How to Solve Sudoku

A guided path from your first Easy grid to Expert and beyond. This page is about order and strategy - when to use which move, and what to do when nothing seems to work. For the rules themselves, see How to Play; for deep dives on each move, see the technique reference.

Start With the Scan

Every Sudoku, at every difficulty, opens the same way: a scan. Before touching a single cell, look at which digits the setter gave you most often. If there are six 5s on the board, the remaining three are usually easy to place - each missing 5 is squeezed by the six already visible. Work the most frequent digits first, then the next most frequent, and only then start examining individual cells.

Scan in a fixed order - rows top to bottom, then columns, then boxes - and you will never wonder whether you missed something. The strongest solvers aren't faster at logic; they're simply more systematic about scanning, so easy placements never hide from them.

The Beginner Loop

Easy and most Medium puzzles fall to a simple three-step loop, repeated until the grid is full.

1. Fill the near-complete units. Any row, column, or box with one empty cell can be completed instantly - that's Last Free Cell, and one fill often unlocks a chain of others. Then flip the question around: for each digit, is there a unit where it can only fit in one spot? That's Last Remaining Cell, and together with cell-by-cell elimination via Last Possible Number it covers roughly 80% of all Easy-puzzle moves.

2. Write your notes. The moment direct placements dry up, stop and pencil in candidates using Notes. This feels slow the first few times, but it is the single habit that separates players who plateau at Easy from players who keep climbing - every advanced technique reads the grid through notes.

3. Harvest the singles. With notes in place, sweep for cells whose candidate list has shrunk to one digit - Obvious Singles. Place them, update the notes of every neighbour, and loop back to step 1, because each placement creates new near-complete units. Most puzzles are solved not by one clever move but by riding this loop.

Breaking Through Medium

The wall most players hit on Medium has a name: they scan cells, but not digits. A Hidden Single is a cell that still shows several candidates - yet for one of those digits, this is the only legal home in its row, column, or box. You will never see it by staring at the cell; you find it by asking, digit by digit, "where can the 7 go in this box?" Switching to that digit-first scan is the biggest single skill jump in Sudoku.

The second Medium-to-Hard tool is the pair. When two cells in a unit hold exactly the same two candidates, those digits are locked there and can be erased from every other cell in the unit - Obvious Pairs. Its sneakier sibling, the Hidden Pair, hides the same lock under extra candidates. Pairs rarely place a digit directly; their job is to thin your notes until new singles appear.

Hard and Beyond

From Expert upward, single units stop being enough - you have to read patterns that span the whole grid. The gateway move is the X-Wing: one digit confined to the same two columns across two rows forms a rectangle that wipes the digit from everywhere else in those columns. It looks intimidating, but it's just the pair concept stretched across the board.

Master and Extreme puzzles add the three-row version, Swordfish. Don't reach for it early - on a typical Expert grid an X-Wing or a missed Hidden Pair is far more likely to be the key than a Swordfish. Advanced solving is mostly the discipline of exhausting simple tools before deploying expensive ones.

When You're Stuck

Being stuck almost always means a missed simple move, not a missing advanced one. Run this checklist in order:

1. Re-scan for near-complete rows, columns, and boxes - placements you made earlier create new ones.
2. Verify your notes: did you erase candidates after every recent placement? One stale pencil mark hides real singles.
3. Do a full digit-by-digit sweep for Hidden Singles - the most commonly missed move at every level.
4. Hunt for pairs you can use to thin the notes, then re-check for singles.
5. Only then look for fish patterns, digit by digit.

Whatever you do, don't guess - here's why guessing backfires. If you've run the full checklist twice and the grid still won't move, paste your position into the Sudoku solver as a last resort and study which technique it applies next; treating the solver as a teacher rather than an answer key turns a stuck puzzle into a lesson.

Practice Path

Each difficulty on Sudoku.by is tuned to train a specific layer of the skills above. Climb in order, and move up once you finish three puzzles in a row without hints:

• Easy - drills the scan and the three direct-placement moves until they're automatic.
• Medium - forces the notes habit and the digit-first scan behind Hidden Singles.
• Hard - trains pair-spotting and disciplined note maintenance under pressure.
• Expert - introduces grid-wide pattern reading with the X-Wing.
• Master - demands Swordfish and full-checklist discipline on every sticking point.
• Extreme - tests everything at once, with long droughts between placements that reward calm, systematic re-scanning.

Frequently Asked Questions