A chair sets a dimension, not just a count — height from cover, gap between tiers, then the count
← Back to the calculatorA chair does two jobs and only one of them is being counted. It holds the bar up, and it holds a dimension: the cover under it and, in a double-layer mat, the clear gap between the two tiers. Height first: chair = cover + db/2 = 1.5 + 0.3125 = 1.81 in, so order a 2 in chair. Then the count on a support grid: 7 across × 6 along = 42 chairs, against the 120 a flat one-per-4-sq-ft rule gives — nearly three times too many.
Open the calculator for the bar counts and weight, or read the four steps below for the chairs themselves.
A rebar chair is a small stand that holds reinforcing bar at a set height above whatever it sits on. Every page about them stops there and moves straight to counting, and that is where the mistakes start, because a chair is doing two jobs at once.
The first job is support: keep the bar from sagging to the bottom or floating to the top before and during the pour. The second job is dimension: a chair is the only thing in a slab that holds a distance — the cover beneath the bottom mat, and the clear gap between the two tiers of a double-layer mat. Get the count right and the height wrong and you have still built the wrong slab, because the height is what sets the effective depth of the steel doing the work.
Search for a rebar chair calculator and almost everything that comes back gives you one flat number: chairs equal to area divided by some figure per square foot. That rule answers "how many" and never "how tall", and even the count it gives is usually far too high. This page does all three numbers on a 24 ft × 20 ft slab with #5 bar both ways at 12 in o.c. in a double-layer mat.
A chair is sold by height, and its height is measured from the surface it stands on to the underside of the bar it carries. That makes it the clear cover plus half a bar diameter, because the cover is measured to the outside face of the bar, not to the bar's centre:
The half-diameter term is the one people drop, and it is small enough to ignore on one bar and large enough to matter across a heavy mat. It also explains why the bar size barely moves the chair you order: chairs come in 1 in steps, so from #4 to #6 at the same cover you are still rounding to the same 2 in chair. What moves the chair size is the cover, and cover is set by exposure, not by the bar.
| Clear cover (in) | Typical situation | #4 height | #5 height | #6 height | Chair ordered |
|---|---|---|---|---|---|
| 0.75 | interior slab or wall, #11 and under | 1.00 in | 1.06 in | 1.13 in | 1.5 in |
| 1.5 | exposed to weather, #5 and under | 1.75 in | 1.81 in | 1.88 in | 2 in |
| 2.0 | exposed to weather, #6 and larger | 2.25 in | 2.31 in | 2.38 in | 2.5 in |
| 3.0 | cast against and permanently in contact with earth | 3.25 in | 3.31 in | 3.38 in | 3.5 in |
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The cover figures along the left are the work of the concrete rebar page, which derives them from ACI 318-19 Table 20.6.1.3.1 and works the same cover-to-chair chain in the context of a full takeoff. Here the point is simply that the chair height follows the cover, and the cover follows the exposure, in that order.
In a single-layer mat the chairs only set the cover. In a double-layer mat — a suspended slab, a raft, a heavily loaded slab on grade with top steel — the chairs carry a second job that no per-area count even looks at: they hold the two mats apart. That distance is the slab depth minus both covers minus one bar diameter, and it is a designed number, because the top steel is only as good as the depth it ends up at.
Run the same sum at a range of slab depths and the gap the chairs have to hold open grows with the slab:
| Slab depth | Clear gap between tiers | Top-steel effective depth | Chair to carry the top mat |
|---|---|---|---|
| 6 in | 2.38 in | 4.19 in | 2 in + a 2.5 in stand |
| 8 in | 4.38 in | 6.19 in | 4.5 in (or a 2 in chair on a 2.5 in base) |
| 10 in | 6.38 in | 8.19 in | 6.5 in |
| 12 in | 8.38 in | 10.19 in | 8.5 in |
| 14 in | 10.38 in | 12.19 in | 10.5 in |
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This is where chair height stops being a consumable and becomes a structural input. Take the 10 in slab: the top steel should sit at 8.19 in effective depth, and the chair is what puts it there. Let the top mat sit 1 in low — a chair one size short, or a few chairs too far apart so the bar sags between them — and the effective depth drops to about 7.19 in, roughly 12 per cent less. Section moment capacity moves with the effective depth, so that is a material loss in capacity that is completely invisible once the slab is poured.
