Every block you walk in the Nether covers 8 blocks of ground back in the Overworld, which is exactly why a base at Overworld coordinate 1000 lines up with a Nether portal built at coordinate 125. Get that ratio wrong by even a little over a long trip and your return portal can end up hundreds of blocks from where you actually wanted it, this calculator does the division (or multiplication) for you in both directions, plus works out how much obsidian a given portal frame size actually needs.
The 1:8 Ratio Explained
The Nether is compressed relative to the Overworld by a fixed factor of 8 on both the X and Z axes. Walking one block in the Nether is equivalent to covering eight blocks back in the Overworld, which is the entire reason experienced players build Nether highways in the first place: an 800-block Overworld journey collapses into a 100-block Nether sprint. To convert an Overworld coordinate into its matching Nether coordinate, divide by 8. To go the other direction, from a Nether coordinate back to Overworld, multiply by 8. Both conversions apply independently to X and Z, the Y (vertical) coordinate isn't affected by the ratio at all, since the two dimensions don't share a height scale the same way.
How Portal Linking Actually Decides Where You Arrive
When you step through a portal, the game doesn't search for an existing portal at the mathematically exact converted coordinate, it searches within a radius (128 blocks in recent Java versions) around that point for an already-existing linked portal first. If it finds one, you arrive there and the two portals become permanently linked to each other. If it doesn't find one, the game generates a brand new portal as close as it can to the ideal converted coordinate, subject to the terrain actually being buildable there, which is why portals occasionally pop up embedded in a cave wall or floating over lava instead of on flat ground. This search-and-link behavior is also the source of the classic "portal linking bug" frustration: build a second Overworld portal too close to where an existing Nether portal's search radius reaches, and you can accidentally link to the wrong one instead of generating a fresh, correctly placed portal.
Minimum, Maximum, and Everything Between
The smallest functional portal has a 2-block-wide, 3-block-tall interior opening, framed by obsidian, for a 4x5 outer frame footprint. That's the size most speedrunners and early-game players default to, since it's the cheapest to build and still teleports you exactly the same as a larger one. On the other end, portals can be built up to a 21x21 interior, useful mainly for a wide, dramatic-looking gateway or for lining up multiple destinations, though it costs vastly more obsidian for zero functional benefit over the minimum size. There's no in-between requirement, any interior width from 2 to 21 and height from 3 to 21 works, the frame always adds exactly one block of obsidian border on every side of whatever interior you choose.
Getting the Obsidian Without a Diamond Pickaxe
Obsidian only drops as an item when mined with a diamond or netherite pickaxe, but there are two well-known ways around that early in a playthrough. The classic method uses a lava source block poured over flowing water (or the reverse), which turns the contact block into obsidian instantly, letting you bucket-farm your way to the 10 blocks a minimum portal needs without any diamond gear at all. The other route, once you've got a Nether portal already built, is finding a naturally generated portal ruin or Nether fortress that already has obsidian nearby, though that assumes you got into the Nether in the first place, which is the exact chicken-and-egg problem the water-lava method solves.
A Worked Example
Say your base sits at Overworld X:1000, Z:-240. Dividing both by 8 gives Nether X:125, Z:-30. Build your Nether portal anywhere within roughly 128 blocks of that point (the game's search-and-link radius mentioned above gives you real flexibility here, you don't need to hit the exact block), and stepping back through will generate or link to an Overworld portal close to your original 1000, -240 starting point. For the frame itself, stick with the minimum 2-wide, 3-tall interior, meaning a 4x5 outer frame, and per the obsidian formula this calculator uses (twice the frame width plus twice the frame height, minus 8 for the four corner blocks the game never requires), that's 2(4) + 2(5) minus 8, exactly 10 blocks of obsidian.
How to Use This Calculator
Use the coordinate converter to plan where to build a Nether portal for a given Overworld destination, or check where an existing Nether portal will surface once you step through it, in either direction. Use the obsidian and frame size section separately to work out exactly how many obsidian blocks a portal of any interior size will actually cost before you start mining or bucket-farming for it.
Frequently Asked Questions
Does the Y coordinate change between dimensions too?+
No, the 1:8 ratio only applies to X and Z. Height (Y) works independently in each dimension and isn't multiplied or divided when you cross over.
Why did my new portal not link to the one I expected?+
The game links to the nearest existing portal within its search radius rather than generating a new one there, so if an old or unintended portal happens to be closer to the converted coordinate than the one you meant to reach, you'll surface there instead.
What's the actual minimum obsidian for a portal?+
10 blocks, for the smallest working 2x3 interior (4x5 outer frame). Corner blocks of the frame are never required by the game.
Can I get obsidian without a diamond pickaxe?+
Yes. Pour a lava source block against flowing water (or flowing water against a lava source), and the contact block turns into obsidian, which you can then mine with any pickaxe once it's already water-flowed into place, or via the standard diamond-pickaxe method if you have one.
Divide by 8, build 10 blocks, done.
The Nether's compression ratio is simple once it clicks, the tricky part is usually remembering it mid-build. If you're optimizing other in-game numbers too, the DPS Calculator and DPI Calculator cover combat math and mouse sensitivity for other games in the same spirit.
Convert your coordinates before you mine a single block of obsidian.
Sources and References
- Minecraft Wiki. Nether Portal. minecraft.wiki. Accessed August 2026.