Nether Portal Calculator
Convert Overworld coordinates to the Nether.
One block in the Nether equals eight in the Overworld, so divide coordinates by 8.
The math behind it
Nether X = Overworld X / 8, Nether Z = Overworld Z / 8. Build a linked portal at these coordinates.
Worked example
Overworld (800, -240) maps to Nether (100, -30).
FAQ
Why does this work?
Traveling 1 block in the Nether covers 8 blocks of Overworld distance.