What a 20×20 slab actually takes to buy — and why it is the exception
← Back to the calculatorA 20×20 slab is the one size where the bar almost fills the stock. Each bar is 19.5 ft — just 6 in shorter than a 20 ft stick — so the offcut is 0.5 ft per bar, the lowest of any slab size from 10×10 to 30×40. The "add 10%" rule that works for a typical slab over-orders this one by 4.0 times.
Three numbers, three names, and none of them borrow each other's. Placed steel is the exact feet of bar in the concrete. Area method is the estimate from the formula, and on this slab it lands 2.6% high — that is the 15 ft of cover it never subtracts. Buy steel is the whole bars you order, also 2.6% high but for the opposite reason: the same 15 ft comes back as offcut. They agree in total here because 19.5 ft against 20 ft stock makes the cover trim and the offcut the same length — a coincidence of this size, not a licence to use either. The gap between 585 ft and 600 ft is 15 ft you pay for and do not pour.
Every slab takeoff has three numbers, and the 20×20 is the cleanest case to see them side by side, because the gaps between them are small enough to measure precisely.
| Placed steel | Area method | Buy steel | |
|---|---|---|---|
| Linear feet | 585.0 ft | 600.0 ft | 600.0 ft |
| Weight, #4 | 390.78 lb | 400.80 lb | 400.80 lb |
| How far off | — | +2.6% | +2.6% |
| Can order from it? | No — steel, not a product | No — estimate only | Yes — count sticks |
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Placed steel is the exact total: 30 bars, each 19.5 ft, which is the net span after 3 in of cover comes off each edge. Area method is 2 × area ÷ spacing = 2 × 400 ÷ 1.333 = 600 ft, and it is high for one reason only: it counts every bar as the full 20 ft slab dimension. Thirty bars × 0.5 ft of cover is the whole 15 ft between 585 and 600 — there is no +1 anywhere in this gap. Buy steel is 30 whole 20 ft bars = 600 ft, because every bar fits one stick with 0.5 ft to spare. None of the three borrow each other's name: the placed steel is what ends up in the concrete, the area method is an estimate, and the buy steel is what leaves the yard.
Run the offcut percentage across slab sizes from 10×10 to 30×40, all at 16 in on centre, #4 bar, 3 in cover, 20 ft stock, and one number stands out:
| Slab size | Bars | Placed ft | Buy ft | Offcut ft | Offcut % | Cost |
|---|---|---|---|---|---|---|
| 10×10 | 16 | 152.0 | 160.0 | 8.0 | 5.3% | $128 |
| 12×12 | 18 | 207.0 | 360.0 | 153.0 | 73.9% | $288 |
| 16×16 | 24 | 372.0 | 480.0 | 108.0 | 29.0% | $384 |
| 20×20 | 30 | 585.0 | 600.0 | 15.0 | 2.6% | $479 |
| 24×24 | 36 | 846.0 | 1440.0 | 594.0 | 70.2% | $1,151 |
| 20×30 | 38 | 891.0 | 1060.0 | 169.0 | 19.0% | $847 |
| 30×40 | 53 | 1793.5 | 2120.0 | 326.5 | 18.2% | $1,694 |
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Two sizes come out cheap and the middle of the table is where the money goes. The 20×20 at 2.6% is the lowest, and the 10×10 at 5.3% is second — but they get there by opposite routes. The 20×20 nearly fills a stick with one bar (19.5 of 20 ft). The 10×10 fills a stick with two bars (9.5 × 2 = 19.0 of 20 ft, 1.0 ft spare). Everything between them fails both tests: a 16×16 bar is 15.5 ft, too long to pair up and 4.5 ft short of filling a stick, so 29.0% of what you buy never goes in the slab. A 12×12 is the worst of the small sizes at 73.9%, and a 24×24 sits at 70.2% for a different reason — its bar is 23.5 ft, so even on 40 ft stock one stick yields one piece and gives back 16.5 ft.
Here is the arithmetic that every other number on this page follows from. The slab is 20 ft square. Cover is 3 in at each edge, so the clear span in each direction is:
Each bar is 19.5 ft long. The stock bar is 20 ft. The offcut per bar is:
Thirty bars, each leaving that 6 in. The offcut as a share of the steel that goes in the slab:
That is the whole story. Not the spacing, not the bar count, not the slab area — the 6 in of cover at each edge turns a 20 ft slab into a 19.5 ft bar, and 19.5 ft is the closest any common slab size gets to filling a 20 ft stick with a single piece. Change the slab to 24×24 and the bar becomes 23.5 ft: past the 20 ft stick, so it moves to 40 ft stock and still takes a whole stick for one piece, giving back 16.5 ft — 70.2%. Change it to 16×16 and the bar is 15.5 ft: too long to cut two from one stick, 4.5 ft short of filling it, so 29.0% of the purchase is offcut. Change it to 10×10 and the bar is 9.5 ft, which pairs up two to a stick and lands at 5.3%. Only 19.5 ft both fills the stick and stays a single piece.
