What quantifying reinforcing steel means
Quantifying reinforcing steel (or “taking off the rebar”) means working out how much steel, in kilograms, a reinforced-concrete structure takes: footings, columns, beams, slabs, tie-columns and tie-beams. The result is almost always expressed in kilograms (kg) or metric tons, because that is how rebar is priced and paid for, even though the drawing gives the dimensions in meters.
The conversion between what you see on the drawing (bars of a given length) and what you buy (kilos of steel) is done with the weight per linear meter of each bar size. That is why a steel takeoff has two parts: measuring the length of each bar correctly (geometry) and applying the right weight for the bar size (conversion).
The quantity you get feeds the unit price analysis for the steel item and, through it, the estimate: miscounted kilos carry all the way to the final cost. That is why it pays to keep the takeoff sheet —the record of how you counted each element— so you can review it later.
Reading the reinforcement on the drawing
It all starts with interpreting the structural drawing. The reinforcement is specified with a standard notation: the number of bars, the bar size and, for repeating reinforcement, the spacing.
A callout like “4 #6” means four #6 bars. In stirrups or transverse reinforcement, something like “S#3 @ 20 cm” means #3 stirrups placed every 20 centimeters. The “#” gives the bar size in eighths of an inch: #3 is 3/8", #4 is 4/8" = 1/2", #5 is 5/8", and so on.
- Number of bars: how many bars in the layer or mat (e.g. “6 #5” = six #5 bars).
- Bar size: the “#” in eighths of an inch; it sets the diameter and therefore the weight per meter.
- Spacing: in temperature reinforcement, stirrups or mats, the distance between bars (e.g. “@ 20 cm”), from which you work out how many bars fit in the element.
- Cover: the distance from the steel to the edge of the concrete (typically 4 to 7.5 cm depending on the element); it shortens the usable length of the bar but is offset by the hooks.
Lengths: development, hooks and lap splices
The most common mistake when quantifying steel is to measure only the clear span of the element and forget the developments. The real length of a bar is almost always greater than the clear dimension of the element, for three reasons.
- Development length (anchorage): the length of bar that has to stay embedded so it works without slipping. It depends on the bar size and the f’c; a common field rule is on the order of 40 bar diameters (e.g. a #5 at 1.59 cm anchors about 64 cm).
- Hooks: the bend at the ends of the bar. A standard 90° hook adds roughly 12 bar diameters per end; a 180° hook, on the order of 4 diameters plus the semicircle. In stirrups, each closing hook adds about 10 cm as a typical figure.
- Lap splices: when a bar does not reach the full length (a commercial bar is 12 m long), it is spliced to another by overlapping them. A typical lap is on the order of 40 diameters and is added to the total length once for every splice.
Weight per bar size
Once you have the total length in linear meters for each bar size, you convert it to kilos by multiplying by the weight per meter. These are the nominal weights of the most common deformed rebar sizes:
- #2 (1/4", 6.4 mm): 0.248 kg/m
- #3 (3/8", 9.5 mm): 0.559 kg/m
- #4 (1/2", 12.7 mm): 0.996 kg/m
- #5 (5/8", 15.9 mm): 1.552 kg/m
- #6 (3/4", 19.1 mm): 2.235 kg/m
- #8 (1", 25.4 mm): 3.973 kg/m
- #10 (1 1/4", 31.8 mm): 6.225 kg/m
- #12 (1 1/2", 38.1 mm): 8.938 kg/m
Step-by-step procedure
The process is done element by element, and within each element, bar size by bar size:
- 1Identify the element and its reinforcement
Find the element on the drawing (footing, column, beam, slab) and read its reinforcement: number of bars, bar size and spacing for each layer or mat.
- 2Work out the unit length of each bar
Take the dimension of the element, subtract the cover and add the developments: hooks and anchorage length. If the bar is longer than 12 m, add the lap length for every splice.
- 3Count how many bars there are of each size
For reinforcement given by number, use the quantity shown. For reinforcement given by spacing (stirrups, mats), divide the available length by the spacing and add 1 (number of bars = length/spacing + 1).
- 4Get the linear meters per bar size
Multiply number of bars × unit length. Add up the linear meters of all the bars of the same size in that element.
- 5Convert to kilos with the weight per meter
Multiply the linear meters of each bar size by its weight per meter (table above). You get the kilos of steel of that size in the element.
- 6Add up the whole job by bar size
Repeat for each element and accumulate by bar size. The total is the steel takeoff for the job; it is best to report kilos per bar size and the grand total in metric tons.
Worked example: steel in a footing
Take an isolated footing 1.50 m × 1.50 m in plan, with a reinforcement mat of #4 bars @ 15 cm both ways. Side cover of 5 cm and a standard 90° hook at each end.
Length of each bar: the side is 1.50 m; subtracting 5 cm of cover on each side leaves 1.40 m, and adding two hooks of about 12 cm each gives ≈ 1.64 m per bar. Since it is the same both ways, each bar is 1.64 m.
Number of bars per direction: over 1.50 m with a 0.15 m spacing, 1.50 / 0.15 = 10 fit, plus 1 = 11 bars per direction. With two directions that is 22 #4 bars.
Linear meters: 22 bars × 1.64 m = 36.08 linear meters of #4 bar.
Conversion to kilos: 36.08 m × 0.996 kg/m = 35.94 kg of #4 steel in this footing. If the job has 8 identical footings, that is 8 × 35.94 ≈ 287.5 kg of #4 from footings alone, which is then added to the rest of the steel (columns, beams, slabs) for the job total.
The most common mistakes when quantifying steel
These are the ones that distort the result the most:
- Measuring only the clear span and forgetting cover, hooks, anchorage and lap splices: it underestimates the real steel.
- Mixing up the bar size and applying the wrong weight per meter (a #5 weighs 56% more than a #4).
- Not adding the 1 when dividing length by spacing in stirrups and mats (you lose one bar per side).
- Forgetting the lap splices on long bars (columns and beams that exceed the commercial 12 m).
- Not keeping a takeoff sheet of the count, which makes it impossible to review or audit the quantity later.