In the Total Length column, enter the formula: =C2*D2, where C2 is the bar length and D2 is the number of bars.
In the Weight column (if included), enter the formula: =E2*F2, where E2 is the total length and F2 is the weight per unit length (you can add a separate column for this value).
Use Excel's built-in functions, such as SUMIF or INDEX/MATCH, to calculate the total weight or length of all bars.