Rebar Calculator

A rebar calculator lays out the reinforcing grid for a slab or footing: how many bars run each way, the total linear feet, and how many standard 20 ft bars to buy — including an allowance for lap splices. Enter the slab size, your grid spacing and the edge clearance.

Your measurements

Result

It turns 'rebar on 12-inch centres' into an actual shopping list, so you order the right number of lengths instead of guessing.

How it is calculated

Bars each direction = (usable dimension ÷ spacing) + 1, where usable = slab size minus twice the edge clearance. Total linear feet = bars × the run length each way. We add 10% for lap splices and round up to 20 ft bars. Bar size comes from the structural design.

Steps

Enter the slab length and width.
Set the grid spacing (12 in on-centre is common for residential).
Set the edge clearance (about 3 in so bars sit inside the slab edge).
Read the bars each way, total linear feet, and 20 ft bars to buy.
Support the grid on chairs at mid-depth and lap splices generously.

Worked example

A 20 × 12 ft slab on a 12 in grid (3 in clearance) needs about 20 bars one way and 12 the other — roughly 480 linear feet, or 27 twenty-foot bars with laps.

Common questions

How do I calculate rebar for a slab?

Divide each slab dimension by the grid spacing, add one for the starting bar, and multiply by the run length. The calculator does both directions and adds lap allowance.

What spacing should rebar be?

12-18 in on-centre is typical for residential slabs; your engineer or code sets the exact spacing and bar size.

How much overlap at splices?

Lap splices are usually at least 40 bar diameters (about 20 in for #4). We add 10% to the total to cover laps.

Rebar or wire mesh?

Mesh suits thin, non-structural slabs; rebar is used for thicker or load-bearing slabs and footings. This sizes a rebar grid.