How the rebar grid is calculated
A slab grid has two sets of bars at right angles. Bars that run along the length are spaced across the width, and bars that run across the width are spaced along the length. Each bar stops short of the slab edge by the edge clearance, so it stays covered by concrete.
- Bar run = slab dimension − 2 × edge clearance.
- Number of bars in one direction = (run in the other direction ÷ spacing), rounded up, + 1.
- Laps: when a run is longer than one stock bar, bars are overlapped. Each splice adds one lap length, here 40 × bar diameter.
- Bars to buy: short runs are cut from full bars, as many per bar as fit; long runs use whole bars plus spliced extensions.
bars each way = ⌈(other side − 2 × clearance) ÷ spacing⌉ + 1
lap length = 40 × bar diameter (#4: 40 × 0.5 in = 20 in)
Worked example
A 24 × 24 ft garage slab with #4 bars at 18 in on center and 3 in edge clearance. Each run is 24 − 0.5 = 23.5 ft. 23.5 ft ÷ 1.5 ft = 15.7, so 16 spaces and 17 bars each way. A 23.5 ft run is longer than a 20 ft bar, so each run needs one splice with a 20 in lap, which means two bars per run. That is 68 bars of 20 ft, about 856 linear ft including laps, 34 splices, 289 tie points where bars cross, and about 572 lb of steel.
A metric example: a 6 × 4 m slab, 12 mm bars at 300 mm, 75 mm clearance, 6 m bars. The runs fit in one bar, so you need 14 bars one way and 21 the other: 35 bars, 163 m of bar and about 144 kg of steel.
Common bar sizes
| US bar | Diameter | Weight | Metric bar | Weight |
|---|---|---|---|---|
| #3 | 3/8 in (9.5 mm) | 0.376 lb/ft | 10 mm | 0.617 kg/m |
| #4 | 1/2 in (12.7 mm) | 0.668 lb/ft | 12 mm | 0.888 kg/m |
| #5 | 5/8 in (15.9 mm) | 1.043 lb/ft | 16 mm | 1.58 kg/m |
More sizes and the formula behind these weights are in the rebar sizes and weights chart.
Choosing the spacing
When plans call for a rebar grid in a residential slab on grade, #3 or #4 bars at about 12 to 18 in each way are a common sight. Closer spacing or larger bars mean more steel and a stiffer slab. Many patios and walks use no rebar at all, or use wire mesh or fibers instead, and rely on joints to control cracking. The spacing you enter should come from your plans, your engineer or local practice.
Placement tips
- Support the grid on chairs or concrete blocks so it sits in the slab, not on the ground. Bars lying on the subgrade do little. In a typical slab on grade the steel goes at or slightly above mid-depth; follow your plans.
- Tie crossings with wire. Many crews tie every other crossing on a slab; follow your plans or inspector.
- Stagger splices so they do not all fall in one line across the slab.
- Keep the edge clearance and cover. Steel too close to the surface rusts and spalls the concrete.
- Bent bars at corners and dowels into footings or existing slabs are not included. Add them separately.
Safety and code notes
Bar size, spacing, cover and lap length for any structural slab, footing or wall are set by code and engineering, not by a calculator. Use this tool to estimate quantities from your plans and check local code or a structural engineer for the design. On site, fit protective caps on vertical bar ends, wear gloves and eye protection, and lift long bars with a partner.
Work out the concrete for the same slab with the concrete calculator.
Frequently asked questions
How much rebar do I need for a 24 × 24 slab?
With #4 bars at 18 in each way and 3 in clearance, you need 17 bars each way. Using 20 ft stock with 20 in laps, that is 68 bars and about 856 linear ft.
How long should a rebar lap be?
A common rule of thumb is 40 bar diameters: 15 in for #3, 20 in for #4 and 25 in for #5. Codes and engineers often require more for structural work.
How many rebar ties do I need?
There is one crossing for each pair of bars. A grid of 17 by 17 bars has 289 crossings. Many crews tie every other one on a slab.