Cable size calculator
This sizes for voltage drop only. A real cable selection also has to satisfy current-carrying capacity after derating for grouping, insulation, ambient temperature and installation method, plus disconnection times for fault protection.
The minimum conductor area for a voltage-drop limit is 2 × ρ × L × I ÷ (limit × V). Twenty amps over 40 m at 230 V within 3% needs 3.99 mm², so the next standard size up is 4 mm², which then drops 2.99%.
How to size a cable
Cable selection is two separate tests and the bigger answer wins. The first is current-carrying capacity: can the cable carry the load without its insulation exceeding its rated temperature, after derating for how it is installed, how many cables share the route, and how hot the surroundings get. The second is voltage drop, which is what this page computes. On short runs the current test almost always dominates; past twenty or thirty metres, voltage drop takes over and starts driving the size up quickly, because drop is linear in length.
Questions
Take the larger of two answers: the size that carries the current after derating, and the size that keeps voltage drop inside the limit. This page answers the second.
In metric: 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50 mm² and up. AWG runs the other way; a smaller number is a thicker wire.
For voltage drop, yes, and linearly. Doubling the length doubles the drop, so it doubles the area needed to hold the same percentage.
Because cables come in fixed steps. The theoretical minimum is shown alongside so you can see how much headroom the next size up gives.
Use it to understand the problem, not to sign off an installation. Fixed wiring must be designed to the regulations that apply where you live.