Enter your slab's length, width, and thickness to get the concrete you need — in cubic yards, cubic feet, and bags.
Order 5–10% extra to cover spillage and uneven subgrade. Ready-mix is usually cheaper than bags above about 1 cubic yard. Estimate only.
Concrete is ordered by volume. Multiply the slab's length, width, and thickness (converted to feet), then convert cubic feet to cubic yards — the unit ready-mix trucks use. For small jobs, bags are easier; each 80 lb bag yields about 0.6 cubic feet.
Cubic feet = L × W × (thickness ÷ 12) · Cubic yards = cubic feet ÷ 27
Concrete is ordered by volume, so every estimate starts by turning your slab’s dimensions into cubic feet, then cubic yards (the unit ready-mix trucks use). The formula is cubic feet = length × width × (thickness in inches ÷ 12), then cubic yards = cubic feet ÷ 27. Getting the thickness conversion right is where most DIY estimates go wrong — a 4-inch slab is one-third of a foot deep, not 4.
A 10 ft × 10 ft slab at 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 33.3 ÷ 27 = 1.23 cubic yards. Because you never want to run short mid-pour, order 5–10% extra for spillage and uneven subgrade — call it about 1.35 yards. In bags, an 80 lb bag yields ~0.6 cubic feet, so this slab needs roughly 56 bags — which is exactly why, above about a cubic yard, ready-mix by the truck is usually cheaper and far faster than mixing dozens of bags by hand.
Framing the forms rather than pouring? The stud calculator sizes wall studs and plate lumber the same evidence-based way.
Under about a cubic yard, bags are convenient. Above that, ordering ready-mix by the yard is usually cheaper and faster than mixing dozens of bags.
Add 5–10% for spillage, uneven ground, and waste — running short mid-pour is far costlier than a little extra.
If you're studding out a wall, use the stud calculator to get the number of studs and plate lumber from your wall length and on-center spacing.
Related: stud calculator · square footage calculator · gravel calculator · cost per square foot calculator · all free calculators.
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