Why D² ÷ 162
Reinforcing steel is sold by weight but drawn and cut by length, so every site needs to turn bar lengths into kilograms. Steel weighs 7,850 kg per cubic metre. A round bar of diameter D millimetres has a cross-section of π/4 × D² square millimetres, and one metre of it weighs π/4 × D² × 7,850 ÷ 1,000,000 kg. Those constants collapse into the rule every foreman knows: weight per metre = D² ÷ 162. This tool uses the exact divisor, 162.2, so a 12 mm bar comes to 0.888 kg/m and an 8 mm bar to 0.395 kg/m.
Enter each bar size, the length of one bar and how many bars of that length you have. Lengths can be in metres or feet; the tool converts feet at exactly 0.3048 m. Add a row for every size in the bar bending schedule and the tool sums the weight per size and overall.
Ordering bars
Bars are supplied in 12 m lengths. The bar count for each size is the total length of that size divided by 12, rounded up, which is what a supplier will bill. Bar bending schedules add laps of 40 to 50 bar diameters, hooks and bends, and 2–3 % cutting waste; include those lengths in the rows if you want the ordered quantity rather than the drawn one.
If you enter a rate in rupees per kilogram, the tool multiplies it by the total weight. District rate books list steel per kilogram, so the rate from the current fiscal year's book gives an estimate that matches the norms.
Tolerances
Rolled bars vary from the nominal size. NS 191 and IS 1786 allow ±7 % on the mass of bars up to 10 mm, ±6 % for 12 mm and ±5 % above 16 mm, so a delivery weighed on a bridge can differ from this theoretical figure by a few percent and still be within specification.