10×12 Foot Patio Slab — Bags vs Ready-Mix Cost Breakdown

A 10×12 foot patio slab is one of the most popular backyard concrete projects. It is large enough for a small dining set and a couple of chairs, yet manageable for a competent DIYer working over a weekend. In this scenario, we walk through the complete calculation, compare the cost of bagged concrete versus ready-mix delivery, and highlight the key decisions you need to make before pouring.

Project Profile

• Slab dimensions: 10 ft × 12 ft × 4 in (0.333 ft) thick
• Location: Suburban backyard, flat ground, good truck access
• Budget priority: Cost-effective without cutting quality corners
• Experience level: Intermediate DIY — has poured small footings before

Step 1: Calculate Volume

Volume = 10 × 12 × 0.333 = 39.96 cubic feet. Convert to cubic yards: 39.96 ÷ 27 = 1.48 cubic yards. In metric: 39.96 × 0.0283 = 1.13 cubic metres.

Step 2: Add Waste Factor

This is a flat, well-prepared site with sturdy 2×4 formwork, so a 10% waste factor is appropriate. Adjusted volume: 1.48 × 1.10 = 1.63 cubic yards (1.24 m³).

Step 3: Bags Estimate

Using 80 lb bags (coverage ~0.6 cubic feet each): 39.96 × 1.10 = 43.96 cubic feet ÷ 0.6 = 74 bags. At $5.50 per bag: 74 × $5.50 = $407.00.

Step 4: Ready-Mix Estimate

At $150 per cubic yard: 1.63 × $150 = $244.50. Even with a potential short-load fee of $50 (since this is under 3 yards), the total is $294.50.

Results Comparison

Key Takeaways

• Ready-mix saves over $100 and several hours of back-breaking mixing labour for this project size.
• At 1.63 cubic yards, you are well above the crossover point where ready-mix becomes the better option.
• If truck access is a problem, you would need a concrete pump (additional $150-300) or would default to bags.
• For the complete formula breakdown, see our formula guide.

Formwork & Sub-Base Preparation Tips

Proper preparation is critical for a durable, crack-resistant patio slab. Begin by marking the outline with string lines and spray paint, ensuring perfect right angles using the 3-4-5 triangle method. Install formwork with pressure-treated 2×4s anchored every 2–3 feet with wooden stakes and levelled to within ±1/8 inch over the full 10×12 ft span. Beneath the formwork, lay down 4–6 inches of compacted gravel base—this improves drainage, prevents settling, and reduces the risk of slab cracking due to frost heave or soil shift. A well-compacted sub-base also minimizes the need for thickening the slab edges, keeping concrete usage and cost efficient. For a 4-inch slab, a uniform base ensures even load distribution, especially important if the patio will support furniture or a grill.

Mixing Methods: Wet vs Dry Bagged Concrete Considerations

When using bagged concrete for a project of this size, it's important to decide between wet-mix (pre-blended with water at the job site) or dry-mix (where water is added during mixing). Wet-mix concrete (e.g., Quikrete 5000) is more consistent and achieves higher strength faster, but requires a wheelbarrow and multiple helpers to handle 74 bags efficiently. Dry-mix methods—where bags are dumped into a mixer and water added gradually—can save labour but risk inconsistent water ratios, potentially weakening the final pour. For a weekend DIY job, wet-mix is recommended for uniformity, especially since the slab will be exposed to weather. Also consider that bagged concrete sets in 20–40 minutes depending on temperature, so timing batches and having all tools ready (trowels, floats, edgers) is essential to avoid cold joints or uneven finishes.

Finishing & Curing Best Practices

Once the concrete is placed and struck off, the finishing stage determines both appearance and longevity. Begin with initial screeding using a straight 2×4, followed by floating with a magnesium or wood float to bring up the cream layer. Edging all exposed edges with an edging tool prevents chipping and gives a clean profile. For slip resistance, apply a broom finish with a stiff-bristle broom at a 30-degree angle once the bleed water has evaporated. Curing is just as important: keep the slab moist for at least 7 days using plastic sheeting or a concrete curing compound. This slows evaporation, allowing full hydration of cement particles and reducing shrinkage cracks. In hot or windy conditions, start curing within 30 minutes of finishing. A properly cured 4-inch slab can achieve 90% of its 28-day strength in just 7 days.