How to build and light a Nether portal

A Nether portal needs a 4×5 obsidian frame (10 blocks if you skip the corners, since they're decorative and never required) and a light source dropped inside it. You don't need a diamond pickaxe to get that obsidian either: pour a water bucket over a lava pool and it turns to obsidian on the spot, ready to place straight into the frame.

The frame: 10 blocks is enough, 14 is what the game builds

The portal is a vertical rectangle, outer dimensions 4 blocks wide by 5 blocks tall at minimum, and up to 23×23 at the maximum — any size in between works, as long as it stays rectangular and hollow in the middle. The four corner blocks are never required structurally; a frame built from just the two side columns and the top/bottom bars — 10 obsidian blocks total — lights and works exactly like a full one. It's only that portals the game generates for you (in ruined portal structures, for instance) always come with corners filled in, which is why "14 blocks" is the number people often quote instead.

Once the frame is up, drop fire inside it — with flint and steel, a fire charge, a ghast fireball, or even a lightning strike — and it has to be the very last block placed. Fire on an unfinished frame just burns out; the game only checks for a valid, complete rectangle at the moment the flame lands.

Getting obsidian without a diamond pickaxe

Obsidian normally needs a diamond or netherite pickaxe to mine, but you can skip mining it altogether. Lava turns into obsidian the instant flowing water touches it. Find a lava pool, pour a water bucket onto it from the edge, and wherever the water reaches you'll hear a hiss and see solid obsidian where the lava was — no pickaxe involved because you never broke a block, you cast a new one.

That's also how speedrunners and early-game players build a portal from scratch with nothing but two buckets:

1. Place a bucket of lava on the ground and immediately pour water over it — one water source per lava block turns the whole thing to obsidian.
2. Repeat to lay down the four-block bottom row.
3. Build the two side columns the same way, one lava-then-water block at a time, up to four blocks high.
4. Cap it with the top bar, and you've got a complete 4×5 frame without ever swinging a pickaxe at it.

It's slower than mining a natural obsidian deposit with a diamond pickaxe, but it's the only route open to a player who hasn't reached diamonds yet — which is exactly why this trick shows up so often in early-game and no-diamond speedrun guides.

If it won't light

Two things cause a frame to refuse fire: the rectangle isn't actually complete — a gap hiding behind another block, or a corner block accidentally filling in what should be open space along an edge — or the fire wasn't the last block placed, which happens if you're standing on a block that itself completed the frame. Walk the whole perimeter and count the obsidian blocks before trying again; a portal frame with the wrong interior width (3 blocks instead of the required minimum of 2, for example) simply won't activate at all, no matter how much fire you throw at it.

The second portal: why it doesn't land where you expect

Once you've got one portal, coordinates in the Nether don't map 1:1 to the Overworld — they're compressed 8 to 1. Move one block in the Nether and you've covered the equivalent of eight blocks back home; to find where a Nether portal will surface in the Overworld (or vice versa), divide your current X and Z coordinates by 8 (Y stays the same). It's why a portal built carelessly in the Nether can dump you out in the middle of an ocean or on top of a mountain on the other side — the ratio is fixed, but your Nether-side position rarely lines up with anything convenient back in the Overworld. If you're building a second portal on purpose, do the division first and place the frame at the resulting coordinates rather than guessing.

Worth knowing before you start digging: the biome you cast your obsidian in on the Nether side can matter for what's nearby when you step through — check the biomes database if you're planning where a return portal should land, and the mobs database if you want to know what's waiting on the other side before you light the fire.