Concrete Slab with Rebar Calculator
Get the concrete volume for a slab plus the rebar for a grid — total bar length, number of bars each way, and tie count.
How it works
concrete = L × W × (T ÷ 12) ÷ 27 · bars each way = floor(clear span ÷ spacing) + 1 · rebar = Σ bars × their run length
The concrete side is the usual slab volume. For the mat, bars are laid both directions on a square grid inside an edge clearance (concrete cover), so the count each way is the clear span divided by the spacing, rounded down, plus one for the closing bar. Multiplying each set by the length of a bar and adding the two directions gives the total steel to order; every crossing gets a tie wire.
Worked example
A 20 × 12 ft, 4 in slab is 80 cu ft = 2.96 cubic yards. With 3 in edge cover the clear field is 19.5 × 11.5 ft; at 16 in (1.33 ft) spacing that is 10 bars lengthwise and 16 bars widthwise, giving 10 × 19.5 + 16 × 11.5 = 379 ft of rebar and 160 tie points.
Frequently asked questions
What rebar size and spacing does a slab need?
A typical 4 in residential slab uses #3 or #4 bar on a 16–18 in grid, or welded wire mesh for light-duty pads. Thicker structural slabs and anything carrying vehicles need an engineer’s call on size and spacing.
How much cover should rebar have?
Keep steel at least the code cover from every face — commonly 3 in against earth, 1.5–2 in elsewhere. Support the mat on chairs so it sits in the middle third of the slab; rebar lying on the subgrade does nothing.
How long should rebar overlaps be?
Lap splices are usually at least 40 bar diameters — about 20 in for #4 bar — and staggered. Standard stock comes in 20 ft lengths, so plan splice locations before you cut.
Related calculators
Estimates only — add for waste and confirm against local building codes and supplier specs.