Thickness is the one dimension that changes your concrete order the most, and the easiest to get wrong. Here’s what the published guidance says, and what it means for how much you buy.
What the code and the bag makers say
- The International Residential Code (Section R506.1) requires concrete slab-on-ground floors to be at least 3½ in thick.
- SAKRETE’s slab instructions call for at least 4 in of concrete over the gravel base.
- QUIKRETE’s Concrete Mix is made for pours 2 in thick or more, so it isn’t meant for thin toppings.
These are minimums. Your plans, your local building department or a professional should set the thickness for anything that carries vehicles or structural loads. The calculator warns you if you enter a slab thinner than 3½ in.
Thickness changes the order more than you’d think
Volume scales directly with thickness, so going from 4 in to 6 in adds 50% more concrete. For a 10 × 10 ft slab, that’s the difference between about 62 and 92 80-lb bags (with 10% extra). Use the 10 × 10 slab page to see 4, 5 and 6 in side by side.
Measure the concrete, not the base
Enter only the thickness of the concrete. The gravel or crushed stone underneath is a separate material. If the ground under your forms isn’t flat, measure the depth in several places and use the average, then keep the waste allowance at 10% or more.
Thickened edges
Many shed bases and garage slabs are thicker around the edge. Use “Slab with thickened edge” in the slab calculator: enter the slab thickness, the total depth at the edge, and how wide the thick strip is.