Rebar Explained: Reinforcement Steel Sizes, Weight, and Spacing

Concrete is strong in compression and weak in tension. Reinforcement steel — rebar — carries the tension, and its weight is the quantity every estimate, order, and delivery starts from. The weight formula is one division: square the bar diameter in millimetres and divide by 162 to get kilograms per metre. This guide covers the bar sizes, the formula and its approximation, how to read a bar schedule, spacing and cover in plain words, and the hard limit: a weight calculator does not design reinforcement, and this guide does not either.

What reinforcement steel is, and why concrete needs it

Rebar is hot-rolled steel bar, usually with ribs — deformations — rolled into its surface so it bonds with the concrete around it. The ribs are the point: smooth bar would slip inside concrete under load, and ribbed bar grips it, so the two materials act together. The steel is normally grade 60 in the United States (60,000 psi yield strength) or the equivalent grade in other codes, and it is supplied in standard diameters and in lengths, commonly 6 m or 12 m, or cut and bent to a schedule.

Concrete without reinforcement cracks under tension: a beam loaded from above stretches at its underside, and plain concrete cannot carry that stretch. Steel carries it. The reinforcement is placed where the tension is — the bottom of a simply supported beam, the top over a support, the faces of a wall in bending — and the concrete protects the steel from fire and corrosion while carrying the compression. The two materials’ thermal expansion is close enough that they move together with temperature, which is a fortunate accident of chemistry that the whole of reinforced concrete depends on.

The vocabulary matters because the schedule uses it. A bar mark identifies each distinct bar type on a drawing. A schedule lists the mark, the size, the shape, the dimensions, and the quantity. Laps are overlaps where two bars join to continue a length; hooks are bent ends that anchor a bar in the concrete. Cover is the concrete thickness between the bar’s surface and the outside face, protecting the steel. None of these is a weight concept, but all of them determine how much steel a job actually needs, which is why the weight is only the start of an estimate.

Bar sizes and weights: the table, and the formula behind it

US bars are numbered by eighths of an inch of diameter: #3 is 3/8 in (9.5 mm), #4 is 4/8 in (12.7 mm), #5 is 5/8 in (15.9 mm), up to #11 at 1.41 in (35.8 mm), with #14 and #18 larger still. Metric practice names the bar by its diameter: 10 mm, 12 mm, 16 mm, 20 mm, 25 mm, 32 mm. The two systems overlap approximately — #4 (12.7 mm) sits near a 12 mm metric bar but is not the same bar — and they should not be substituted for each other on a design without the engineer’s say-so. The table below gives the standard weights; the calculator applies the same figures.

The formula: weight per metre in kg/m = D² ÷ 162.2, usually written ÷ 162, with D in millimetres. It comes from steel’s density (7,850 kg/m³) and the bar’s cross-sectional area (πD²/4): weight = 7,850 × π × D² ÷ 4 ÷ 1,000,000 kg per metre of bar, which simplifies to D² ÷ 162.2. In imperial units the equivalent is weight in lb/ft = (diameter in inches)² × 2.67, or by bar number, (bar number² ÷ 24) approximately — #4 gives 0.668 lb/ft, #8 gives 2.670 lb/ft. The ÷ 162 form is an approximation, accurate to about 1%, because real bars vary slightly in diameter and the ribs add a small amount of steel. For ordering, the approximation is fine; for a certified weight, the mill certificate governs.

Worked example from the calculator: a 12.7 mm bar gives 12.7² ÷ 162 = 0.996 kg per metre. Ten bars at 6 metres each: 0.996 × 6 × 10 = 59.74 kg. A custom 20 mm bar: 20² ÷ 162 = 2.469 kg per metre, 14.81 kg for a 6-metre bar, 148.15 kg for 10 bars. The same steps in reverse give a check: total weight divided by weight per metre gives total length, and total length divided by bar length gives the bar count. Any schedule whose weight, length, and count do not reconcile has an error in one of the three.

Standard bar weights (ASTM A615 nominal diameters and unit weights, rounded).

