Rebar Calculator

Lay out a rebar grid for a slab and get everything you need to order: bar counts in each direction, total length with waste, total weight, and how many standard-length sticks that translates to.

Rebar Grid Details

Slab Length

Slab Width

Spacing (on-center)

Rebar Size

Waste / Overlap Allowance

Enter your slab dimensions and spacing to see total rebar needed.

How the grid is calculated

Bars run in both directions at your on-center spacing. The calculator computes how many bars fit each way (rounding to include the closing bar at each edge), multiplies by the run lengths, adds your waste percentage for lap splices and trimming, and converts total length to weight using the standard weight of your bar size.

Bar sizes at a glance

US bar numbers describe diameter in eighths of an inch: #3 is 3/8 in (9.5 mm) at 0.376 lb/ft, #4 is 1/2 in (12.7 mm) at 0.668 lb/ft, and #5 is 5/8 in (15.9 mm) at 1.043 lb/ft. Slabs commonly use #3 or #4 at 12–18 inches on-center; structural elements use larger bars per design.

Frequently Asked Questions

What rebar spacing should I use in a slab?

12 to 18 inches on-center each way is typical for residential slabs, with 12 in being the more conservative choice. Structural slabs follow the engineer’s drawings.

How much does rebar overlap at splices?

Lap splices are commonly 40 bar diameters — 20 inches for #4 bar. The calculator’s waste percentage (default 10%) covers typical splice and trim losses.

How long is a standard stick of rebar?

20 ft sticks are the common stock length in the US; 6 m bars are standard in metric markets. The calculator reports whichever matches your selected unit system.