Pick a wire size and a drop budget, and this tool tells you exactly how far that conductor can run before the loss exceeds your limit.
The question on every long run — to a detached garage, a well pump, a gate, a far solar panel — is rarely "what is the drop" but "how far can the wire I have actually go." The relationship is linear: half the current doubles the distance, and each step up in AWG size cuts resistance by roughly 21%, extending the run by about 26%.
One-way. The round trip through both conductors is already built into the formula's factor of 2 (or √3 for three-phase).
Raise the system voltage if you can (240 V doubles the distance of 120 V for the same load power), reduce the current, or accept a higher % budget where the load tolerates it.