Concrete Patio Calculator
Calculates the cubic yards of concrete needed for a patio, walkway, or slab, including a waste allowance. Use it when ordering ready-mix concrete or estimating bags for a DIY pour.
About this calculator
Concrete volume is measured in cubic yards because that is how ready-mix trucks and bulk suppliers price it. The formula converts imperial dimensions to cubic yards: volume (yd³) = (length × width × (thickness ÷ 12)) ÷ 27 × waste_allowance. Dividing thickness by 12 converts inches to feet; dividing by 27 converts cubic feet to cubic yards. The waste_allowance multiplier (e.g., 1.05 for 5% waste) compensates for spillage, subgrade irregularities, and formwork overfill — standard practice is 5–10% extra. Slab thickness is the most influential variable: a 4-inch residential patio uses 33% less concrete than a 6-inch driveway slab. Choosing the right concrete mix (2,500 psi for patios, 3,000–4,000 psi for driveways) affects durability but not volume.
How to use
A 12 ft × 20 ft patio slab, 4 inches thick, with a 1.05 waste allowance (5%). Step 1 — convert thickness: 4 ÷ 12 = 0.333 ft. Step 2 — cubic feet: 12 × 20 × 0.333 = 80 ft³. Step 3 — cubic yards: 80 ÷ 27 ≈ 2.96 yd³. Step 4 — apply waste: 2.96 × 1.05 ≈ 3.11 yd³. Order 3.5 cubic yards (minimum truck load is often 1 yd³, so round to the nearest half yard). That is approximately 63 bags of 60 lb concrete mix as an alternative.
Frequently asked questions
How many bags of concrete do I need for a 10x10 patio slab?
A 10×10 ft slab at 4 inches thick contains (10 × 10 × 0.333) ÷ 27 ≈ 1.23 cubic yards, or about 33 cubic feet. A 60 lb bag of pre-mix concrete yields approximately 0.45 cubic feet, so you would need roughly 74 bags. An 80 lb bag yields about 0.60 cubic feet, reducing the count to about 55 bags. Adding a 5–10% waste buffer bumps those figures to 78–82 and 58–61 bags respectively. For slabs over 1 cubic yard, ordering ready-mix is usually cheaper and faster than mixing bags.
What thickness should a concrete patio slab be?
Residential patios and walkways are typically poured at 4 inches thick, which provides adequate strength for foot traffic and outdoor furniture. If you plan to park vehicles on the slab or expect heavy loads, increase thickness to 5–6 inches and consider adding steel reinforcement or fiber mesh. Slabs thinner than 3.5 inches are prone to cracking under freeze-thaw cycles in northern climates. Local building codes may specify minimum thickness, especially for attached patios near the foundation — always check before pouring.
How much does a yard of concrete cover at 4 inches thick?
One cubic yard (27 cubic feet) of concrete poured at 4 inches (0.333 ft) thick covers 27 ÷ 0.333 = 81 square feet. At 3 inches thick it covers 108 sq ft, and at 6 inches thick it covers only 54 sq ft. This relationship is why slab thickness has such a large impact on the total concrete ordered and cost. A quick rule of thumb: at 4 inches thick, budget roughly one cubic yard per 80 square feet of slab area before waste.