Now the count, and it is a grid count rather than an area count, for the same reason bar count was a step function and not an average: one chair supports a patch of mat, and chairs are laid out on a regular support grid, normally 3 to 4 ft on centre. Count the grid points:
The + 1 is a chair at the start of each run, exactly as it is a bar at the start of a row on the spacing page. Drop it and you lose a whole edge line of support, which is the line that carries the cantilever over the formwork. Add a small breakage allowance on top, because chairs get stood on and knocked over during placing:
| Support grid | Across 24 ft | Along 20 ft | Chairs (net) | Per sq ft | With 5% breakage |
|---|---|---|---|---|---|
| 5 ft | 5 | 5 | 25 | 0.052 | 27 |
| 4 ft | 7 | 6 | 42 | 0.088 | 45 |
| 3 ft | 9 | 7 | 63 | 0.131 | 67 |
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Read the middle line as the default and the two outer lines as the adjustments. Tighten toward 3 ft — 63 chairs, half again as many — where the mat is heavy, where the bars are large enough to sag under their own weight across a 4 ft span, or where the top mat will be walked over during placing. Go to 5 ft only on light single-layer mesh where nothing is being carried on the steel.
The shortcut most sites publish, and the one that turns up in a lot of forum answers, is one chair per 4 sq ft. On this 480 sq ft slab that is 120 chairs — against 42 from the grid. It is not a rounding difference; it is nearly three times the chairs you will place, and it comes from treating a chair as though it supports one point of mat instead of a patch.
| Way of counting | Rule | Chairs for 480 sq ft | Versus the grid |
|---|---|---|---|
| Support grid, 4 ft | grid points | 42 | — |
| Support grid, 3 ft | grid points | 63 | +50% |
| Flat rule | 1 per 4 sq ft | 120 | +186% |
| Flat rule, tight | 1 per 3 sq ft | 160 | +281% |
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Why the flat rule is tempting is that it is almost the right shape — chairs do scale with area, so more area does mean more chairs — but it is the wrong function. A grid count grows with the square root of the area per unit area, not linearly, and on any slab of realistic size the two diverge hard. 42 chairs is also not a compromise: it is the count that puts a support within 2 ft of every point on the mat, which is what the 4 ft grid is for.
Two other numbers worth carrying down from the rest of the site. The chairs here are one line item in a takeoff that also includes the lap allowance on the slab page and the tie wire, which follows the intersection count and not the chair count. And when you order, the chairs are bought as units while the bar is bought as whole stock lengths, so the chair figure stays a unit count while the steel figure has to survive the division by stock length on the estimator page.
Chair height is the clear cover plus half the bar diameter: height = cover + db/2. For a #5 bar at 1.5 in cover that is 1.5 + 0.3125 = 1.81 in, so you order a 2 in chair. The half-diameter term is there because the chair sits under the bar while the cover is measured to the outside of the bar. Chairs are stocked in 1 in steps, so round up to the next size.
Count grid points, not area. On a 24 ft × 20 ft slab at 4 ft on centre that is (24 ÷ 4) + 1 = 7 by (20 ÷ 4) + 1 = 6, so 42 chairs. At 3 ft it is 63. The flat rule of one per 4 sq ft gives 120 on the same slab, nearly three times as many, because it treats each chair as supporting a point of mat rather than a patch.
The slab depth and the chairs, together. The clear gap is depth − bottom cover − top cover − one bar diameter. On an 8 in slab with 1.5 in cover each way and #5 bar that is 8 − 1.5 − 1.5 − 0.625 = 4.38 in. The top chairs are what hold that gap open, so they are setting the effective depth of the top steel, not just holding steel off the base.
The top mat drops and its effective depth drops with it. On a 10 in slab the top steel should sit at 10 − 1.5 − 0.3125 = 8.19 in. One inch low and it is about 7.19 in, roughly 12 per cent less, and section capacity falls with the effective depth. The slab looks identical after the pour, which is why chair height is specified rather than left to the placer.
Normally yes, and they do a different job: bottom chairs set the bottom cover, holding the lower mat off the base or the vapour barrier, and their height is just the bottom cover plus half a bar. Where the slab is cast against earth the bottom cover is larger, so the bottom chairs end up taller than the top ones. The tall chairs that set the tier gap carry the top mat.
Three to four feet on centre is the usual grid, taken toward 3 ft or tighter where the mat is heavy, the bars are large, or the top mat will be walked on during placing. The limit is how far a bar can span between supports without sagging out of position — it is not the bar spacing of the mat, and tightening the bar spacing does not by itself call for more chairs.
All three position bar, in different directions. A chair is a vertical stand that sets the height of a mat, and it is what sets cover and tier gap. A spacer sits between parallel bars to hold the horizontal spacing when bars will not stay at drawn centres. A dobie, a small concrete block, is a bearer for getting cover under a bottom mat on ground. A double-layer slab normally needs chairs for height and, where the mat is loose, spacers for spacing.