Engineers often ask how to calculate the resistance of a cable, there is a very simple formula that works well within an ohm or so with copper cable.
If you take the number 19 the divide by the cross sectional area of the cable this will give you the resistance over a kilometre. (This works for copper cable)
Here are a couple of simple examples:
1, the cable has a CSA of 2.5mm²
19 ÷ by 2.5mm² will give you 7.6 ohms per km. ÷ by 1000 for resistance per meter.
2, the cable has a CSA of 4mm²
19 ÷ by 4mm² will give you 4.75 ohms per km.
Then by using ohms law you can calculate volt drop.
On a different note, have you downloaded the RS Tool Box from the apple store? This is an engineering reference at your fingertips, certainly worth keeping handy for your day to day working.