What a rebar layout actually consists of
A slab grid is defined by four numbers: the bar size, the spacing each way, the cover, and the plan dimensions. Everything else follows. The bar count in one direction is the clear band between the covers divided by the spacing, rounded down, plus one — the bar at the far edge. Bar length is the plan dimension less the cover at each end. Multiply and you have lineal feet; divide by stock length with an allowance for laps and you have sticks to order.
The fence-post rule is where most takeoffs go wrong. Twenty feet of clear band at 12 in spacing does not hold 20 bars, it holds 21, because you are counting posts and not gaps. On a 24 ft × 24 ft slab at 12 in on centre the difference is two bars each way — trivial. On a 200-bar highway pour it is not.
The other number worth understanding is the steel area per foot. A grid is not really "#4 at 12 inches"; structurally it is 0.20 square inches of steel in every foot of slab width. That is what an engineer specifies and what an inspector checks, and it is why #4 at 12 in and #5 at 18 in are close cousins — 0.200 against 0.207 in²/ft. Knowing the equivalence lets you substitute intelligently when a size is out of stock, subject to the engineer agreeing.
Cover, spacing limits and lap splices under ACI 318-19
Cover is the clear distance from the surface of the concrete to the nearest bar, and it exists to protect the steel from corrosion and fire. ACI 318-19 sets minimums by exposure in Table 20.5.1.3.1: 3 in where concrete is cast against and permanently in contact with ground, 1.5 in for #5 and smaller exposed to earth or weather, and less for interior members not exposed. On a slab on grade poured over a vapour barrier the sides are typically formed, so 1.5 in side cover is common; a footing poured against a trench wall gets 3 in.
Maximum spacing has two different limits and they get confused constantly. For shrinkage and temperature reinforcement, ACI 318-19 caps spacing at the lesser of five times the slab thickness and 18 in, with a minimum reinforcement ratio of 0.0018 of the gross concrete area for Grade 60 deformed bars. For flexural reinforcement in one-way slabs the cap is tighter — the lesser of three times the thickness and 18 in. This calculator checks your spacing against both and tells you which one you have crossed.
Lap splices transfer force from one bar to the next by bond with the surrounding concrete. ACI 318 Chapter 25 requires the splice length to be calculated from the development length, and it depends on bar size, concrete strength, cover, confinement, epoxy coating and whether the bar is a top bar. The 40-diameter figure this calculator defaults to is a detailing rule of thumb that is often conservative for #4 and #5 bars in ordinary conditions and is not a substitute for the calculation. Use whatever the drawings specify.
Worked example: a 24 ft × 24 ft slab with #4 at 12 in each way
A 24 ft square, 5 in thick slab on grade, formed edges with 3 in cover, #4 bars at 12 in on centre both ways, 20 ft stock, 40-diameter laps.
- Clear band. 24 ft × 12 = 288 in, less 2 × 3 in cover = 282 in in each direction.
- Bar count. floor(282 ÷ 12) + 1 = 23 + 1 = 24 bars each way, 48 bars in total.
- Bar length. 24 ft − 2 × 3 in = 24 − 0.5 = 23.5 ft per bar.
- Splices. 23.5 ft exceeds the 20 ft stock, so each run needs a second stick. Lap = 40 × 0.5 in ÷ 12 = 1.667 ft. Reach of the second stick = 20 − 1.667 = 18.333 ft, and 23.5 − 20 = 3.5 ft remains, so one extra stick does it: 2 sticks per run.
- Order. 48 runs × 2 = 96 stock bars of 20 ft.
- Placed steel. 48 × 23.5 = 1,128 ft of bar, plus 48 splices × 1.667 ft = 80 ft of lap, giving 1,208 ft.
- Weight. 1,208 × 0.668 lb/ft = 807 lb, about two-fifths of a ton.
- Ties. 24 × 24 = 576 intersections. Tying every second one inside the field and every one around the perimeter is typical practice.
- Steel area. 0.20 in² × 12 ÷ 12 = 0.200 in²/ft each way. Against a 5 in slab, that is 0.200 ÷ (12 × 5) = 0.0033 of the gross section, comfortably over the 0.0018 minimum.
Note how much the 20 ft stock length costs you here: 96 bars of 20 ft is 1,920 ft of steel bought to place 1,208 ft. Ordering 30 ft stock would let each 23.5 ft run come from a single stick — 48 bars, no splices, no lap waste and a much faster tie-out.
How to read the result
Look at sticks per run before anything else. If it reads 2, ask whether a longer stock length would make it 1. Splicing doubles the handling, adds lap steel you pay for and never use structurally, and creates a congested plane in the slab where every bar laps at the same station — which is precisely what detailers stagger splices to avoid. If splices are unavoidable, stagger them so no more than half the bars lap at any one section.
