How to estimate rebar quantity — and why the offcut is a remainder, not a percentage
← Back to the calculatorThe step estimators get wrong is the third. The offcut is not a percentage — it is the remainder left on each stock bar after the pieces are cut. On the same 20 ft stock it runs from 0% on a 10 ft line to 40% on a 12 ft line.
A 7 ft line is the clearest case: 30% waste from 20 ft bars, 12.5% from 40 ft bars. Same steel, same job, different bar length. Work it out below.
Four steps, in this order. Each one changes the number the next one works on, so getting them out of order is how an estimate goes wrong.
Most estimating advice says to add a waste factor. The trouble is that there is no such factor: the waste is decided by arithmetic, and the arithmetic has only two inputs — your line length and the length of bar you buy.
Everything else follows from those four lines. Two jobs with identical steel can have wildly different waste, purely because of how the line length divides into the bar.
Every row below is the same bar, cut to a different line length. This is why a flat waste percentage cannot be right.
| Line length | Pieces per 20 ft bar | Offcut per bar | Offcut % |
|---|---|---|---|
| 4 ft | 5 | 0 ft | 0% |
| 5 ft | 4 | 0 ft | 0% |
| 6 ft | 3 | 2 ft | 10% |
| 7 ft | 2 | 6 ft | 30% |
| 8 ft | 2 | 4 ft | 20% |
| 9 ft | 2 | 2 ft | 10% |
| 10 ft | 2 | 0 ft | 0% |
| 12 ft | 1 | 8 ft | 40% |
| 16 ft | 1 | 4 ft | 20% |
| 18 ft | 1 | 2 ft | 10% |
| 20 ft | 1 | 0 ft | 0% |
Swipe the table sideways for more columns →
Nothing in that table is a rule of thumb — it is division. Change the stock length and every row changes.
Twenty lines, each 7 ft long. That is 140 ft of steel to place, whichever bar you buy.
| Stock bar | Pieces per bar | Bars to buy | Length bought | Offcut | Waste |
|---|---|---|---|---|---|
| 20 ft | 2 | 10 | 200 ft | 60 ft | 30.0% |
| 30 ft | 4 | 5 | 150 ft | 10 ft | 6.7% |
| 40 ft | 5 | 4 | 160 ft | 20 ft | 12.5% |
Swipe the table sideways for more columns →
Three answers between 6.7% and 30% for the same 140 ft of steel. Note that the longest bar is not automatically the best: 30 ft wins here because 7 ft divides into it more cleanly, not because it is bigger. Ask the yard which lengths it actually stocks, then run all of them.
An offcut is a piece of bar, not scrap. On the 20 ft example above each bar leaves 6 ft — and if the same job also has a 6 ft trim line, that offcut is the trim line, and the waste drops to zero without buying anything extra.
This is the part a percentage never captures. Before you accept a waste figure, look down the whole takeoff for short lines that will fit in the leftovers. It is the cheapest saving on any estimate, because it costs nothing but attention.
Work it out rather than adding a flat percentage. The offcut is what is left of each stock bar after the pieces are cut, so it depends on how your line length divides into the bar you buy. On 20 ft stock the same job wastes nothing on a 10 ft line, 10% on a 6 ft line and 30% on a 7 ft line. Count pieces per bar, take the remainder, then compare stock lengths before you order.
A flat percentage is the usual shortcut and the arithmetic shows why it is unreliable. On 20 ft stock a 4 ft line wastes nothing, a 7 ft line wastes 30% and a 12 ft line wastes 40%. The factor that decides your waste is how the line length divides into the stock length, not a number taken from a table.
Change the stock length you buy, not the design. A 7 ft line wastes 30% of a 20 ft bar and 12.5% of a 40 ft bar, because 40 ft divides into five 7 ft pieces with 5 ft left over. Ask the yard which lengths it stocks and run the remainder for each. Where the same job has a short line, a long offcut can become a piece instead of scrap.
No. A lap is steel you install. It is a real part of the finished slab, so it belongs in the order. Offcut is steel you buy and never place. Both make the order bigger, but only the offcut is waste.