How much concrete do I need for a 20x20 foot patio slab?

A 20×20 foot patio slab at 4 inches thick requires approximately 5 cubic yards of concrete. Here is the full calculation and what it means for your NB project.

The formula is straightforward: Length × Width × Thickness (all in feet), then divide by 27 to convert cubic feet to cubic yards.

20 ft × 20 ft × 0.33 ft (4 inches) = 133.3 cubic feet ÷ 27 = 4.94 cubic yards, rounded to 5 cubic yards.

Always order 5–10% extra to account for ground irregularities, spillage, and the fact that you never want to run short mid-pour. For this slab, ordering 5.5 cubic yards is prudent. Running out of concrete with a partially finished slab is a serious problem — a cold joint formed between the first and second pours creates a weak plane and is highly visible.

Thickness considerations for NB: A 4-inch (100 mm) slab is the minimum for a residential patio that will only see foot traffic. If you anticipate any vehicle access, lawn tractors, or heavy loads, go to 5 inches, which increases your volume to about 6.2 cubic yards. NB's frost heave makes thin slabs (3 inches or less) especially risky — the soil movement combined with a weak slab is a recipe for cracking in the first few seasons.

Cost estimate for a 20×20 patio in NB (2025–2026): Air-entrained ready-mix at 32 MPa runs $200–$240 per cubic yard delivered in the Moncton, Fredericton, and Saint John areas. For 5.5 cubic yards, that is $1,100–$1,320 in materials. Note: Ready-mix suppliers typically charge a short-load surcharge of $75–$150 for orders under 4 cubic yards — 5.5 yards avoids that extra cost. Labour, base preparation (6 inches of compacted gravel), forming, finishing, and sealer brings the total installed cost for a 400 sq ft patio to roughly $4,000–$7,500 depending on finish type.

For DIY volume estimating, use this quick formula: multiply square footage by the thickness in inches, then divide by 12. That gives cubic feet. Divide by 27 for cubic yards. For a different size patio — say 12×16 feet at 4 inches — the math is: 192 sq ft × 4 ÷ 12 = 64 cubic feet ÷ 27 = 2.4 cubic yards. At that volume, you are paying the short-load surcharge, which makes bagged concrete worth considering for very small jobs.

Need help finding a contractor for your NB patio? New Brunswick Concrete can match you with local professionals for a free estimate.