Rebar Calculator for Slabs (Grid Spacing & Quantity)

Enter the slab size, bar spacing, clearance from the edges and the bar size. The calculator lays out a grid of bars running both ways, adds laps where a run is longer than one stock bar, and tells you how many sticks to buy and what they weigh.

Material estimate only. Bar size, spacing, edge cover and lap length come from your plans, your local building code or an engineer; this calculator counts and weighs the bars for the layout you enter. Check with your building department or a licensed professional before you pour.

Rebar grid calculator

How it works

Bars running lengthwise = (width − 2 × edge clearance) ÷ spacing, rounded up, + 1
Bars running widthwise = (length − 2 × edge clearance) ÷ spacing, rounded up, + 1
Rounding up means the actual spacing is never wider than the spacing you enter.
Splices per run = (run − stock length) ÷ (stock length − lap), rounded up, when a run is longer than one bar
Weight = total feet × nominal weight per foot (#3 0.376, #4 0.668, #5 1.043, #6 1.502 lb/ft, from ASTM A615)

Short runs are nested: if two runs fit in one stock bar, the calculator counts one bar for both.

Worked example

A 20 × 12 ft slab with #4 bar at 18 in on center, kept 3 in from the edges:

Slab length: 20 ft
Slab width: 12 ft
Bar spacing (on center): 18 in
Clearance from slab edges: 3 in
Bar size: #4 (1/2 in)
Stock bar length: 20 ft
Lap splice length: 24 in

The calculator gives:

Stock bars to buy: 23 × 20 ft
Bars running lengthwise: 9 at 19′ 6"
Bars running widthwise: 14 at 11′ 6"
Total length: 336.5 ft including laps
Weight: 225 lb of #4 bar
Intersections to tie: 126

Across the 138 in of usable width, 18 in spacing needs 7.67 spaces, rounded up to 8, so there are 9 bars running lengthwise (actually 17-1/4 in apart), each 19 ft 6 in; along the 234 in of usable length there are 13 spaces and 14 bars running across, each 11 ft 6 in. That is 336.5 ft of bar, 225 lb, and 23 sticks of 20 ft bar.

Material and waste notes

Bar sizes, diameters, metric equivalents and weight per foot for #3 to #18 bar are in our rebar size chart.
Lap length is not a fixed number. It depends on bar size, concrete strength and the design; the IRC points to Table R608.5.4(1) for concrete walls. The 24 in default is only a placeholder, so enter the lap on your plans.
Keep bars in the right place with chairs or bricks. Bars lying on the ground do little.
Concrete cast against earth needs more cover than concrete placed in forms; the edge clearance here is a layout input, not a cover requirement.
Whether a slab needs rebar, wire mesh or fibers, and at what spacing, is a design decision.

Frequently asked questions

How many rebar do I need for a 20 x 20 slab?

At 18 in each way with 3 in edge clearance, 14 bars each way: 28 bars of 19 ft 6 in, 546 ft in all, or 28 twenty-foot sticks. At 12 in spacing it is 21 bars each way, 42 sticks.

How much does #4 rebar weigh?

0.668 lb per foot, so a 20 ft stick weighs about 13.4 lb (ASTM A615 nominal weight).

What spacing is typical for a slab?

It depends on the slab and its use; 12 to 18 in grids are common in residential flatwork, but follow your plans.

Sources

Last reviewed: 26 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.