How much concrete for a 20 × 20 slab?
A 20 × 20 ft slab (400 sq ft) poured 4 inches thick takes 4.94 cubic yards of concrete. With 10% extra for uneven ground and spillage, order 5.43 cubic yards of ready-mix or buy 245 bags of 80 lb mix.
| Thickness | Exact volume | Order (+10%) | 80 lb bags | 60 lb bags | Typical use |
|---|---|---|---|---|---|
| 4 in | 4.94 yd³ | 5.43 yd³ (4.15 m³) | 245 | 326 | Patio, walkway, shed floor |
| 5 in | 6.17 yd³ | 6.79 yd³ (5.19 m³) | 306 | 408 | Hot tub pad, light driveway |
| 6 in | 7.41 yd³ | 8.15 yd³ (6.23 m³) | 367 | 489 | 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
146.7 cubic feet = 5.43 cubic yards = 4.15 m³. Above about 1 cubic yard, ready-mix delivery is usually cheaper and faster than bags.
Rough material cost (cheapest option)
$923Ready-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 20x20 slab?
At 4 inches thick, 245 bags of 80 lb concrete mix or 326 bags of 60 lb mix, including 10% extra. At 6 inches, 367 bags of 80 lb mix.
Should I use bags or ready-mix for a 20x20 slab?
Ready-mix is usually cheaper and much less work above about 1 cubic yard, and this slab needs 5.43. Under 4–5 yards many suppliers add a short-load fee, so compare a quote with 245 bags.
How do you calculate concrete for a 20 by 20 slab?
Multiply length × width × thickness in feet: 20 × 20 × (4 ÷ 12) = 133.3 cubic feet. Divide by 27 to get 4.94 cubic yards, then add 5–10% extra.