The rule that gets repeated on most rebar material pages — including several of the top-ranking pages for "rebar calculator" — is "add 10% for waste". On a typical slab the real offcut runs 10% to 30%, so 10% is in the ballpark. On a 20×20 at 16 in it is not. Follow the method all the way to what you would actually order, because 10% of a number is not a number of sticks:
| Method | Order ft | Offcut vs 585.0 ft placed | Extra vs the correct order |
|---|---|---|---|
| Count whole bars (30 × 20 ft) | 600.0 ft | 15.0 ft (2.6%) | — |
| 585.0 + 10% = 643.5, round to 33 sticks | 660.0 ft | 75.0 ft (12.8%) | +60.0 ft (4.0×) |
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Take the first row, because it is the method most people follow. Ten percent of 585.0 ft is 643.5 ft, and 643.5 ft cannot be bought — bars come in 20 ft lengths, so it becomes 33 sticks, 660.0 ft. The correct order is 30 sticks, 600.0 ft. The difference is 60.0 ft, or 40.08 lb, or $47.94 at $0.799 a foot — and 60.0 ÷ 15.0 is 4.0. That one ratio is the whole argument: the rule does not add a safety margin on top of the real offcut, it multiplies it.
Run the same rule on the area-method figure instead and it lands in exactly the same place: 600.0 + 10% = 660.0 ft, which is 33 sticks again. That is not a second opinion — on this slab the area method already equals the stock-bar total, so both routes multiply the same 15 ft. Thirty-three sticks either way, and neither number was ever a count of bars.
Change the spacing on a 20×20 slab and the bar count moves, the placed steel moves, the cost moves — but the offcut percentage stays at 2.6%. That is because every bar is still 19.5 ft long, regardless of how many bars there are.
| Spacing | Bars | Placed ft | Buy ft | Offcut ft | Offcut % | Cost | Area vs placed |
|---|---|---|---|---|---|---|---|
| 12 in | 40 | 780.0 | 800.0 | 20.0 | 2.6% | $639 | +2.6% |
| 16 in | 30 | 585.0 | 600.0 | 15.0 | 2.6% | $479 | +2.6% |
| 18 in | 28 | 546.0 | 560.0 | 14.0 | 2.6% | $447 | −2.3% |
| 24 in | 20 | 390.0 | 400.0 | 10.0 | 2.6% | $320 | +2.6% |
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Every row in that table has the same offcut percentage because the offcut is set by one thing — 20 ft stock minus 19.5 ft bar — and that does not change with spacing. What does change is the total steel: going from 24 in to 12 in nearly doubles the bar count (20 to 40) and the cost ($319.60 to $639.20). The 16 in spacing is the residential default and sits in the middle at $479.40. The spacing choice is about how much steel you put in the slab, not about how much you waste buying it.
The size table from the answer block, expanded with the bar length, the stock each size is cut from, how many pieces one stick yields, and the stick count that follows. Each row uses the shortest stock that holds the bar in one piece, so no row carries a splice.
| Slab | Bar length | Stock cut from | Bars | Pieces per stick | Sticks | Buy ft | Offcut % |
|---|---|---|---|---|---|---|---|
| 10×10 | 9.5 ft | 20 ft | 16 | 2 | 8 | 160.0 | 5.3% |
| 12×12 | 11.5 ft | 20 ft | 18 | 1 | 18 | 360.0 | 73.9% |
| 16×16 | 15.5 ft | 20 ft | 24 | 1 | 24 | 480.0 | 29.0% |
| 20×20 | 19.5 ft | 20 ft | 30 | 1 | 30 | 600.0 | 2.6% |
| 24×24 | 23.5 ft | 40 ft | 36 | 1 | 36 | 1440.0 | 70.2% |
| 20×30 | 19.5 / 29.5 ft | 20 / 40 ft | 38 | 1 / 1 | 23 + 15 | 1060.0 | 19.0% |
| 30×40 | 29.5 / 39.5 ft | 40 ft | 53 | 1 | 30 + 23 | 2120.0 | 18.2% |
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Read the stick column rather than the bar column, because that is what you pay for. The 20×20 buys 30 sticks and puts 585 of 600 ft in the concrete. The 16×16 buys 24 sticks and puts only 372 of 480 ft in — it uses less steel overall but wastes a quarter of what it buys. And the 24×24 buys 36 sticks of 40 ft to place 846 ft, the worst ratio on the page: each bar is 23.5 ft, so a 40 ft stick yields exactly one piece and gives back 16.5 ft.
