Two spacings on one panel is two grids — count each zone, share the boundary bar, and never average the spacings
← Back to the calculatorTwo spacings means two grids. Tightened zone: clear 118 in ÷ 6 in = 19 spaces + 1 = 20 bars. Main zone: 118 in ÷ 12 in = 9 spaces + 1 = 10 bars. The boundary bar belongs to both zones, so subtract it once: 20 + 10 − 1 = 29 bars of 8 ft. Averaging the two spacings instead gives 27 — two bars short.
Open the calculator for a uniform grid, or read the four steps below for a zoned one.
A rebar grid is two sets of parallel bars crossing at right angles. Count one set across the distance the other set runs, and you get the classic pair of numbers — rows and columns — whose product is the number of intersections in the mat. That is the whole model, and it works perfectly as long as both sets are evenly spaced.
Real drawings are not that tidy. A slab on grade is rarely one uniform grid: there is a normal field zone and then a zone where the spacing tightens — under a bearing wall, around a column pad, along a thickened edge, or wherever the drawing drops from #4 at 12 in to #5 at 6 in. A panel with two spacings is not one grid with a compromise spacing. It is two grids sitting side by side, and they share a bar at the seam.
Search for a rebar grid calculator and almost everything that comes back assumes a single uniform spacing, or lets you pick one spacing and one panel size and nothing else. This page works the case those tools skip: a 20 ft × 8 ft slab with the main zone at 12 in on centre and the last 10 ft tightened to 6 in, in #5 bar.
Do not run one spacing down the whole panel and do not extrapolate from the main zone. Work each zone separately, with the clear distance measured inside that zone:
Two things to be deliberate about before you move on. First, each zone keeps its own end bar at each end: the tightened zone has a bar 1 in in from the boundary and a bar 1 in in from the far edge, and so does the main zone. Second, the + 1 is a bar at the start of the zone, not a spacer — if you drop it you will be one bar short per zone, and on a tight zone that is a visible gap.
This is the step every calculator skips, and it is worth exactly one bar per seam. The two zones are laid out along the same run, so the last bar of one zone and the first bar of the other want to land in the same place: at the boundary. There is one bar there, not two.
Some drawings do show a deliberately doubled bar at the joint — a bar each side of the seam. That is a real detail and you may have to build it, but it still counts as two bars in the total, so either settle it on the drawing or settle it in the cut list. What you cannot do is leave it as an unstated assumption, because it moves the order by one bar and it moves the seam position by 6 in.
10 + 20 and moving on over-orders by one bar; counting an average spacing under-orders by two.The temptation when a panel carries two spacings is to blend them: half the run at 12 in and half at 6 in, so call it 9 in and calculate once. That is the single most expensive shortcut on this page, and it is easy to show why.
Bar count is a step function, not a straight line. Every time you step down a spacing you add whole bars at whole intervals; you never add a fraction of a bar. Average the spacings and you are travelling along a smooth line that the real answer only touches at the ends:
| Way of counting the 20 ft run | Spacing used | Bars | Error vs real |
|---|---|---|---|
| Real zoned layout | 12 in + 6 in | 29 | — |
| Average of the two spacings | 9 in | 27 | −2 bars |
| One spacing for the whole panel | 12 in | 20 | −9 bars |
| Tighten the whole panel instead | 6 in | 40 | +11 bars |
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Two bars missing does not sound like much until you picture where they are: the entire tightened zone is the part of the slab that was designed for a load, and the two missing bars are the ones that were supposed to be closest together. Averaging systematically under-reinforces every tightened zone, because the average always sits between the two spacings and the tight half of the panel is the half that carries the work.
The same step-function arithmetic shows up wherever spacing changes mid-panel, which is why the spacing page warns against averaging there too — it just does it for a change at the edge rather than a designed zone.
Once the zoned count exists, it is worth putting it next to the two alternatives, because the numbers are what make the case for zoning. All three lines below are the same 20 ft × 8 ft slab in #5 bar:
| Layout | Bars (tightened dir.) | Total length | Weight | Per 100 sq ft | Intersections |
|---|---|---|---|---|---|
| Uniform 12 in | 20 | 320 ft | 333.8 lb | 208.6 lb | 160 |
| Zoned 12 in / 6 in | 29 | 392 ft | 408.9 lb | 255.5 lb | 232 |
| Uniform 6 in | 40 | 640 ft | 667.5 lb | 417.2 lb | 640 |
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Read the middle line against the bottom one. Tightening the full panel doubles the steel — 333.8 lb becomes 667.5 lb, and the intersections nearly quadruple to 640, which is the tie wire and the labour. Zoning instead gets the tight spacing onto the loaded 10 ft for 22.5% more steel and 232 intersections. That is the whole point of a zoned drawing: put the density where the moment is, and leave the field at the lighter mat.
The corners worth counting yourself, all from the same two-zone geometry: the zoned mat carries +22.5% steel and +45% intersections against the uniform 12 in mat, and −39% steel and −64% intersections against the uniform 6 in mat. Densities add steel roughly linearly but add tying roughly with the square, so a tightened zone always costs more in labour than its share of the panel suggests.