Then check the steel area per foot against the drawing, not against the bar callout. Substitutions are common on site and the safe way to make one is to keep the area per foot at or above what was specified. Going from #4 at 12 in (0.200 in²/ft) to #5 at 18 in (0.207 in²/ft) preserves the area but nearly doubles the spacing, which can breach the maximum-spacing rule and does change crack control. Area is necessary but not sufficient; spacing limits still apply.
Finally, treat the tie count as a labour figure, not a materials figure. Wire ties are cheap; the hours are not. A 576-intersection grid tied at every second crossing is roughly 300 ties, and a competent placer works through them quickly with a twister — but on a big pour this is exactly the number a foreman needs to schedule the crew.
Steel area per foot of slab width
| Spacing | #3 (0.11 in²) | #4 (0.20 in²) | #5 (0.31 in²) | #6 (0.44 in²) | #7 (0.60 in²) | #8 (0.79 in²) |
|---|---|---|---|---|---|---|
| 6 in | 0.220 | 0.400 | 0.620 | 0.880 | 1.200 | 1.580 |
| 8 in | 0.165 | 0.300 | 0.465 | 0.660 | 0.900 | 1.185 |
| 10 in | 0.132 | 0.240 | 0.372 | 0.528 | 0.720 | 0.948 |
| 12 in | 0.110 | 0.200 | 0.310 | 0.440 | 0.600 | 0.790 |
| 16 in | 0.083 | 0.150 | 0.233 | 0.330 | 0.450 | 0.593 |
| 18 in | 0.073 | 0.133 | 0.207 | 0.293 | 0.400 | 0.527 |
Bar areas are the nominal values in ASTM A615. Spacings above 18 in are outside the ACI 318-19 maximum for slab reinforcement regardless of the area they deliver.
This lays out a grid; it does not design one
Bar size and spacing come from a structural design that accounts for loads, span, subgrade support, concrete strength and serviceability. Nothing on this page determines whether a given grid is adequate. Two specific traps: reinforcement in an unreinforced slab-on-ground does not stop cracks forming, it only holds them tight, so it is no substitute for correctly spaced control joints; and steel placed on the ground instead of on chairs at the specified depth does close to nothing, because its lever arm is gone. Placement tolerance matters as much as quantity.
Mistakes that cost bars, time or an inspection
- Forgetting the +1. Dividing the span by the spacing counts gaps, not bars. You always need one more bar than gaps.
- Measuring cover from the form to the bar centreline. Cover is clear cover, to the outside surface of the bar — and for a grid, to the outermost bar, which is usually the one in the lower layer.
- Ignoring lap steel in the weight. Splices add real tonnage. On the worked example above the laps are 80 ft of the 1,208 ft total, about 7%.
- Lapping every bar at the same station. Stagger splices so no more than half the bars are spliced at any one section.
- Substituting by bar size instead of by area per foot. Keep steel area per foot at or above the specified value, and check the maximum spacing rule separately.
- Placing bars on the subgrade. Steel in the bottom of a slab on grade with no chairs contributes almost nothing. Use chairs, dobies or bolsters at the spacing the specification calls for.
- Assuming 40 diameters is a legal lap. ACI 318 Chapter 25 requires the splice to be calculated. Use the drawings' figure.
Where this fits with the rest of the pour
The rebar takeoff is one line on a concrete estimate. Once the grid is set, the tonnage from this page goes straight into the rebar weight calculator for pricing and freight, and the volume of concrete around it comes from the concrete slab calculator or, for strip and spread footings, the concrete footing calculator. If the pour is small enough to mix on site, the concrete bag calculator converts the volume to bags.
For reinforced masonry rather than concrete, the layout logic is different — bars go in cells at fixed 8 in centres, so spacing is quantised — and the quantities come out of the concrete block calculator along with the grout that surrounds them. Where the footing size itself is in question, the soil bearing footing size calculator is the place to start, because a footing that is too narrow cannot be rescued with more steel.
A word on welded wire reinforcement: for a residential slab on grade, sheets of WWR are often specified instead of a bar grid, and the area-per-foot table above is the right way to compare them. Sheet reinforcement is faster to place and much easier to leave sitting on the subgrade, which is why bar on chairs remains the more reliable detail wherever the steel actually has to work.
Key terms
- On centre (o.c.)
- Spacing measured from the centreline of one bar to the centreline of the next, not the clear gap between them.
- Clear cover
- The shortest distance from the concrete surface to the outside of the nearest bar. Set by ACI 318-19 Table 20.5.1.3.1 according to exposure.
- Lap splice
- An overlap of two parallel bars long enough for force to transfer through the surrounding concrete. Its length is calculated, not assumed.
- Chair / bolster
- A support that holds reinforcement at the specified height while concrete is placed. Without it the bar ends up on the subgrade and does nothing.
- As per foot
- Steel cross-sectional area in each foot of slab width — bar area times twelve divided by the spacing in inches. The quantity an engineer specifies.
- Shrinkage and temperature reinforcement
- Steel provided to control cracking from volume change rather than to carry load. ACI 318-19 sets its minimum ratio at 0.0018 of the gross section for Grade 60 bars.