Calculator
Rebar Grid Calculator
A 20 ft stick of rebar doesn't advance a slab's reinforcing grid by 20 ft once it has to be spliced to the next stick. Two bars lapped together need to overlap by a set multiple of the bar's own diameter to actually transfer load between them, and that overlap is bar that's doing nothing but joining two pieces — on a common 40-diameter lap with #4 bar, a 20 ft stick only advances the run by about 18 ft 4 in, and every splice along a long run repeats that loss.
This calculator builds a two-way slab grid from your spacing, counts how many stock-length sticks each direction actually needs once lap splices are accounted for, and adds the ties, chairs and total linear footage that go with it. A slab's total rebar order is rarely just its area divided by a coverage rate — it's a grid with a shape, and the shape is what decides how many splices, and how much overlapping bar, the job actually needs.
Calculate your quantity
In bar diameters. 40 is a common minimum.
Rebar sticks
30 20 ft bars
Total rebar
600 lin ft
- Bars running lengthwise
- 8 bars
- Bars running crosswise
- 14 bars
- Lap length
- 20 in
- Effective length per stick
- 18.33 ft
- Splices
- 8 splices
- Grid intersections
- 112 intersections
- Wire ties
- 56 ties
- Chairs
- 28 chairs
- A 40-diameter lap on a 0.5 in bar is 20 in, so every 20 ft stick only advances the run by 18.33 ft. Over 8 splices that is 13.3 ft of bar that is doing nothing but overlapping.
Shopping summary
- • 30 x 20 ft lengths of rebar
- • 0.84 lb of tie wire, 28 rebar chairs
This is an estimate — confirm structural work with a professional.
How this calculation works
Two sets of bars run perpendicular to each other across the slab: one set runs the length of the slab, with its count determined by how many fit across the width at the entered spacing; the other runs the width, with its count determined by how many fit along the length. Each individual bar in the lengthwise set is as long as the slab's usable length (inside the edge cover), and each bar in the widthwise set is as long as the usable width — and it's those individual bar lengths, compared against the stock length available minus one lap, that decide how many sticks each single bar actually needs.
The lap itself is a fixed multiple of the bar's diameter — commonly 40 diameters for a standard tension lap — converted to a length and subtracted from the stock stick length to get how much run each stick actually adds once it's tied to the next one. A bar that needs multiple sticks to span its own length needs one fewer splice than the number of sticks, since the first stick doesn't splice to anything behind it.
- Bar count in each direction depends on the usable dimension in the OTHER direction — bars running lengthwise are counted by how many fit across the width, and vice versa, which is easy to transpose by mistake when reading the breakdown.
- Splices only occur where a single bar's own length exceeds what one stock stick can cover; a bar shorter than a stick minus its lap needs no splice at all.
- Ties are estimated at roughly every other grid intersection, and chairs at roughly a quarter of intersections, both standard rough site allowances rather than an engineered tie schedule.
- Edge cover reduces the usable slab dimensions in both directions before any bar count is calculated, which is why a slab with more cover has slightly fewer bars at the same nominal spacing.
The formula
effectivePerStick = barStockLength − (lapDiameters x barDiameter ÷ 12); sticksPerRun = ceil(barLength ÷ effectivePerStick)
- usableLength / usableWidth
- Slab dimensions reduced by twice the edge cover — the actual span the rebar grid covers once the required clearance from the slab's outer edge is subtracted on both sides.
- lapDiameters x barDiameter
- The lap length in inches — a fixed number of bar diameters, converted using the bar's actual diameter, since a lap on thicker bar is a longer overlap than the same diameter-count on thinner bar.
- effectivePerStick
- Stock stick length minus the lap length — the actual run one stick contributes to a spliced bar once it's overlapped with the next one, always shorter than the stick's own nominal length wherever a splice is needed.
- sticksPerRun
- The individual bar length (usable length or width, depending on direction) divided by effectivePerStick, rounded up — how many stock sticks one continuous bar in that direction actually needs.
- splices
- One fewer than sticksPerRun, per bar, summed across every bar in that direction — the count of actual overlap joints the grid needs, each one consuming a lap length of bar that adds no new run to the slab.
Lap length is set by bar diameter, not by stock stick length, so switching to a shorter stock length (10 ft instead of 20 ft) increases the number of splices needed to span the same run without changing how much any individual splice costs in overlapped bar.
Where these numbers come from
- A 40-bar-diameter tension lap as a common planning multiple
- A commonly cited rule-of-thumb multiplier for a standard tension lap splice in normal-weight concrete, not a fixed code figure — the lap length an actual structural drawing calls for depends on concrete strength, bar spacing, cover, and the specific splice classification, and can be longer than this default. Treat this page's figure as a planning estimate, and confirm the exact lap called for on any structural drawing before ordering against it.
- 10 and 20 ft stock bar lengths
- Common retail and yard-counter stock lengths for smaller rebar sizes. Mills and fabricators often supply longer stock (40 or 60 ft) that a supplier custom-cuts, but 10 and 20 ft are the common lengths sold over the counter for smaller residential jobs.
- 3 in default edge cover
- A commonly required minimum concrete cover figure for slabs cast on or near soil, less on formed surfaces. Required cover varies by application and by code; confirm the specific figure your inspector or drawings call for rather than relying on this page's default for a job with strict cover requirements.
- Ties at roughly half of grid intersections, chairs at roughly a quarter
- A rough site-practice allowance for a typical slab-on-grade tie-off, not an engineered fastening schedule. Actual tie frequency and chair spacing are commonly set by project specification or by standard practice on the specific job, and can be tighter than this default for a heavily loaded or inspected pour.