A zoned mat has two bar marks by definition: the short bars in the tightened direction, and the long bars across it. Write them down before you touch stock length, and note where the seam sits, because the seam is the one place a placing crew will get the layout wrong.
| Mark | Size | Pieces | Cut length | Runs | Notes |
|---|---|---|---|---|---|
| A | #5 | 29 | 8 ft | across the 20 ft run | 12 in in the main zone, 6 in in the tightened zone |
| B | #5 | 8 | 20 ft | along the 20 ft run | one bar each, 12 in o.c. across the 8 ft width |
| Total | #5 | 37 pieces | — | — | 392 ft net, 408.9 lb |
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Then apply 20 ft stock. Mark B is already 20 ft and needs no thinking. Mark A is 8 ft, so two pieces come off every stock bar:
| Item | Uniform 12 in | Zoned | Uniform 6 in |
|---|---|---|---|
| Mark A pieces (8 ft) | 20 | 29 | 40 |
| Mark B pieces (20 ft) | 8 | 8 | 16 |
| 20 ft bars for mark A | 10 | 15 | 20 |
| Total bars bought | 18 | 23 | 36 |
| Cut allowance | 25.0% | 17.3% | 12.5% |
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Notice the direction of the last row. The zoned mat uses more steel than the uniform 12 in mat, but it wastes less of what it buys, because 29 short pieces pair up on 20 ft stock better than 20 do. Counting pieces rather than feet is what exposes that: the estimator page shows why the leftover is a remainder from the division and not a flat percentage, and the footing page runs the same 20 ft versus 40 ft trade the other way round.
A 6 in error in the boundary costs nothing: 29 bars, 392 ft, 408.9 lb, the same order. A 3 in error adds a bar and leaves a 1 in space at the seam — 0.375 in of clear between two #5 bars.
One of those numbers is harmless and the other cannot be built, and the bar count does not distinguish them. What decides it is the remainder each zone ends on, and the remainder always falls at the seam.
Every zoned layout ends each zone on a remainder, and the remainder always falls at the seam. That is already true at the nominal boundary: 118 in at 12 in is nine spaces of 12 with 10 in left over, and 118 in at 6 in is nineteen spaces of 6 with 4 in left over. A drawing that says 12 in and 6 in does not produce 12 in and 6 in where the zones meet — it produces 10 in and 4 in.
So the boundary is not a dimension that carries a tolerance; it is a setout line with a remainder hanging off it. Move the line and the remainder changes, in a pattern that repeats every 6 in because 6 in divides the tight spacing:
| Boundary moved | Main zone bars | Tight zone bars | Seam space main / tight | Total bars | Weight |
|---|---|---|---|---|---|
| −12 in | 9 | 22 | 10 / 4 in | 30 | 417.2 lb |
| −9 in | 10 | 22 | 1 / 1 in | 31 | 425.5 lb |
| −6 in | 10 | 21 | 4 / 4 in | 30 | 417.2 lb |
| −3 in | 10 | 21 | 7 / 1 in | 30 | 417.2 lb |
| 0 — as drawn | 10 | 20 | 10 / 4 in | 29 | 408.9 lb |
| +3 in | 11 | 20 | 1 / 1 in | 30 | 417.2 lb |
| +6 in | 11 | 19 | 4 / 4 in | 29 | 408.9 lb |
| +9 in | 11 | 19 | 7 / 1 in | 29 | 408.9 lb |
| +12 in | 11 | 18 | 10 / 4 in | 28 | 400.5 lb |
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Read the total column first, because it is the one that gets checked. Across a foot of movement it runs 30, 31, 30, 30, 29, 30, 29, 29, 28 — not monotonic, and three different boundaries give exactly 29 bars and exactly 408.9 lb: the line as drawn, 6 in into the tight zone, and 9 in into it. A count, a weight or an order confirmation cannot separate those three layouts.
Read the seam column second. At 0, +6 and +12 in the seam is 10/4, 4/4 and 10/4 — tight, but buildable. At +3 and +9 in both remainders land on 1 in, and 1 in centre to centre on a #5 bar is 0.375 in of clear gap against the 1 in minimum clear spacing the site uses everywhere else:
Nobody builds the 1 in space. Either the extra main bar comes out or the boundary bar gets shoved, and both fixes end at the same place: 121 in with 10 bars leaves 13 in at the seam, 1 in wider than the 12 in the drawing specifies. That is an 8.3% local miss, and it is invisible afterwards because the count is back to 29.
The 6 in error is the one that sounds worse and is not. Push the boundary 6 in into the tight zone and that zone is 112 in instead of 118 in, so it takes 19 bars instead of 20; the main zone is 124 in and takes 11 instead of 10. Total 29, weight 408.9 lb, seam 4 in on both sides — buildable, and nothing on the order moves. What did move is 8.3 lb of bar out of the tightened zone and into the field, and the strip that lost its bar is the 6 in at the boundary, not the strip at the wall.
