How much concrete for a 12 × 20 slab?
A 12 × 20 ft slab (240 sq ft) poured 4 inches thick takes 2.96 cubic yards of concrete. With 10% extra for uneven ground and spillage, order 3.26 cubic yards of ready-mix or buy 147 bags of 80 lb mix.
| Thickness | Exact volume | Order (+10%) | 80 lb bags | 60 lb bags | Typical use |
|---|---|---|---|---|---|
| 4 in | 2.96 yd³ | 3.26 yd³ (2.49 m³) | 147 | 196 | Patio, walkway, shed floor |
| 5 in | 3.7 yd³ | 4.07 yd³ (3.11 m³) | 184 | 245 | Hot tub pad, light driveway |
| 6 in | 4.44 yd³ | 4.89 yd³ (3.74 m³) | 220 | 294 | Driveway, garage floor |
Bag counts include the 10% extra and assume 0.60 ft³ per 80 lb bag and 0.45 ft³ per 60 lb bag.
Change the size, thickness or price
88 cubic feet = 3.26 cubic yards = 2.49 m³. Above about 1 cubic yard, ready-mix delivery is usually cheaper and faster than bags.
Rough material cost (cheapest option)
$554Ready-mix often has a short-load fee under 4–5 yd³. Rough 2026 US prices, edit to match local quotes.
FAQ
How many bags of concrete for a 12x20 slab?
At 4 inches thick, 147 bags of 80 lb concrete mix or 196 bags of 60 lb mix, including 10% extra. At 6 inches, 220 bags of 80 lb mix.
Should I use bags or ready-mix for a 12x20 slab?
Ready-mix is usually cheaper and much less work above about 1 cubic yard, and this slab needs 3.26. Under 4–5 yards many suppliers add a short-load fee, so compare a quote with 147 bags.
How do you calculate concrete for a 12 by 20 slab?
Multiply length × width × thickness in feet: 12 × 20 × (4 ÷ 12) = 80 cubic feet. Divide by 27 to get 2.96 cubic yards, then add 5–10% extra.