Rebar weight:
kilograms per metre, per bar and per tonne

Why steel is bought by weight but cut by length

Reinforcement is drawn on plans in metres and bar sizes, yet suppliers invoice by the tonne. To order the right quantity, or to check a bill, you must convert a bar diameter and a cut length into kilograms. The conversion is simple geometry plus the density of steel.

A solid round bar is a cylinder. Its volume is the cross-sectional area times the length, and mass is volume times density. Everything on a bar-bending schedule can be reduced to that one chain of multiplication.

Deriving the weight from first principles

Because bar diameters are quoted in millimetres and lengths in metres, it helps to fold the constants together. Putting d in millimetres and the density in, π/4 × 7,850 ÷ 1,000,000 works out to roughly 1/162.2, which gives the site shortcut below.

One more check is to remember a few anchor values. A 10 mm bar is about 0.62 kg per metre and a 16 mm bar about 1.58 kg per metre. If a quick calculation gives you a figure far from those anchors for a similar size, recheck the units before ordering.

Worked example: a 12 mm bar, 6 m long

Both routes agree to two decimals, which is the quickest way to catch a slipped decimal point on site. The shortcut is accurate enough for estimating; the first-principles route is for when you are working with a different density or a hollow section.

Weight chart for common bar sizes

Notice how fast weight climbs with size. Doubling the diameter from 10 mm to 20 mm quadruples the weight per metre, because the area scales with the square of the diameter.

Slab, beam and column examples

Weight alone does not tell you whether a design is adequate, but it helps with ordering and with sanity checks on a drawing. Consider a small slab 4 m by 3 m reinforced with 10 mm bars at 150 mm centres each way. In the 4 m direction you need bars spaced across the 3 m width, which is 3 ÷ 0.15 = 20 spaces, so 21 bars of about 4 m. In the 3 m direction, 4 ÷ 0.15 gives about 27 spaces, so 28 bars of about 3 m.

Total length is roughly 21 × 4 + 28 × 3 = 168 m. At 0.617 kg per metre for 10 mm bar, that is about 104 kg before laps, bends and cover adjustments. A figure in that region is reasonable for a single-layer mesh, and a result ten times larger or smaller would point to a slip in spacing or units.

Beams and columns work the same way, with main bars running the full length and stirrups cut to the perimeter of the cage. Stirrups are where estimators often undercount, because a small stirrup is repeated hundreds of times and each one is a separate cut piece.

Turning a schedule into a quantity

1. 1List every bar mark with its diameter, cut length including hooks and laps, and the number of pieces.
2. 2Multiply each cut length by the number of pieces to get total metres of that diameter.
3. 3Convert total metres to kilograms using the weight per metre for that diameter.
4. 4Add the totals for each diameter to get the overall steel weight.
5. 5Add a wastage allowance for cutting offcuts, usually a few percent, then divide by 1,000 for tonnes.

Order in standard stock lengths where you can. Cutting a 12 m bar into pieces that add up to 11.5 m leaves a half-metre offcut, and across hundreds of bars that waste is real money. Ask the bar-bending schedule preparer to optimise cuts against the stock length.

Grades, ribs and what the weight tells you

Bar grade, such as Fe 500 or Fe 550, describes strength, not weight. Two grades of the same diameter weigh the same per metre, so the weight chart applies to either. What the grade changes is how much steel the engineer needs, because stronger bar can carry the same load with less area.

Weight can still be a useful quality check. If a bundle of 12 mm bar is consistently well below the theoretical mass, the actual cross-section may be undersized, which means less strength than the drawing assumes. Standards set mass limits for this reason, and a reputable supplier labels each bundle with its size and heat or batch number.

Why the delivered weight differs

Rolled bars are produced to a permitted mass tolerance, so a bundle can weigh slightly more or less than the theoretical figure. The ribs on TMT bar also mean the actual area is not a perfect circle of the nominal diameter. Standards allow a small percentage either way, and the allowed band is wider for thinner bars.

For procurement, use the theoretical figure to plan and the weighbridge slip to settle. For structural checks, rely on the engineer's drawing and the grade of steel specified, not on a weight estimate.

Common slips when estimating steel

• Entering the diameter in centimetres or metres into the shortcut, which expects millimetres.
• Forgetting hooks, bends and lap lengths, which add a lot of length to stirrups and main bars.
• Using the clear span instead of the cut length including the development length into the support.
• Mixing up bars per bundle with bar length, since 12 m is a standard stock length but not universal.
• Rounding each line early; carry the decimals until the total.