That is why a zoned mat needs two checks and not one. The count check asks whether 29 is the right number; the position check asks whether the bar nearest the boundary sits where the drawing put it. The first is arithmetic and the second is a tape measure, and only the second catches a 6 in shift.
Two different things get called a 3 in error, and they are not the same failure. An error at the first bar of a zone shifts that row and moves one remainder. An error at the boundary moves the line both zones are measured to, so it moves two remainders at once and it moves a bar from one zone into the other. The boundary is the one to string.
Nothing in section moves with any of this. Cover is still 0.75 in, the chair height is still 1.75 in, and where a second layer is called for the layer spacing is still one bar diameter plus 1 in of clear. A boundary error is entirely in plan. The perpendicular direction is untouched too — its 8 bars at 12 in do not care where the boundary sits, which is why the intersection count follows the tightened direction alone: 232 at the line as drawn, 240 at 3 in.
Treat each spacing as its own grid over its own zone, then subtract the one bar the two zones share at the boundary. On the 20 ft × 8 ft slab with the main zone at 12 in and the last 10 ft at 6 in, the main zone takes 10 bars and the tightened zone 20 bars over a 118 in clear distance. Because both sets are spaced along the same run, the bar at the joint is counted twice, so the real total is 10 + 20 − 1 = 29 bars of 8 ft.
No. A 20 ft run with 10 ft at 12 in and 10 ft at 6 in averages 9 in, which would give floor(238 ÷ 9) + 1 = 27 bars against a correct 29. Bar count is a step function, so averaging understates a tightening by two bars here — and those two bars are the ones nearest the load. It is worse if you extrapolate from the field spacing across the whole panel: that loses nine bars.
On the 20 ft × 8 ft slab in #5 bar, keeping both directions at 12 in is 320 ft and 333.8 lb. Tightening the last 10 ft of the run to 6 in takes it to 392 ft and 408.9 lb, a 22.5 per cent increase. Tightening the whole panel to 6 in is 640 ft and 667.5 lb, a 100 per cent increase. Zoning buys the density where the load is for roughly a quarter of the extra steel, not all of it.
Where the load, the bearing or the thickening is — this page does not set them. In practice a zone boundary follows a construction joint, a step in slab thickness, the edge of a bearing pad, or the line where the drawing switches bar mark. For the count, what matters is that you measure the clear distance inside each zone separately instead of running one spacing across the panel.
The seam carries one bar, not two, so lay the zones out so the last bar of the tight zone and the first of the main zone are the same bar. Where the drawing deliberately shows a doubled bar at the joint, count both and say so on the cut list. Either way the difference is exactly one bar — which is why it has to be settled before the order, not after.
Multiply the count in one direction by the count in the other, zone by zone, and add. On the 20 ft × 8 ft slab the tightened direction carries 29 bars and the perpendicular direction 8, so the mat has 29 × 8 = 232 intersections, against 160 for the uniform 12 in mat. That is 45 per cent more tying for 22.5 per cent more steel and 17.3 per cent less cutting waste.
Yes, and a zoned layout often cuts better than a uniform one. The 8 ft bars come two to a 20 ft stock bar, so 29 pieces need 15 whole bars; the eight cross bars are already 20 ft each. That is 23 bars of 20 ft, 460 ft, for a mat that uses 392 ft — a 17.3 per cent cut allowance against 25 per cent on the uniform 12 in mat.
Chairs follow the area and the cover, not the bar spacing, so tightening a zone does not by itself add chairs. What it does change is the number of intersections to tie, because the crossing count goes up with the bar count in both directions. Where the tightened zone is also doubled or carries a different cover, the chair height is set separately — the concrete rebar page works that chain.
The bar has one and the boundary does not. ACI 117 allows ±¼ in on placement for members under 12 in deep, which is a tolerance on where each bar sits; the boundary line has no tolerance of its own because it inherits the tight spacing. That gives it 6 in of real freedom, and only the marks that are a whole number of the tight spacing are buildable. On the 20 ft × 8 ft slab, moving the boundary 3 in either way puts 1 in at the seam — 0.375 in of clear on a #5 bar — while moving it 6 in leaves the total at 29 bars and 408.9 lb, so no count catches it. Set the boundary on a 6 in mark measured from the zone’s own anchor and check the seam space, not the bar count.
Yes, and it is a clearance check, not a spacing choice. One engineer’s forum rule puts it plainly: “You need 1.33 inches spacing between your rebar” when the aggregate is 1 in (r/StructuralEngineering) — the four-thirds rule, 4/3 of the aggregate size as clear space between bars. On this page’s 6-in zone with a #5 bar (0.625 in), the clear space is 6 − 0.625 = 5.375 in, four times that requirement; the check only starts to bite when spacing drops toward 2 in. A zoned layout is where tight spacings live, so check the clear space in the tight zone, not the nominal spacing.