Switch from 20 ft to 40 ft stock bars and the 20×20 slab does not change its waste. Each 40 ft stick yields two 19.5 ft bars (2 × 19.5 = 39.0 ft), with 1.0 ft left over:
| Stock | Pieces per stick | Sticks | Buy ft | Offcut ft | Offcut % | Est. cost |
|---|---|---|---|---|---|---|
| 20 ft | 1 | 30 | 600.0 | 15.0 | 2.6% | $479.40 ($15.98/stick) |
| 40 ft | 2 | 15 | 600.0 | 15.0 | 2.6% | $479.40 (~$32/stick) |
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The waste is identical because 19.5 ft fits 20 ft stock almost exactly, and it fits 40 ft stock almost exactly twice. What 40 ft stock buys you is not less waste — it is fewer sticks to transport, handle and cut: 15 instead of 30. Whether that is worth it depends on the yard's stock and the truck, not on the arithmetic. The rebar length page covers the full menu of stock sizes and what each does to an order.
The area method is the shortcut that most online calculators and material pages use: total slab area times two (for both directions), divided by the spacing. For a 20×20 at 16 in:
The actual placed steel is 585.0 ft. The gap is 15.0 ft, which is 2.6%, and it has one cause, not two:
floor(clear ÷ spacing) + 1 = 15 bars each way. The area formula implies the same 30 bars here (600 ÷ 20 = 30), so the two agree on how many bars the job takes and differ only on how long each one is.Fifteen feet, all of it cover. That also explains why the error changes sign with slab size instead of staying a fixed percentage: on a 10×10 the area method comes out low (150.0 ft against 152.0 ft placed), on this slab it is 2.6% high, and on a 24×24 at 24 in it is 2.1% high. One rule of thumb cannot cover a number that moves in both directions.
floor, not ceil — and write the layout down. Clear span here is 234 in, and 234 ÷ 16 = 14.625. The layout is what settles it: first bar inside the cover, then one every 16 in. That is 14 spaces at exactly 16 in, 15 bars, and the leftover 10 in falls at the far edge as a short closing space (with the 3 in cover behind it). No space between bars is wider than 16 in, which is all the spacing rule asks. Round the division up instead and you get 16 bars on a 15.6 in grid: two extra bars, 39 ft of steel you did not need, and the same over-count in every table above.A 20×20 slab with #4 bar at 16 in on centre both ways, 3 in of cover on each edge, takes 30 bars (15 each way), each 19.5 ft long. The placed steel is 585.0 ft / 390.78 lb. On 20 ft stock you buy 30 whole bars = 600.0 ft / 400.80 lb, and the offcut is 15.0 ft, which is 2.6%. The cost at $15.98 per 20 ft stick is $479.40.
Because the net span is 19.5 ft — only 6 in shorter than the 20 ft stock bar. Every bar leaves just 0.5 ft of offcut, so the waste is 2.6%. The only size that comes close is a 10×10, and it gets there a different way: its bar is 9.5 ft, so two bars share one stick (9.5 × 2 = 19.0 ft, 1.0 ft spare) and 18 bars need 9 sticks = 180 ft, which is 5.3% offcut. In between, the ratio collapses — a 16×16 bar is 15.5 ft, too long to pair and 4.5 ft short of filling a stick, so 29.0% is offcut. 19.5 ft is the only length that both fills the stick and stays one piece.
No. Adding 10% to the 585.0 ft placed steel gives 643.5 ft, and that has to become whole bars: 33 sticks of 20 ft is 660.0 ft, against the correct order of 30 sticks, 600.0 ft. So the rule buys 60.0 ft of extra bar — 40.08 lb, $47.94 — which is 4.0 times the real 15.0 ft offcut, not 10% of it. Work the order in whole stock bars instead: the 2.6% offcut is already inside the 30-bar count, and a handling allowance of a stick or two for cutting error is all that sits on top.
The area method (2 × area ÷ spacing) gives 600.0 ft for a 20×20 slab at 16 in OC, which is 2.6% above the actual 585.0 ft of placed steel. It is high for one reason: the formula counts each bar at the full 20 ft slab dimension, while the real bar is 19.5 ft because 3 in of cover comes off each end. Thirty bars × 0.5 ft is the whole 15.0 ft gap, and there is no +1 in it — the count is floor(clear ÷ spacing) + 1 = 15 bars each way whichever way you reason it. The area method tells you the order of magnitude; it cannot tell you how many bars to buy.
No. On 40 ft stock you get 2 bars per stick (2 × 19.5 = 39.0 ft, with 1.0 ft left), so you need 15 sticks. The buy total is still 600.0 ft and the offcut is still 15.0 ft = 2.6%. The waste does not change because 19.5 ft fits 20 ft stock almost exactly, and it fits 40 ft stock almost exactly twice. What 40 ft stock does save is the number of sticks to handle: 15 instead of 30.
At 12 in on centre the bar count goes up to 40 (20 each way), the placed steel is 780.0 ft, the buy steel is 800.0 ft, and the offcut is 20.0 ft — still 2.6%. The offcut percentage does not change with spacing, because every bar is still 19.5 ft long and leaves 0.5 ft per stick. What changes is the total steel: 12 in spacing adds 195 ft of placed steel and $159.80 to the cost compared to 16 in.