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 |
Swipe the table sideways for more columns →
The cover figures along the left are the work of the concrete rebar page, which derives them from ACI 318-19 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 |
Swipe the table sideways for more columns →
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 |
Swipe the table sideways for more columns →
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% |
Swipe the table sideways for more columns →
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.
Every page says three to four feet, and none of them show the arithmetic. The reason is that the arithmetic does not support the number: a bar under its own weight barely sags at four feet, and at five it is still inside a tenth of an inch. So the rule comes from somewhere else, and knowing where is what tells you when your job can break it.
sag = 5 w L⁴ / 384 E I
With w the bar's own weight in lb per inch, L the clear span in inches, E = 29 × 10⁶ psi and I = πd⁴/64. For #4 that is w = 0.668 ÷ 12 = 0.0557 lb/in and I = 0.003068 in⁴.
| Bar | I | Sag at 3 ft | Sag at 4 ft | Sag at 5 ft |
|---|---|---|---|---|
| #3 | 0.000971 in⁴ | 0.024 in | 0.077 in | 0.188 in |
| #4 | 0.003068 in⁴ | 0.014 in | 0.043 in | 0.106 in |
| #5 | 0.007490 in⁴ | 0.009 in | 0.028 in | 0.068 in |
| #6 | 0.015532 in⁴ | 0.006 in | 0.019 in | 0.047 in |
Swipe the table sideways →
A #4 spanning four feet sags 0.043 in; at five feet, 0.106 in. A #6 at four feet sags 0.019 in. The bar does not need a chair every three feet to hold itself up — it would sit within a sixteenth of an inch of the drawn position at five. So the 3–4 ft figure is not carrying the bar's weight, and two things it is actually carrying are these:
Now the part that explains why nobody's chair quote matches anybody else's. Hercules publishes 2–3 ft in the US and one chair per square metre in the same article. Those are not the same rule. Two feet is 0.25 chairs per sq ft, three feet is 0.111, and one per square metre is 0.093 — the top of their own range is 2.7 times the bottom, and their rule of thumb sits below both. On a 480 sq ft slab that is 120 chairs, 53, or 45, from one page.
Spacing and height are one decision, because the chair that sets the height is the same object that sets the spacing. Read down the column for the member you are pouring, not across the whole table.
| Where the chair sits | Cover to bar | Height, #4 | Height, #5 | Spacing | What sets the spacing |
|---|---|---|---|---|---|
| Slab on formwork | 1.5 in | 1.75 in | 1.81 in | 3–4 ft | walked during placing |
| Slab on ground or membrane | 2 in | 2.25 in | 2.31 in | 3–4 ft | walked, plus an uneven base |
| Double-layer slab, top mat | per drawing | depth − covers − db | same | 3 ft | the tier gap must not close |
| Footing or pier base | 3 in | 3.25 in | 3.31 in | 4–5 ft | nobody walks it; single mat |
Swipe the table sideways →
Drawings and suppliers outside the US often call the same thing chair reinforcement spacing, and it is the same measurement: the distance between the supports holding a mat at a set height. The number does not change with the name — 3–4 ft for bar chairs in a slab, 4–5 ft in a footing, and the height still comes from cover plus half a bar.
What does change is the fitting. A mesh chair has a groove that traps wire at a crossing; a bar chair has a saddle sized for a specific diameter. Put a #4 in a mesh groove and it sits on the edges of the groove rather than in it, so the bar is held a little high and can roll. That is why the same spacing on a wire-mesh mat and a #4 mat calls for two different chairs, and why the count is the last thing to decide rather than the first.
A spacer is not a chair. A chair stands a mat up and sets its height; a spacer holds the other direction — it keeps a bar off the form face, or keeps two parallel bars at their drawn centres. Most searches for “rebar spacer” mean the second thing, the plastic clip or the wheel, and most pages answer with chairs. This is the other half.
| Device | What it holds | Where it sits | What sizes it |
|---|---|---|---|
| Clip-on spacer (plastic) | Side cover | Snapped on the bar, foot against the form face | Bar size and cover — stocked in ¾ to 2 in |
| Wheel or roller spacer | Clear gap between two parallel bars | Between the bars, line wire threaded through | The gap itself, from 1 in up |
| Concrete dobie | Bottom cover on ground | Under the bottom mat, on the subgrade | The cover — 2 in or 3 in |
| Slab chair or bolster | The height of the mat | Under the bar, on form or ground | cover + db/2 — the chair arithmetic |
Swipe the table sideways →
What sets spacer spacing is the same sag that sets chair spacing, and it runs the other way. A bar's own weight goes as its area, so as d², while its stiffness goes as d⁴ — which means the small bar sags most, not the big one. Read the sag table in Step 5 again with that in mind: at four feet a #3 sags 0.077 in and a #4 sags 0.043 in, so the #3 is the one that needs its spacers closest.
That is what makes four feet a floor rather than a habit. Open a #3 run to six feet and the sag is 0.39 in — outside the ¼ in that ACI 117 allows on cover, before anybody has stood on the mat. A #4 at six feet is 0.22 in and still inside it; a #5 is 0.14 in. So four feet is safe for every bar, six feet is safe from #4 up, and the small-bar, loose-spacing combination is the one that quietly eats the cover allowance.
Count them off the perimeter, not the area, because a spacer only exists where a bar meets something. A 20 ft × 40 ft slab has 120 ft of form edge; at 4 ft that is 30 spacers for the bottom mat, 60 if the top mat is also clipped to the forms. The same slab takes 66 chairs on a 4 ft grid — (20÷4 + 1) × (40÷4 + 1) — so the spacer count is under half the chair count, and it is the number nobody bothers to work out.
One material rule that is easy to get wrong: on formwork, plastic is fine, and on ground it is not. A plastic clip or wheel left in the soffit of a slab on grade becomes a void in the bottom of the slab — use a concrete dobie there, because the spacer and the slab have to be the same material. The cover table that sets the size in the first place is on the concrete cover 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.
It is the same measurement as chair spacing, under the name used on drawings and by suppliers outside the US: the distance between the supports that hold a mat at a set height. The number does not change with the name — 3–4 ft for bar chairs in a slab, 4–5 ft in a footing where there is no top mat to walk on, and the height is still cover plus half a bar. What does change is the fitting: a mesh chair has a groove for wire, a bar chair has a saddle sized to one diameter, so the same spacing on a mesh mat and a #4 mat takes two different chairs.
On a 4 ft support grid it is 0.088 chairs per sq ft — one chair per 11.4 sq ft, so a 10 ft × 10 ft bay takes 9. At 3 ft it is 0.131 per sq ft, and at 5 ft 0.052. The three numbers are the same rule read at three spacings, which is why the flat "one per 4 sq ft" figure is so far off: it is 0.25 per sq ft, nearly three times the 4 ft grid, and it corresponds to a 2 ft grid nobody uses.
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.
Count them off the form perimeter rather than the area, because a spacer only exists where a bar meets a form or another bar. A 20 ft × 40 ft slab has 120 ft of edge; at 4 ft on centre that is 30 spacers for the bottom mat, and 60 if the top mat is clipped as well. That is well under the 66 chairs the same slab takes on a 4 ft grid, which is why the two counts are never quoted together. Four feet is safe for every bar: a #3 sags 0.077 in at four feet and 0.39 in at six, and the six-foot case is already outside the ¼ in that ACI 117 allows on cover.