A 4-inch slab suits patios and shed floors; driveways and load-bearing pads usually want 5–6 inches. This view defaults to 4 inches — set your thickness first, then order 5–10% over for an uneven subgrade.
Why order extra? The headline number is the exact material your project needs. When you actually order, add about 5–10% on top — the “with 5% waste” figure in the result is a safe starting point. That margin covers what you lose to compaction, cuts around edges and corners, and ordinary spillage during handling. It is cheaper to have a little left over than to stop mid-job and wait on a second delivery.
Field notes
Past about 50 bags, stop bagging and order ready-mix — hand-mixing that volume risks cold joints between batches and eats a whole day. Order 5–10% over; you can't pause a pour to run for more, and a short load means a visible seam. Dampen the subgrade and set your forms dead level first, because concrete finds every low spot and won't forgive a sloppy base.
How the concrete slab calculator works
The math behind the tool:
cubic yards = (length ft × width ft × thickness ft) ÷ 27
80 lb bags = cubic feet ÷ 0.6Material density
| Material | Density | Per cubic yard |
|---|---|---|
| Cured concrete | 150 lb/ft³ | 2.02 tons |
Sources: Portland Cement Association (concrete density and mix guidance).