Concrete Calculator

Running short on concrete mid-pour is one of the most costly mistakes a DIY builder can make — once the mix starts setting, you can't just pause and run to the store. Our concrete calculator cubic yards for slab and concrete calculator bags of mix needed gives you a defensible number in seconds, with a 10% waste buffer already baked in.

The tool handles the three most common shapes homeowners and small contractors work with. For a driveway or patio it acts as a concrete slab cost calculator by thickness — enter length, width, and slab thickness in inches. For deck footings and stair pads it functions as a concrete calculator for footings and posts. For deck piers and structural columns, it becomes a concrete calculator round columns that uses the standard cylinder volume formula.

The math itself is simple but easy to fumble under time pressure. One cubic yard equals 27 cubic feet. A 60-lb bag of pre-mix yields roughly 0.45 cubic feet; an 80-lb bag yields about 0.60 cubic feet. That means a 10 × 10 × 4-inch slab (33.3 cu ft) needs roughly 82 bags of 60-lb mix, 61 bags of 80-lb, or 1.35 cubic yards of ready-mix. Anyone who's asked how much concrete do I need calculator quickly realizes bag counts add up fast — a two-car driveway is almost always cheaper delivered.

Ready-mix from a truck usually beats bagged mix on both cost and quality once you cross about 1.5 cubic yards. Trucks have a short-load minimum (typically 3 yards with a fee for less), and delivered concrete has consistent water content, better cure strength and no mixing labor. Use our ready mix concrete calculator volume output plus your local $/yd rate to decide.

For driveways specifically, the recommended slab thickness is 4 inches for cars and 5–6 inches if you'll ever park an RV or contractor truck on it. A 20 × 20 driveway at 4 inches needs about 4.9 cubic yards — use this as a concrete calculator for driveway. Patios are usually fine at 4 inches. Sidewalks are typically 3.5–4 inches. Structural garage floors follow the same 4-inch minimum, with rebar or fiber mesh strongly recommended.

For stairs, break each step into individual rectangles (rise × run × width) and total them. Our tool also serves as a concrete calculator stairs and steps when you enter the total volume as the "slab" input. A 3-step porch stair typically consumes around 12–20 cubic feet, or roughly 20–35 bags of 80-lb mix.

For a patio, the concrete calculator for patio slab use case, remember that a 4-inch slab across a 12 × 14 patio is about 1.75 cubic yards — right at the sweet spot where ready-mix delivery and bag counts converge in price. If you're doing decorative work (stamped or colored), always order the mix from a supplier who can add the color at the batch plant rather than on site.

Always over-order by 10% (which this calculator does automatically). Uneven ground, spillage, over-dug forms and the reality that no wheelbarrow ever gets fully emptied all conspire to eat into your calculated volume. It's much cheaper to have slightly too much concrete you can dump into extra footings than to be 10 bags short at 4pm with the mix beginning to set.