Convert Overworld coordinates to the Nether and back to line up linked portals. Free, no signup.
The Nether is 8x compressed - divide Overworld X/Z by 8 for the Nether spot (Y stays the same). Perfect for linked portals.
The Nether is 8x compressed: one block in the Nether equals eight in the Overworld. Divide your Overworld X and Z by 8 to find the matching Nether spot (Y is the same).
Build the second portal at the calculated coordinates so both portals map to each other, giving you a fast Nether highway between two Overworld locations.