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Sudoku Solver — step by step

Free Sudoku solver online. Paste any 9×9 puzzle, check uniqueness, and get every next move explained with the real technique name — not a silent fill-in.

Instant solve Named techniques Uniqueness check 100% free

0 / 81 clues

01

Paste any puzzle

81-char string, dots or zeros for empties. Or click cells and type. Works on phone and desktop.

02

Hint with technique name

Every suggestion is a human move — Naked Single, Hidden Single, Pointing Pair, X-Wing — with a plain-English why.

03

One-click Solve

Need only the answer? Solve fills the unique completion and flags boards with zero or multiple solutions.

04

Learn as you go

Jump from any hint into the techniques library for annotated examples of the same pattern.

Watch it work

The Hint button never lies.

Every move this Sudoku solver suggests is one a human could plausibly find — never a silent brute-force dump. Twelve seconds of the engine reading a board and pointing at the next step.

Try the solver →

What is a Sudoku solver?

A Sudoku solver is a tool that takes a partially filled 9×9 grid and either completes it or shows the next logical placement. Most online solvers only dump the finished grid. Zeloku does both: a one-click Solve for when you need the answer, and a Hint engine that names the technique — so you learn the pattern instead of just copying digits.

How to use this free Sudoku solver

  1. Enter the puzzle. Click any cell and type 1–9, or paste an 81-character string (use 0 or . for empty cells). Click Load sample puzzle if you just want to see the engine work.
  2. Ask for a Hint. The solver scans for the cheapest human technique that places a digit, highlights the cell, and explains why.
  3. Or press Solve. Uniqueness is checked first. If the board has exactly one solution, every empty cell fills in sage green so you can tell givens from answers.
  4. Learn the move. Open the matching page in the techniques library and study an annotated example of the same pattern.

Techniques this Sudoku solver knows

Hints are attempted in pedagogical order — easiest first — so the move you see is usually the one a careful human would find next:

  • Naked Single — only one candidate left in a cell
  • Hidden Single — a digit has only one legal home in a unit
  • Naked Pair — two cells share the same two candidates
  • Pointing Pair — a box locks a digit into one line
  • X-Wing — a rectangular digit pattern that eliminates candidates

Harder patterns (Swordfish, full chain reasoning) fall through to Solve when no simple step remains. The engine will say so rather than invent a guess.

Sudoku solver vs. playing the daily puzzle

Use the Sudoku solver when you have a puzzle from a newspaper, app or friend and want either the answer or a nudge. Use the daily Sudoku when you want a fresh, rated board with a timer, mistake tracking and a win card. Same engine underneath; different job.

Why Zeloku's Sudoku solver is different

  • Explains, doesn't just fill. Technique name + plain-English reason on every Hint.
  • Uniqueness-aware. Flags zero-solution and multi-solution boards instead of silently lying.
  • Runs in your browser. No account, no upload of your puzzle to a server queue you hit an API yourself.
  • Tied to a real library. Every technique the solver names has a long-form page with diagrams.
  • Free, forever. No freemium gate on Solve or Hint.

Tips for getting better with a solver

  • On a stuck board, take one Hint, write the digit yourself, then try to find the next move unassisted.
  • When a technique name is new, open its library page before taking the next Hint.
  • If Solve reports multiple solutions, your givens are incomplete or mistyped — fix the source puzzle, don't trust the fill.
  • For learning from scratch, start with how to play Sudoku and an Easy board, then come back to the solver when you stall.

FAQ

Sudoku solver — common questions

How does this Sudoku solver work? +

Zeloku first checks that your givens are valid and that the puzzle has a unique solution (backtracking with MRV ordering). The Hint engine then walks pedagogical techniques in order — Naked Single, Hidden Single, Naked Pair, Pointing Pair, X-Wing and more — so every suggested move is one a human could find.

Is this Sudoku solver free? +

Yes. The full solver — paste, hint, solve, uniqueness check — is free forever. No account, no ads, no daily limit.

Can the solver explain each step? +

Yes. Press Hint and the engine returns the next logical placement plus the technique name and a plain-English explanation. Press again for the step after that. Solve fills the rest in one click when you only need the answer.

What puzzle formats can I paste? +

Any 81-character string of digits. Use 0 or a dot (.) for empty cells. Spaces and line breaks are stripped automatically. You can also click cells and type 1–9 by hand.

What if my puzzle has no solution or multiple solutions? +

The solver detects both. No-solution boards show an error. Multiple-solution boards warn you and still display one valid completion — a well-formed Sudoku should always have exactly one solution.

Does the Sudoku solver use guessing? +

For the full Solve button, a fast exhaustive search finds the unique completion. For Hint, the engine never guesses — it only surfaces named human techniques. If no simple technique applies, it tells you so instead of planting a trial digit.

Can I use this solver to learn Sudoku? +

That's the main reason it exists. Use Hint on a stuck board, read the technique name, then open the matching page in the techniques library to see annotated examples. Over a few sessions the patterns stick.

Techniques library

Every move the solver names, explained.

Daily Sudoku

A fresh rated board every day.

How to play

Rules + first two techniques.

Hardest Sudoku ever

Paste Arto Inkala's 2012 grid into the solver.

More free Sudoku to play