18 in is the ceiling, 12 in is the default, and the minimum steel decides which one you can use
← Back to the calculatorThat is the whole answer, and it is two numbers, not one: a gap you are not allowed to exceed and an area you are not allowed to fall under. On a thin slab the area bites first. On a thick slab it bites harder, because the minimum grows with thickness while the ceiling does not.
What follows is the arithmetic behind those two gates, a 4 / 5 / 6 in comparison, and a worked 20 ft by 10 ft grid carried all the way to the number of 20 ft sticks you order.
Almost every page that answers this question quotes a single number: "12 inches on center for a 4 inch slab", or "18 inches maximum". Both are real, and neither is the rule. Spacing on a slab is bounded on both sides, and the two bounds come from different clauses.
The ceiling is the shrinkage and temperature spacing limit: the lesser of five times the slab thickness and 18 in. At 4 in thick that is the lesser of 20 in and 18 in, which is 18 in. The ceiling is about crack control — how far the concrete can be trusted to go between bars.
The floor is the minimum area of shrinkage and temperature steel: 0.0018 times the gross concrete section per foot of width for Grade 60 deformed bar. On a 4 in slab that is 0.0018 × 4 in × 12 in = 0.0864 sq in per foot. The floor is about having enough steel in the section at all, whatever the gap.
A layout has to satisfy both. That is why the answer is a band and not a number, and why the band closes up as the slab gets thicker.
Five combinations, each scored against both gates. provided = area × 12 / gap, so a #3 (0.11 sq in) at 12 in gives 0.11, and the same bar at 18 in gives 0.0733.
| Layout | Provided sq in/ft | Minimum 0.0864 | Gap ceiling 18 in | Verdict |
|---|---|---|---|---|
| #3 @ 12 in | 0.110 | pass +27% | pass | use it |
| #3 @ 18 in | 0.0733 | fail −15.1% | pass | short of steel |
| #4 @ 18 in | 0.1333 | pass +54% | pass | use it |
| #4 @ 24 in | 0.100 | pass +16% | fail, over 18 in | gap too wide |
| #4 @ 12 in | 0.200 | pass +132% | pass | use it |
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Read rows two and four together, because they are the two ways to be wrong. #3 at 18 in is inside the ceiling and under the minimum: the concrete is not cracking too far between bars, there is simply not enough steel. #4 at 24 in is the mirror: plenty of steel, spread too far apart. One fails on area, the other on gap, and a page that only quotes "18 in maximum" cannot tell them apart.
The ceiling stops moving as soon as the slab reaches 4 in, because 5 × 4 in already exceeds 18 in and the 18 in leg of min(5h, 18) takes over. The minimum keeps moving, because it is proportional to thickness. That is the whole difference between the three slabs below.
| Slab | Ceiling min(5h, 18) | Minimum 0.0018 x h x 12 | #3 @ 12 in | #4 @ 18 in | What changes |
|---|---|---|---|---|---|
| 4 in | 18 in | 0.0864 | 0.110, pass | 0.1333, pass | #3 at 12 in is enough |
| 5 in | 18 in | 0.108 | 0.110, pass by 1.9% | 0.1333, pass | #3 at 12 in still clears, barely |
| 6 in | 18 in | 0.1296 | 0.110, fail −15.1% | 0.1333, pass by 2.9% | #3 at 12 in stops working |
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Three things fall out of that table that a single "4 inch slab" answer hides.
The gates tell you which layouts are legal. They do not tell you how many bars that is, or what you order. This example does, in both directions, so every number on the rest of the page can be checked by hand.
Three numbers, and they are all different: 385.0 ft is the steel in the slab, 400.0 ft is what you buy, and 15.0 ft is the difference. The 15 ft is not waste in the abstract; it is 10 half-foot tails from the long bars plus 10 ft left over from the short ones, because 9.5 ft does not divide 20 ft.
| 20 by 10 ft, #4 @ 12 in | Piece count | Linear feet | Weight | What it is |
|---|---|---|---|---|
| Long bars, 19.5 ft | 10 | 195.0 ft | 130.26 lb | Placed |
| Short bars, 9.5 ft | 20 | 190.0 ft | 126.92 lb | Placed |
| Steel in the slab | 30 | 385.0 ft | 257.18 lb | Net |
| Offcut, long 0.5 ft tails | 10 | 5.0 ft | 3.34 lb | Scrapped |
| Offcut, short-bar remainder | — | 10.0 ft | 6.68 lb | Scrapped |
| Order, 20 sticks | — | 400.0 ft | 267.2 lb | Bought |
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The last two rows are the reason this page carries a piece count at all. A single total of "385 ft" is a good quantity and a bad order: no counter sells 385 ft, and the only way to reach it is to know that 19.5 ft comes one per stick and 9.5 ft comes two per stick. That distinction — one piece versus two per 20 ft — is worth 15 ft here and more on a bigger slab.
