30 sticks, $479.40, 15 ft of offcut — and why the waste cannot be zero
← Back to the calculatorThree numbers, three names: 585.0 ft is the steel in the slab, 600.0 ft is what you buy, and 15.0 ft is the piece of the purchase you cannot use. The last one has a floor on this slab, and the reason is arithmetic, not sloppiness: 19.5 ft of line does not divide 20 ft of stick, and 39 ft of two lines does not fit one either.
The takeoff first, because the order is built on it. One layer, #4 both ways, maximum spacing 16 in on center, and the outer bar's centerline 3 in in from each edge — the cover a slab cast against the ground carries. The clear span is 240 − 2 × 3 = 234 in, and every line in the grid is that long: 19.5 ft.
| 20 × 20 ft, #4 @ 16 in | Piece count | Linear feet | Weight | What it is |
|---|---|---|---|---|
| Bars each way, 19.5 ft | 15 + 15 | 585.0 ft | 390.78 lb | Placed |
| Half-foot tails | 30 | 15.0 ft | 10.02 lb | Scrapped |
| Order, 20 ft sticks | 30 | 600.0 ft | 400.8 lb | Bought |
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One detail of the layout is worth pausing on. Fifteen bars at 16 in start 3 in from one edge and leave a closing gap of 10 in at the far edge. Many crews would split that difference and run 8 in of edge on both sides. That choice changes the placed steel — the lines shorten to 18.67 ft, 560.0 ft in the slab, and the offcut share of the same order rises to +7.1% — and it changes nothing about the buy: 30 sticks either way, $479.40 either way. The order does not care where the odd 10 in goes; the placed number does. That is exactly why the two are named separately.
Quick estimates floating around this slab size sometimes promise a cleaner outcome — 22 sticks, no offcut, around $352. Both numbers come from the same slip: counting the bars as if they ran the full 20 ft with nothing deducted at the edges. Eleven or so lines per direction only covers a 20 ft slab if the gap between bars averages 24 in — which is not a 16 in layout — and a line of exactly 20.0 ft exists only if the bar starts at the form line with zero cover. No buildable slab does that, and a 16 in layout cannot be 22 lines on any reading.
The real geometry is less forgiving. With 3 in of edge, every line is 19.5 ft, and 19.5 ft sits in the worst possible slot of a 20 ft stick:
Same slab, four legal-or-not layouts. The two gates from the spacing page apply: #4 at any of these gaps provides at least 0.1000 sq in per foot against the 0.0864 minimum, so the area gate passes everywhere — the ceiling is what separates the rows. On a 4 in slab the ceiling is 18 in, and 24 in fails it.
| Layout | Bars each way | Placed | Sticks | Order | Offcut | Cost | Per sq ft | Gates |
|---|---|---|---|---|---|---|---|---|
| #4 @ 12 in | 20 + 20 | 780.0 ft / 521.04 lb | 40 | 800.0 ft / 534.4 lb | 20.0 ft | $639.20 | $1.60 | pass / pass |
| #4 @ 16 in | 15 + 15 | 585.0 ft / 390.78 lb | 30 | 600.0 ft / 400.8 lb | 15.0 ft | $479.40 | $1.20 | pass / pass |
| #4 @ 18 in | 14 + 14 | 546.0 ft / 364.73 lb | 28 | 560.0 ft / 374.1 lb | 14.0 ft | $447.44 | $1.12 | pass / at ceiling |
| #4 @ 24 in | 10 + 10 | 390.0 ft / 260.52 lb | 20 | 400.0 ft / 267.2 lb | 10.0 ft | $319.60 | $0.80 | gap fails, 24 > 18 |
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Read the cheapest row with care. 24 in saves $159.80 against 16 in — and on a 4 in slab it is not a legal shrinkage and temperature layout, because 24 in is past the 18 in ceiling. The row is here because the price is exactly why people end up asking about it; the ceiling is the answer. Between the legal rows the money moves the way it always does: 12 in to 16 in is $159.80, and 16 in to 18 in is another $31.96 — each widening buys fewer bars at the same stick price.
Notice the offcut column barely moves in ratio: 20.0, 15.0, 14.0, 10.0 ft are all just half a foot times the stick count, and the order-over-placed percentage is +2.6% on every row. What the spacing decision buys you is fewer sticks, not a cleaner cut.
The shortcut every estimator knows: two feet of bar per square foot per foot of spacing.
Now put those beside the orders in the table above. At 12 in, 16 in and 24 in, the area rule reproduces the purchase to the foot: 800.00 against 800.0, 600.00 against 600.0, 400.00 against 400.0. At 18 in it does not: 533.33 estimated against 560.0 bought, and 533.33 is also 2.3% under the 546.0 ft actually placed — the only row where the shortcut under-counts.
The other lever people reach for is the stick length. Here it does nothing. A 40 ft stick yields two 19.5 ft lines with a 1 ft tail, so fifteen sticks cover the same thirty lines:
600.0 ft bought, 15.0 ft offcut, zero splices — the same +2.6%, in the same 1 ft tails instead of 0.5 ft ones. The steel quantity is decided by the grid and the cutting, not by which stick the yards happen to stock. What 40 ft stock changes is the price per foot, and this page carries no 40 ft price, so no dollar figure is claimed here: whichever stick is cheaper per foot on the day wins, and the arithmetic above holds for either.
In #4 at 16 in on center, one layer both ways, with the outer bar centerline 3 in from each edge, a 20 ft × 20 ft slab takes 15 bars each way: 30 bars at 19.5 ft, which is 585.0 ft of bar and 390.78 lb. At 12 in on center the same slab takes 40 bars, 780.0 ft, 521.04 lb.
At 16 in on center the order is 30 sticks of 20 ft bar. At the $15.98 per stick shelf price observed on 2026-11-07 that is $479.40, about $1.20 per square foot, material only. At 12 in on center it is 40 sticks, $639.20. Prices move; the date on the observation is part of the number.
Not with 20 ft stock. Every bar line is 19.5 ft, one stick per line, so the offcut is 30 half-foot tails: 15.0 ft, 2.6% over the placed steel. Zero offcut would need lines of exactly 20.0 ft, which means bars starting at the form edge with zero cover, and no buildable slab does that.
For a 4 in slab as shrinkage and temperature steel, yes. #4 at 16 in provides 0.1500 sq in per foot against the 0.0864 minimum, and the 16 in gap is inside the 18 in ceiling. 12 in and 18 in also pass; 24 in provides enough area but fails the 18 in ceiling on a 4 in slab.
The steel comes out identical. A 40 ft stick yields two 19.5 ft lines with a 1 ft tail, so 15 sticks cover the same 30 lines: 600.0 ft bought, 15.0 ft offcut, no splices, the same +2.6%. Which stock wins is decided by the per-foot price, and this page prices only the 20 ft stick, so no dollar figure is claimed for 40 ft.
No. The 20 by 20 ft slab, the 16 in spacing, the 3 in edge and the shelf price are example inputs chosen so the arithmetic can be checked by hand. Your spacing, bar size and layer count come from the drawings and the engineer; use the method to check a quote, not to replace one.