Free concrete slab calculator: enter slab size and thickness to get cubic feet, cubic yards, 80 lb bags, and 60 lb bags with waste.
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Measure the slab inside the forms, in feet, and enter the thickness in inches. The calculator finds the area, turns it into cubic feet, adds your waste allowance for uneven ground and spillage, then converts to cubic yards for a ready-mix order and to whole bags for a bagged order. An 80 lb bag yields about 0.60 cubic feet and a 60 lb bag about 0.45 cubic feet, so bag counts are always rounded up.
Concrete is sold by volume, so a slab is a shallow box: length times width times thickness. Convert thickness from inches to feet by dividing by 12, then divide cubic feet by 27 for cubic yards. A 10 foot by 10 foot slab at 4 inches is 100 times 0.333, which is 33.33 cubic feet, or 1.23 cubic yards before waste. With the default 10 percent waste allowance that becomes 36.67 cubic feet, or 1.36 cubic yards, which needs 62 bags of 80 lb mix, rounded up from 61.11, or 82 bags of 60 lb mix. A 12 by 16 foot patio at 4 inches with 10 percent waste is 70.4 cubic feet, 2.61 cubic yards, and 118 bags of 80 lb mix, matching the worked example competitors publish.
Measure length and width inside the forms, not outside the boards. Enter thickness in inches, keep the waste allowance at 10 percent unless the ground is very level, and read cubic yards for ready-mix and bags for a bagged mix. For an L-shaped slab, split it into two rectangles, calculate each, and add the results. For several identical pads, run the calculator once and multiply by the number of pads. Always confirm the bag yield printed on the bag you buy, because high-strength and fast-setting mixes can differ.
Standard yields are 0.60 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag, and 0.30 for a 40 lb bag. That means one cubic yard takes 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb mix. Bagged concrete suits small jobs under about 1 cubic yard: fence posts, small pads, and repairs. Past that size, every bag must be carried, opened, and mixed, and the first batches start to set before the last ones are placed. Ready-mix delivery, ordered by the cubic yard and usually rounded up to the next quarter yard, is faster, more consistent, and often cheaper per yard on larger pours.
Patios, walkways, and shed floors are usually poured at 4 inches. Sidewalks are at least 4 inches. A driveway for passenger cars is 4 to 5 inches, and a driveway for trucks, RV pads, and hot tub pads needs 5 to 6 inches with reinforcement. House floor slabs are at least 3.5 inches under the residential code, and structural slabs, poor soil, and heavy loads can call for more. These are planning minimums, not a structural design: check local code and a professional for load-bearing work.
At 4 inches thick before waste, an 8 by 8 foot slab needs 0.79 cubic yards, a 10 by 10 foot slab needs 1.23, a 12 by 12 foot slab needs 1.78, a 16 by 20 foot slab needs 3.95, and a 20 by 20 foot slab needs 4.94. At 6 inches those figures rise to 1.19, 1.85, 2.67, 5.93, and 7.41 cubic yards. Add waste before ordering, and remember that a slab planned at 4 inches that averages 4.5 inches needs 12.5 percent more concrete, which is why measuring depth at several points matters.
Order 5 to 10 percent extra on well-prepared level ground and 10 to 15 percent on uneven subgrade, thickened edges, or pumped pours, because running short mid-pour leaves a weak cold joint. Space control joints no more than about 8 feet apart on a 4 inch slab, keep panels close to square, and cut joints at least 1 inch deep. Support mesh or rebar at mid-depth so it stays inside the slab, and cure for 5 days at 70 degrees or warmer, or 7 days at 50 to 70 degrees, using a curing compound, sealed plastic, or wet burlap. Protect fresh concrete from freezing for the first 48 hours.
Confusing cubic feet with cubic yards is the classic error, followed by forgetting to convert thickness from inches to feet. Using a generic bag yield instead of the yield on your bag, ignoring uneven subgrade, and assuming a round pier hole is rectangular all understate the order. Another mistake is leaving the default dimensions unchanged after switching units. Measure inside the forms, in one unit system, add waste, and round bags up, and the order will match the pour.
A 10 by 10 foot slab at 4 inches needs 33.33 cubic feet, or 1.23 cubic yards, before waste. With 10 percent waste that is 1.36 cubic yards, or 62 bags of 80 lb mix.
One cubic yard takes 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix, because those bags yield 0.60, 0.45, and 0.30 cubic feet each. Check the yield on your bag.
Use 4 inches for patios, walkways, and shed floors, 4 to 5 inches for car driveways, and 5 to 6 inches for truck driveways, RV pads, and heavy loads. Check local code for structural work.
Bags suit jobs under about 1 cubic yard. Past that, ready-mix delivery is usually faster, more consistent, and cheaper per yard, and it avoids cold joints from slow hand mixing.
Order 5 to 10 percent extra on level ground and 10 to 15 percent on uneven subgrade or pumped pours. Running short mid-pour creates a weak joint that is hard to repair cleanly.
For a 4 inch slab, space control joints no more than 8 feet apart, keep panels close to square, and cut joints at least 1 inch deep, or one quarter of the thickness, whichever is deeper.
Cubic feet equals length times width times thickness in inches divided by 12. Cubic yards equals cubic feet divided by 27. Bags equal cubic feet divided by the yield per bag, rounded up.