Multiplies the total length by the mass per metre, which is fixed for each bar size by standard: 0.995 kg per metre for D13. An allowance is added for the offcuts left when bars are cut from stock lengths.
The weight of reinforcing bar follows from the mass per metre fixed for each size. There is no need to measure the thickness.
is the mass per metre, the length of one bar and the number of bars. The standard sets these masses: 0.560 kg/m for D10, 0.995 for D13, 1.56 for D16 and 2.25 for D19.
In D13, the D marks it as deformed bar and 13 is the nominal diameter. Ribs on the surface push the real outside diameter to about 14 mm, but the bar is named for the plain round bar of equal cross-section. The mass per metre is the value in the standard, 0.995, rather than anything derived from the nominal figure.
Fifty D13 bars of 4 m each come to 200 m in total, weighing kg. Allowing 5% for offcuts when cutting from stock lengths brings that to 209 kg.
Mass per metre goes with the cross-sectional area, so roughly with the square of the nominal diameter. Going from D10 to D32 multiplies the diameter by 3.2 but the weight by 11. At the same length, a change of size changes the order of magnitude.
Bends, hooks and anchorage lengths are not included. A real takeoff works from the total length once those are added.