Note these facts ;

Density of Steel = 7850kg/m³

Density = mass / volume

Volume of steel= πr²L, where r is the cross sectional radius of the steel and L is the length of the steel rod

Reinforcing steel for concrete is sold in lengths of 12m.

1 tonne = 1000kg

1m = 1000mm

Question: How many 12mm  reinforcing steel will be sold to you per tonne.

Calculation:

Cross sectional radius is 12mm /2 = 6mm

Length of steel is 12m, therefore

Volume of steel is πr²L, that is;

3.142×(6/1000)² ×12=  0.001357344m³

Since, Density = mass / volume, it also means that;

Mass = density × volume

That is; 7850kg/m³ ×0.001357344m³

= 10.66kg

Since 1 tonne is 1000kg it implies that 1000kg/ 10.66kg= 93.81 but you will be supplied 93 pieces.