How this calculator works
The Nether is eight times smaller than the overworld in the horizontal directions. A portal at a given overworld X and Z is matched to a Nether point at one eighth of those coordinates, and a portal in the Nether is matched to a point eight times farther out in the overworld. Y is not scaled, so a portal at height 64 has a partner at height 64. The game does not teleport you to the exact calculated point: it searches nearby for an existing portal, which is why a badly placed pair can link to a different destination than the arithmetic suggests.
Formula
Nether X or Z = overworld X or Z / 8. Overworld X or Z = Nether X or Z x 8. Y stays the same in both directions. Obsidian for a frame = 2 x (interior width + interior height) + 4 with the four corners, or 2 x (interior width + interior height) without them. Frame outer width = interior width + 2. Frame outer height = interior height + 2.
Worked example
An overworld portal at X 800, Y 64, Z -1,600 maps to Nether X 100, Y 64, Z -200. Going the other way, a Nether portal at X -30, Z 45 maps to overworld X -240, Z 360. A standard portal with a 2 by 3 opening needs 2 x (2 + 3) = 10 obsidian without corners, or 14 with them. The largest allowed opening, 21 by 21, needs 84 blocks without corners and 88 with them.
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Frequently asked questions
Why is the Nether distance one eighth?
The designers made the Nether compact so travel there covers more overworld ground. One Nether block equals eight overworld blocks horizontally. The vertical axis is not compressed, so climbing in the Nether does not move you higher in the overworld.
Do I need the corner blocks?
No. A portal frame works without its four corners, so a standard 2 by 3 opening needs 10 obsidian rather than 14. Many players add the corners for a cleaner look. Either way, the opening must be at least 2 wide and 3 tall.
Will my portal link to exactly these coordinates?
Not always. The game converts your position, then searches a radius around the converted point for an existing portal before building a new one. If another portal is close enough, you arrive there instead. Keep portals far apart, or build both ends and confirm the link before you rely on it.
What happens to Y?
Y is unchanged by the conversion. The Nether has its own build height and a roof, so a target Y that works in the overworld may be inside bedrock or above the build limit in the Nether. Move the portal vertically to a safe opening if the game places it somewhere unusable.
Are the limits the same in Bedrock Edition?
The one-to-eight ratio and the frame sizes are the same in both editions. Portal linking details, such as the search radius and how portals pair up, have differed between editions and versions. Check the Minecraft Wiki page for your edition if a link behaves unexpectedly.