Speaker Wire Gauge Calculator
Cable sits in series with the load, so every ohm in the run is an ohm the amplifier drives instead of the cone. What matters is not the length on its own but the length against the impedance: the same 50 ft that is harmless into 8 ohms costs twice as much into 4. Enter your run and load for the thinnest copper that still holds the line.
Measure the route you will actually pull, not the straight line.
Units
Use the figure the amplifier actually sees, not the box number.
Standard — the long-standing rule of thumb for a full-range run.