The weight of a round bar is its diameter squared divided by 162, in kilograms per metre. An 8 mm bar is about 0.395 kg per metre, 10 mm about 0.617, 12 mm about 0.888, 16 mm about 1.58, 20 mm about 2.47 and 25 mm about 3.85.
A standard bar is 12 metres, so multiply by twelve for the weight of one length. Mills work to a tolerance on section weight, so a load can be a little under or over theoretical - the tolerance is what your purchase order should name, not the exact figure.
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