Convert between Overworld and Nether coordinates instantly. Type in either side — the other updates live. The Y coordinate is kept separate as it doesn't scale.
For a portal to reliably link, you need a portal within 128 blocks in the Nether at the converted coordinates. Build your Nether portal at the exact coordinates this calculator gives you.
If you have an existing Overworld portal and want to link a second one, both must be within linking range of their respective Nether counterparts — use this tool to make sure they don't accidentally share the same Nether portal.