How to measure a slab
Measure the finished length and width in feet, right inside the forms. Measure thickness in inches — this is where most DIY estimates go wrong, because a "4-inch" slab over soft subgrade often ends up 4.5 inches thick once the gravel settles. When in doubt, measure the actual depth of your forms and keep the 10% waste factor on.
Slab thickness guide
Thickness is the structural decision. The standard rules of thumb: 4 inches for patios, walkways and shed floors; 5–6 inches for residential driveways; 6+ inches with rebar reinforcement for RV pads, workshop floors or anything carrying heavy point loads. Thicker is dramatically cheaper than tearing out and replacing a cracked slab.
Subgrade tips
Concrete is only as good as what's under it. Compact 4 inches of crushed gravel, screed it level, and wet it lightly before the pour so it doesn't suck water out of the mix. Uneven subgrade is exactly what the waste slider covers — on rough ground, nudge it to 12–15%.
The math, worked
A 10×10 slab at 4 inches: 10 × 10 × (4 ÷ 12) = 33.3 ft³ ÷ 27 = 1.23 cubic yards, or about 56 eighty-pound bags (33.3 ÷ 0.60). With 10% waste, order 1.36 yards or 62 bags. The calculator above does this live, including the bag math for 60-lb and 40-lb bags.
Frequently asked questions
How thick should a concrete slab be for a driveway?5 to 6 inches, with wire mesh or rebar reinforcement. Standard 4-inch slabs are fine for patios and walkways but will crack under vehicle loads. For RVs or heavy trucks, go 6+ inches with rebar.
How much concrete do I need for a 10×10 slab?At 4 inches thick: 33.3 cubic feet, or 1.23 cubic yards — about 56 eighty-pound bags. With 10% waste, plan on 1.36 yards or 62 bags.
Do I need rebar or wire mesh in a slab?For patios and walkways, 4-inch slabs often do fine without steel if the subgrade is solid and control joints are cut. Driveways and anything structural should have wire mesh at minimum; rebar is better for heavy loads.
How many 80-lb bags for a 12×12 slab at 4 inches?A 12×12 slab at 4 inches is 48 ft³ (1.78 yd³), which needs 80 eighty-pound bags before waste — about 88 with 10% waste. That's past the half-yard rule, so a ready-mix truck is usually cheaper and far less work.
More concrete calculators
Disclaimer: This calculator provides estimates for planning purposes only — not professional engineering advice. Always verify quantities with your supplier and follow local building codes for footings, structural walls and load-bearing work. Prices shown are 2026 estimates; get a local quote.