft³ (4.44 yd³). Bags: 120 ÷ 0.60 = 200 bags exact, or 220 with the 10% cushion.

When Bags Stop Making Sense: The 1 yd³ Crossover

Bags usually win under about 1 cubic yard. A 12×20 slab is nearly three times that line at any standard thickness: 2.96 yd³ at 4 inches, 4.44 yd³ at 6 inches. So for this particular pour, ready-mix is almost always the right call.

Why the crossover exists: ready-mix suppliers bill a 1 yd³ minimum. On a small pour you pay for a full yard you do not use, plus a delivery fee, and often a short-load surcharge on top. That padding is what makes bags cheaper under a yard. Once your pour is genuinely over a yard, you are using what you paid for and the truck's per-yard price beats the per-bag price.

At the example prices used on the main calculator ($6.50 per 80 lb bag, $160 per cubic yard, $75 delivery):

Then there is labor. 147 bags at 80 lb is 11,760 lb of material you lift, mix, and wheelbarrow; 220 bags is 17,600 lb — almost nine tons. That is several days of mixing even with a rented mixer, and concrete does not wait while you catch up: a slow bag-by-bag pour risks cold joints across a slab this size. A truck places the same pour in about an hour. Bags still make sense for a 12×20 when the truck cannot reach the pour (tight backyard access), when you want to work in stages over several days, or when local ready-mix quotes come in far above the example prices. Run your own quotes through the slab cost comparison before deciding.

Pouring a Different Size?

These numbers are only exact for a 12 × 20 slab. For any other footprint, thickness, or shape, use the free calculator: it figures cubic yards, cubic feet, and bag counts live, and compares bag cost against a ready-mix quote with your local prices. Pouring a different standard size instead? See the 10×20 slab bag guide or the 12×12 slab bag guide.

Open the Concrete Calculator →

12×20 Slab FAQ