Minecraft Nether Portal Calculator
Convert coordinates between the Overworld and the Nether so your portals line up, and check whether two portals will link. Every block you walk in the Nether covers 8 in the Overworld.
Overworld coordinates
Will my portals link?
Overworld portal
Nether portal
Overworld → Nether: you’ll come out at this Nether portal.
Nether → Overworld: you’ll come out at this Overworld portal.
If another portal is closer to the matching spot, Minecraft uses that one instead. Keep portals in the same area at least 32 blocks apart in the Nether so they don't cross-link.