Same slab, same 4 in thickness, three layouts. This is where the spacing decision turns into money, and where the "just go to 18 in and save bar" instinct meets the minimum-steel gate.
| Layout | Bars | Placed | Placed weight | Sticks | Order weight | vs #4 @ 12 |
|---|---|---|---|---|---|---|
| #4 @ 12 in | 10 + 20 | 385.0 ft | 257.18 lb | 20 | 267.2 lb | — |
| #4 @ 18 in | 7 + 14 | 269.5 ft | 180.03 lb | 14 | 187.04 lb | −30.0% |
| #3 @ 12 in | 10 + 20 | 385.0 ft | 144.76 lb | 20 | 150.4 lb | −43.7% |
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Both cheaper rows are legal on a 4 in slab, which is the part people miss. #4 at 18 in passes both gates and cuts placed weight by 30.0%: 7 long bars and 14 short, 269.5 ft, 180.03 lb. #3 at 12 in keeps the identical grid — same 385 ft, same 20 sticks — and cuts weight 43.7% purely by changing the bar, because 385 ft of #3 is 144.76 lb against 257.18 lb of #4.
Those two savings are not the same kind of saving, and the difference is why the piece count matters. #4 at 18 in saves steel and sticks: 14 sticks instead of 20, 280 ft instead of 400 ft, because 14 bars cut at 9.5 ft pack two per stick and 7 long bars take one each, leaving 10.5 ft of offcut. #3 at 12 in saves weight but not sticks: the grid is unchanged, so it still needs 20 sticks, and the offcut is still 15 ft. A lighter bar with the same geometry is cheaper steel and identical cutting.
There is a shortcut that estimators use on slabs, and it is worth knowing both because it is fast and because it produces a number that looks like a different number.
400.0 ft. Which is exactly the quantity of stock bought in the worked example above, to the foot — and that is a coincidence of this slab, not a rule. Two numbers with the same value and different meanings: 400.0 ft is a purchase (20 sticks of 20 ft), while 400.0 ft here is an area estimate of the steel, which does not know anything about stock lengths at all.
The gap between the two is easiest to see on the layout that has no offcut at all: #4 at 18 in buys 280 ft for 269.5 ft placed, 10.5 ft of offcut on 14 sticks, 3.9% too — but for a different reason, the long bars' 0.5 ft tails plus the short bars' remainder. Percentages matching by accident is exactly why a percentage is the wrong mental model; the pieces are the model.
For a 4 inch slab the code ceiling on shrinkage and temperature bar spacing is 18 in, the lesser of 5 times the thickness or 18 in, and 12 in is the common practical default. The floor is set by minimum steel: 0.0018 times the gross section for Grade 60 deformed bar, which on a 4 in slab is 0.0864 sq in per foot of width. #3 at 12 in gives 0.110 and clears it; #3 at 18 in gives 0.0733 and does not.
No. #3 at 18 in provides 0.0733 sq in per foot against a 0.0864 minimum, so it is 15.1% short even though 18 in is inside the spacing ceiling. Passing the spacing limit is necessary but not sufficient. #3 at 12 in gives 0.110 and passes both gates.
No, the opposite. The ceiling min(5h, 18) is already 18 in at 4 in thick, so it stays 18 in for 5 in and 6 in slabs, while the minimum steel rises with thickness: 0.0864, 0.108 and 0.1296 sq in per foot. A thicker slab needs more steel per foot, not less, and #3 at 12 in passes 4 in and 5 in slabs but fails a 6 in slab by 15.1%.
Not as shrinkage and temperature reinforcement. #4 at 24 in gives 0.100 sq in per foot, which clears the 0.0864 minimum, but 24 in is past the 18 in ceiling, so it fails the spacing gate. Both gates have to pass: the provided area and the gap.
In #4 at 12 in on center, with the outer bar centerline 3 in in from each edge, the grid is 10 bars at 19.5 ft and 20 bars at 9.5 ft: 385.0 ft placed, 257.18 lb. Ordered from 20 ft stock that is 20 sticks, 400.0 ft, 267.2 lb, with 15.0 ft of offcut — about 3.9% over the steel in the slab.
No. It shows the code floor and ceiling and works the arithmetic of a layout, so you can check a drawing or a quote. The spacing your slab requires comes from the structural drawings and the engineer; the 20 by 10 ft slab here is an example input, not a design.