From per-metre to per-bar to total

An estimate runs in three multiplications. Weight per metre from the diameter. Weight per bar from the length: per-metre weight × bar length. Total weight from the count: per-bar weight × number of bars. Bigger bars and longer bars are two ways to the same total — doubling the diameter quadruples the weight per metre (diameter squared), while doubling the length only doubles it — which is why a schedule change from #4 to #5 bars raises the tonnage far more than a length change of the same proportion. Two different jobs can weigh the same for entirely different reasons, and the weight alone says nothing about which job is which.

Waste and laps belong in the order, not in the arithmetic. Cutting produces offcuts; laps add length that the drawing’s net lengths do not show; hooks add bends that consume bar. The calculator reports the net weight from the lengths and counts entered. The order quantity should add the project’s waste and lap allowance on top — a percentage set by the estimator, not by the calculator — and the delivery should be checked against the schedule by weight, length, and count together. A delivery that matches on weight but not on count has the wrong bars in it.

Custom diameters use the same formula: enter the diameter directly, and the calculator applies D² ÷ 162 to it. A 20 mm custom bar and a #6 bar (19.1 mm) are close but not identical, and the difference — 2.47 against 2.24 kg/m — is 10% of the weight, which is the difference between a correct order and a short one. Enter the actual diameter, not the nearest named size, whenever the bar is non-standard.

Spacing, cover, and reading a bar schedule

Spacing is the centre-to-centre distance between parallel bars, set by the design: closer spacing with smaller bars and wider spacing with larger bars can give the same steel area per metre of width, with different crack behaviour. The steel area per metre is what the design specifies — for example, 10 mm bars at 150 mm spacing give 523 mm² per metre — and the bar size and spacing are chosen together to meet it while keeping the bars placeable and the concrete pourable. Spacing that is too tight traps aggregate and leaves voids; spacing that is too wide leaves wide cracks. The balance is the designer’s, and it is not visible from the weight.

Cover is the concrete between the bar and the face, and it protects the steel from corrosion and fire. It is set by the exposure — more cover for exterior and ground-contact concrete, less for interior — and it is maintained on site with chairs and spacers. Too little cover and the steel corrodes, expands, and spalls the concrete; too much cover and the bar sits too deep to carry the tension it was placed for. Cover is measured to the bar’s outer surface, including its ribs, and the schedule and drawings state it explicitly. A weight calculator has no opinion on cover, and an order has no business changing it.

Reading a schedule: find the bar mark, read the size and shape, check the dimensions against the drawing, and multiply out the quantity. Shapes are coded — straight, L-bent, U-bent, stirrup — and the bending dimensions are given to the outside of the bends unless the code says otherwise. Check the total by the reconciliation above: weight, length, and count must agree. If they do not, the schedule, not the arithmetic, is wrong, and the fabricator should be asked before cutting starts. Cutting steel is easy; uncutting it is not.

Stirrups and ties: the other steel in the schedule

A beam’s schedule lists more than its main bars. Stirrups — closed loops of smaller bar, usually #3 or #4, that wrap around the main bars — carry the shear: the vertical sliding force that tries to cut the beam near its supports. They are spaced along the beam, closer together near the supports where shear is highest and wider apart toward midspan where it is lowest, and their spacing is set by the design, like everything else. A beam without stirrups, or with stirrups too widely spaced, fails suddenly in shear, without the warning cracks that bending gives — which is why stirrup spacing is inspected as carefully as bar size.

Ties do the corresponding job in columns: they wrap the vertical bars, hold them in position during the pour, prevent them from buckling outward under load, and confine the concrete core so it carries more. A tied column and a spirally reinforced column differ in how the confinement is provided — ties are individual loops, a spiral is a continuous helix — and the spiral confines more effectively, which is why spirally reinforced columns are credited with higher capacity in the codes. The schedule distinguishes them by shape and spacing, and the weight calculation treats them the same way: length per piece, times the count, times the weight per metre for the diameter.

Chairs, spacers, and tying wire are the steel that never appears in the design calculations but appears in the order. Chairs hold the bars at the right height and cover during the pour; spacers hold them off the formwork; tying wire — soft annealed wire, twisted at every intersection or every other one — holds the cage together until the concrete sets. Their weight is small and their cost is smaller, and omitting them from the order is a false economy: a cage that moves during the pour ends up with the wrong cover, and the wrong cover is the defect that no weight calculation can detect. Order the accessories with the bars, and check the cage before the pour, not after.

Frequently asked questions