Every TMT order is bought by weight and specified by diameter, so the number that matters is the mass per metre. IS 1786:2008 tabulates it, and the same standard sets how far a delivered batch may sit either side of it.
What the standard says
Table 1 of IS 1786:2008 gives the nominal mass for each size. The figure is derived at a density of 0.00785 kg per square millimetre per metre, which clause 7.2.1 defines. A 12 mm bar is 0.888 kg per metre; a 16 mm bar is 1.580.
Table 2 sets the tolerance on that nominal mass. It is not one number. It is plus or minus 7 per cent on a batch up to and including 10 mm, plus or minus 5 per cent over 10 and up to 16 mm, and plus or minus 3 per cent over 16 mm.
What that means on a delivery
A tonne of 12 mm bar is about 94 bars of 12 metres. At the 5 per cent batch tolerance, the same tonne could arrive as a little under 90 bars or a little over 98, and every one of them is to specification.
This is the single most common misunderstanding in a steel delivery. A short count is not automatically short weight, and a weighbridge ticket is the thing to argue from, not a bar count.
How to check
Weigh the lot. Compare it against the nominal mass times the length times the count, then allow the tolerance for the size band. If it falls inside, the material is to standard. If it does not, ask for the mill test certificate and the weighbridge slip together.
You can work the figures for a specific order on our weight calculator, which prints the tolerance beside every answer.
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