A valid Nether portal frame is a rectangle whose outside dimensions are at least 4 blocks wide by 5 blocks high and no larger than 23 by 23 blocks. The empty opening inside it ranges from 2 by 3 blocks up to 21 by 21 blocks. The frame is built from obsidian, and corner blocks are optional.
The size limits
The size rule applies to the outer edge of the obsidian frame, not to the space you walk through. Each outside dimension can be any whole number from its minimum to its maximum, and the two dimensions do not have to match. A 4 by 5 frame and a 23 by 23 frame are both valid, and so is a 10 by 6 frame.
| Measurement | Minimum | Maximum |
|---|---|---|
| Outer frame (width by height, in blocks) | 4 by 5 | 23 by 23 |
| Empty portal opening (width by height, in blocks) | 2 by 3 | 21 by 21 |
These figures come from the Minecraft Wiki’s community-maintained article on the Nether portal. The wiki is not an official Mojang or Microsoft publication, but it is the standard reference for these gameplay rules.
How to calculate the opening from the frame
Because the frame sits around the opening, you subtract two blocks from each outside dimension:
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- Opening width = outer width minus 2
- Opening height = outer height minus 2
A 4 by 5 outer frame therefore leaves a 2 by 3 opening, and a 23 by 23 outer frame leaves a 21 by 21 opening. Build the frame to the outer measurements, not the interior ones, or the portal will be one row or column too small.
Corners and obsidian count
The four corner blocks of the frame are optional. Leaving them out does not stop the portal from working, and it reduces the number of obsidian blocks you need. The wiki’s figures for the smallest frame are:
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| Smallest 4 by 5 frame | Obsidian required |
|---|---|
| With all four corners | 14 |
| Without corners | 10 |
If you are short on obsidian, the cornerless 4 by 5 frame is the cheapest valid portal. Larger frames follow the same rule, so omitting corners on any size saves four blocks compared with the same frame built with them.
Shape rules
A portal frame must be rectangular. Each side has to be straight, and both dimensions must stay within the limits above. The wiki’s rule does not cover decorative or irregular shapes, so do not assume that a non-rectangular ring will be recognised as a portal.
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Lighting the portal
The frame is activated by placing fire inside the opening. The steps are:
- Build the obsidian frame to a valid size, with or without corners.
- Stand in front of the opening and make sure the interior is clear of blocks and entities.
- Use flint and steel on any block inside the frame to place fire.
Flint and steel is the method the wiki lists. It is a standard in-game tool, and no other ignition method needs to be assumed for this article.
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Edition and version limits
The dimensions above are general vanilla Minecraft guidance from the wiki article. The article does not tie these figures to a specific Java Edition or Bedrock Edition release, and it does not confirm that every edition or modded build uses the same rules. If you need the exact rules for your version, check the edition and version reference for your game before relying on these sizes.
Bottom line for builders
For the most economical portal, build the 4 by 5 outer frame without corners, which needs 10 obsidian and leaves a 2 by 3 opening. For a larger opening, use any rectangle up to 23 by 23 outer blocks, which leaves an opening up to 21 by 21.
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