Worked examples
A 20 by 12 ft slab on an 18 in grid with #4 bar
| Slab length | 20 ft |
|---|---|
| Slab width | 12 ft |
| Grid spacing | 18 in |
| Bar stock length | 20 ft |
| Bar size | #4 (1/2 in) |
| Lap length | 40 |
| Edge cover | 3 in |
| Rebar sticks | 30 20 ft bars |
|---|---|
| Total rebar | 600 lin ft |
| Bars running lengthwise | 8 bars |
| Bars running crosswise | 14 bars |
| Lap length | 20 in |
| Effective length per stick | 18.33 ft |
| Splices | 8 splices |
| Grid intersections | 112 intersections |
| Wire ties | 56 ties |
| Chairs | 28 chairs |
This slab needs 8 bars running lengthwise and 14 running widthwise. The lengthwise bars, at 19.5 ft each once edge cover is subtracted, each fit on a single 20 ft stick with room to spare — no splice needed. The widthwise bars, at 11.5 ft, also fit on one stick each. The 8 splices in this job all come from the lengthwise direction, where a handful of bars needed a second stick to close out the run.
30 sticks and 600 linear feet of bar for a 240 sq ft slab — 2.5 linear feet of bar per square foot of slab, on this spacing. That per-square-foot ratio is the number worth remembering when a different slab comes up on the same 18 in grid, since it holds roughly steady across similarly proportioned slabs even as the total area changes.
A long, narrow 40 by 6 ft walkway on the same 240 sq ft footprint
| Slab length | 40 ft |
|---|---|
| Slab width | 6 ft |
| Grid spacing | 12 in |
| Bar stock length | 20 ft |
| Bar size | #3 (3/8 in) |
| Lap length | 40 |
| Edge cover | 2 in |
| Rebar sticks | 58 20 ft bars |
|---|---|
| Total rebar | 1160 lin ft |
| Bars running lengthwise | 6 bars |
| Bars running crosswise | 40 bars |
| Lap length | 15 in |
| Effective length per stick | 18.75 ft |
| Splices | 12 splices |
| Grid intersections | 240 intersections |
| Wire ties | 120 ties |
| Chairs | 60 chairs |
This slab covers exactly the same 240 sq ft as the first example — 40 by 6 instead of 20 by 12 — but needs almost double the rebar: 58 sticks against 30, on a tighter 12 in grid and a thinner #3 bar. Shape, not just area, is driving that difference: a long, narrow slab on a tight grid needs far more crosswise bars (40 of them here, against 14 in the first example) than a squarer slab of the same footprint.
Every one of this job's 12 splices falls on the lengthwise bars specifically, because at nearly 40 ft long, each lengthwise bar needs three 20 ft sticks to span the run — the widthwise bars, at under 6 ft each, never need more than one. Two slabs of identical area can have very different rebar orders once shape and grid spacing enter the picture, which is exactly why this page works from the slab's actual dimensions rather than its area alone.
Common mistakes
- Estimating linear feet of rebar from slab area and a flat coverage rate. Two slabs of identical area can need very different total footage depending on their shape and grid spacing, as the two examples on this page show directly.
- Forgetting that lap splices consume bar without adding run. A stick count that ignores lap length overstates how far each stick actually reaches, and under-orders the total footage needed to close out every run.
- Using a shorter stock length to save on delivery length without accounting for the extra splices it creates. Switching from 20 ft to 10 ft stock roughly doubles the splice count on a long run, each one consuming its own lap length of bar.
- Applying the same lap-diameter multiple regardless of bar size without converting to an actual length. A 40-diameter lap on #3 bar and the same 40-diameter lap on #5 bar are meaningfully different physical lengths, even though the multiple is identical.
- Skipping edge cover when calculating usable slab dimensions. Bar count is set from the usable length and width, not the raw slab dimensions, and ignoring cover slightly over-counts bars on a slab with generous required clearance.
Shopping summary
Order the stick count and total linear footage together — a supplier pricing by the stick needs the count, and one pricing by weight or total length needs the footage, and they don't always agree once partial sticks and splices are in the mix.
Concrete volume for the pour itself is a separate calculation from the reinforcing grid — the concrete slab calculator sizes that from the same length, width and thickness, and if the slab also needs edge or perimeter footings, those typically carry their own bar schedule rather than sharing this page's grid count.
FAQ
Why does the walkway example need more rebar than the square slab even though both cover the same area?
Because bar count in each direction depends on how many bars fit across the OTHER dimension at the given spacing, and a long, narrow slab has a much longer dimension to cover with crosswise bars than a squarer slab of the same total area does. Shape changes the grid's bar count independently of area.
Does a tighter grid spacing always mean proportionally more rebar?
Roughly, but not exactly — halving the spacing roughly doubles the bar count in that direction, but splice count and total footage don't scale in perfect lockstep with bar count, since each individual bar's own length (and therefore its stick and splice needs) doesn't change with spacing in the other direction.
Can I use a shorter lap to save on rebar footage?
The lap length used here is a common planning multiple, not the minimum a specific structural design might require — reducing it below what a project's actual specification calls for is a strength decision, not just a cost-saving one, and should only be done against a structural drawing or engineer's direction, not to trim a shopping list.
How accurate is the wire tie and chair count?
It's a rough site-practice estimate — roughly every other intersection for ties and roughly a quarter of intersections for chairs — rather than an engineered fastening schedule. A project with a stricter inspection requirement or a heavier expected load commonly specifies tighter tie frequency and chair spacing than this default assumes.
What happens if my slab isn't rectangular?
This calculator assumes a rectangular grid. For an L-shaped or otherwise irregular slab, the most reliable approach is to split it into rectangular sections, run this page once per section against each section's own dimensions, and add the results — splicing at the seams between sections the way a single continuous slab would.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.