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Sudoku looks simple: fill a 9×9 grid so every row, column, and 3×3 box contains the digits 1 through 9 exactly once. But that simplicity hides real depth. This guide walks through the techniques solvers actually use, in the order you'll need them, plus a bit of the game's history and why so many people make it part of their daily routine. If you're just starting out, the Beginner Tips page covers habits and common mistakes that matter just as much as the techniques below.
Almost every puzzle — even hard ones — starts by clearing out the easy cells first. These three techniques will get you through the opening of any grid.
Pick a number, say 7, and scan each 3×3 box in turn. If two of the three rows (or columns) crossing that box already contain a 7 somewhere else in the grid, and the box has only one empty cell left in the remaining row/column, that cell must be the 7. This is the fastest way to fill in a puzzle's first clues.
Look at a single empty cell and list every number it could legally hold, based on what's already in its row, column, and box. If only one number survives that process, it belongs there — no further reasoning needed.
Sometimes a cell still looks like it has several options on its own, but one of those candidate numbers can't fit anywhere else in that row, column, or box. When a candidate is "hidden" like that — only possible in one spot even though the cell itself allows others — it's forced there.
Once scanning and singles stop making progress, these pattern-based techniques unlock the next layer of the puzzle.
If two cells in the same row, column, or box both have their candidates narrowed down to the exact same two numbers, those two numbers must occupy those two cells — in some order. That means every other cell in that row, column, or box can safely have those two numbers eliminated from its candidate list. The same logic extends to three cells sharing three candidates (a naked triple).
The mirror image of a naked pair: if two candidate numbers only appear in the same two cells within a unit — even if those cells have other candidates cluttering them up — those two numbers must go in those two cells. Every other candidate can be stripped out of those two cells.
If a candidate number within a box only appears in cells that all sit in the same row (or column), that number can be eliminated from the rest of that row outside the box — it's "pointing" out of the box. The reverse also works: if a candidate in a row is confined to a single box, it can be eliminated from the rest of that box.
Expert-level puzzles occasionally need these — they take longer to spot, but they're what separates a stuck grid from a solved one.
If a candidate number appears in exactly two cells in each of two different rows, and those cells line up in the same two columns, they form a rectangle. That number can then be eliminated from every other cell in those two columns — the four corners "lock" the candidate in place.
The same idea as an X-Wing, extended to three rows and three columns instead of two. It's rarer and harder to spot, but the logic is identical.
A chain of three cells, each with exactly two candidates, linked so that one shared number rules out a candidate in a cell that "sees" all three. It's the technique most solvers reach for once pairs, triples, and X-Wings run dry.
In SUDOKU — Premium Edition, difficulty isn't just about how many clues are given — it's about which techniques a puzzle actually requires to finish:
If you're moving up a difficulty and getting stuck, it's almost always a sign to look for the next technique on this list rather than to keep re-scanning what you've already checked.
Sudoku's roots trace back to the 18th century, when Swiss mathematician Leonhard Euler experimented with "Latin squares" — grids where each symbol appears once per row and column. The modern 9×9 puzzle with its added 3×3 box constraint was created much later, by retired American architect Howard Garns, and first published in 1979 by Dell Magazines under the name "Number Place."
The puzzle found its true home in Japan, where puzzle publisher Nikoli introduced it in 1984 and gave it the name we use today — Sudoku, short for sūji wa dokushin ni kagiru ("the digits must remain single"). It stayed largely a Japanese pastime until the mid-2000s, when retired judge Wayne Gould wrote a program to generate puzzles quickly and pitched them to The Times of London in 2004. The puzzle exploded internationally within a year, and newspapers, books, and apps worldwide have carried it ever since.
Sudoku has stuck around for a reason. A few things players consistently mention:
Knowing a technique and using it well are two different things. Most beginners already understand naked singles and scanning within a few puzzles — what trips people up is habits: how they approach a fresh grid, when they reach for a hint, and whether they're patient enough to look twice before guessing. This page covers that side of things. For the actual solving techniques, see the Strategy Guide.
Guessing when stuck, instead of walking away from that cell and working somewhere else.
A guess that's wrong doesn't just cost you a mistake — it can quietly break the logic of cells you haven't even reached yet, making everything after it harder to untangle. If a cell won't resolve, leave it and make progress somewhere else on the board. It's usually solvable a few moves later once other cells fill in.
Never using pencil marks (Notes) until a puzzle is already half-stuck.
Turn on Notes early, especially on Medium and Hard puzzles. Jotting down every candidate a cell could still hold — even when you're not sure yet — is what makes pairs, triples, and pointing patterns visible in the first place. Trying to spot them purely in your head gets unreliable fast.
Scanning the whole board randomly instead of following any order.
Pick a number and check it against every box before moving to the next number, rather than jumping between numbers and boxes at random. It's a small change, but it stops you from re-checking the same cells over and over without realizing it.
When you open a fresh puzzle, working in this order avoids wasted effort:
Hints in this game are limited per puzzle, so it's worth being deliberate about when you use one. A hint is most valuable when you're genuinely stuck — not when you're unsure between two options. If you can narrow a cell down to two candidates through notes, that's often enough to work out the answer yourself without spending a hint.
Also worth remembering: you're allowed up to 5 mistakes per puzzle before it ends. That's not a lot of room on Hard difficulty, so treat mistakes as a resource too — double-check a cell before committing to it rather than tapping through options to see what sticks.
If Medium or Hard puzzles feel like a wall, it's rarely about intelligence — it's almost always a missing technique. Play a stretch of Easy puzzles until scanning and singles feel automatic, then move to Medium and get comfortable spotting naked pairs before you push into Hard. Trying to force a Hard puzzle using only basic techniques is the single most common reason beginners bounce off Sudoku early.
SUDOKU — Premium Edition is a free, browser-based Sudoku puzzle game. Play solo across three difficulty levels — Easy, Medium, and Hard — or challenge friends in real-time multiplayer rooms.
Track your progress on the global leaderboard, add friends, and customize your profile with a display name, country, and photo.
No installs, no sign-up walls — just open the page and start playing. Built and maintained independently by a solo developer, with new features added over time.
Fill the 9×9 grid so that every row, every column, and each of the nine 3×3 boxes contains the digits 1 through 9 — with no repeats.
Fill a cell. Tap an empty cell on the board, then tap a number on the numpad below to fill it in.
Notes. Turn on Notes to pencil in candidate numbers for a cell instead of committing to one — handy while you're narrowing down possibilities.
Hint. Stuck? Use a Hint to reveal a cell. Each puzzle gives you a limited number, so save them for when you really need them.
You can make up to 5 mistakes in a puzzle. Go over that, and the game ends — so double-check before you tap.
Start with rows, columns, or 3×3 boxes that already have the most numbers filled in — they're the fastest to solve. Use Notes to track which numbers could go where, and look for cells where only one number is possible.
Yes, completely — no purchases, no paywalls.
No, not for solo play — just hit Start Game. Setting up a profile is only needed if you want to appear on the global leaderboard or play with friends.
Yes — use the pause button during a game and resume whenever you're ready, as long as you keep the tab open.
Up to 5 per puzzle. Go over that and the puzzle ends.
Each puzzle gives you a limited number of hints. Using one reveals the correct number for a cell of your choice.
No — multiplayer rooms, friends, and the leaderboard are all free.
We'd love to hear it — see the Contact link on the home screen.
Found a bug, have feedback, or just want to say hi? Reach out anytime — happy to hear from you.
Email: mayankthedeveloper@gmail.com