How to use the solver
- Copy your puzzle in. Tap a square, then a number on the pad. On a computer, click the first square and type: each number moves you on to the next square, and 0 or a dot leaves a blank, so you can type the puzzle row by row. The arrow keys move you around and Backspace clears a square. Or tap Paste a puzzle and paste all 81 squares at once.
- Press Next step. The square it fills lights up, the row, column or box it depends on is shaded, and the panel above the grid names the technique and says why the number must go there.
- Keep going, or see the whole route. Press Next step again for each move, or Solve all, step by step for a numbered list of every step. Tap any step in the list to see the grid at that point.
- Turn on Pencil marks to see every number each empty square could still hold. When a step rules numbers out, they show crossed out in small print.
- Check my puzzle tells you whether it has no solution, more than one, or exactly one, and how hard our grader rates it.
The technique ladder
The solver always uses the easiest technique that works, and only climbs the ladder when the easier ones find nothing. These are the rungs, easiest first. They are the same techniques that grade every daily puzzle on Suvi, so the level a puzzle gets (easy, medium, hard or expert) is simply the highest rung it needs.
Naked single (easy)
A square where eight of the nine numbers already appear in its row, column or box, so only one is left. The solver shades all three, so you can see where the other eight are.
Hidden single (easy)
A number that has only one possible square left in a row, column or box, even though that square could hold other numbers too. The solver shades the row, column or box it's about. Most first moves in a real puzzle are hidden singles: pick a number and visit each box in turn.
Naked pair (medium)
Two squares in the same row, column or box that can each hold only the same two numbers. Those numbers must go in those two squares, whichever way round, so you can cross both numbers off every other square in that row, column or box.
Pointing (medium)
Inside one box, every possible square for a number lies in the same row (or the same column). The number has to land on that stretch, so cross it off the rest of that row outside the box.
Box-line (medium)
The reverse of pointing: in one row (or column), every possible square for a number lies inside the same box. The box's copy of that number must be on this line, so cross it off the other squares of the box.
Hidden pair (hard)
Two numbers that can each go in only the same two squares of a row, column or box. Those two squares must hold those two numbers, so every other candidate can be crossed off them. It is the mirror image of a naked pair, and harder to spot because the squares look busy.
X-wing (hard)
Pick one number. If two rows each have exactly two places for it, and those places are in the same two columns, the number sits in opposite corners of that rectangle. Either way round, both columns get their copy from these two rows, so cross the number off everywhere else in those two columns. It works the same with rows and columns swapped.
When logic stalls (expert)
Some puzzles can't be finished with the techniques above, and Suvi's expert dailies are exactly those. When that happens the solver says so rather than guessing behind your back, and offers the answer from a backtracking solver: a program that tries a number, carries on, and backs up when it hits a clash. People do the same thing more carefully: pick a square with only two candidates, try one, and see whether it breaks. There are bigger techniques too (triples, the swordfish, chains) that this solver doesn't use.
Where each step comes from
The solver is the same code that grades every Sudoku on Suvi and writes the game's hints, so the sentence you read here is the one the game's Hint button would show. Each step is worked out fresh from the grid in front of it: the easiest technique that leads to a number being placed. It runs on your device, and the puzzle you enter isn't sent anywhere.
We graded 4,000 of our own daily puzzles with it to see how often each technique is really needed: here's what we found.
More to play
- Sudoku: a new puzzle at four levels every day, with pencil marks, undo and a hint that explains itself.
- Killer Sudoku: sudoku with cages that add up, no starting numbers.
- Sudoku techniques: the full guide, with worked examples.
- Word unscrambler: our other free tool, for when the word puzzles get the better of you.
Questions
- Is the sudoku solver free?
- Yes. It's free, with no sign-up and no download, on a phone, tablet or computer.
- Does it just give me the answer?
- No. It shows one step at a time: the square, the technique and the reason, so you can learn the move and spot it yourself next time. If you only want a nudge, press Next step once and carry on by yourself.
- Which techniques does it know?
- Naked and hidden singles, naked pairs, pointing, box-line, hidden pairs and the X-wing. That is enough for every easy, medium and hard puzzle on Suvi. Expert puzzles need more, and the solver tells you when it gets there.
- Why does it say logic stalls?
- Because none of its techniques can place another number. The puzzle can still have exactly one solution; it just needs a guess or a bigger technique at that point. Press Show the answer to see the rest, filled in by a backtracking solver.
- Can I paste a puzzle?
- Yes. Tap Paste a puzzle and paste the 81 squares row by row: digits 1 to 9, and 0 or a dot for each blank. Spaces and line breaks don't matter.
- Why does it say my puzzle has more than one solution?
- A proper sudoku has exactly one. If yours has more, a number is probably missing or mistyped. A puzzle with fewer than 17 numbers can never have just one solution.
- Is my puzzle sent anywhere?
- No. The solver runs in your browser and the puzzle never leaves your device.
Who made it
Sudoku solver is a free tool from Suvi, written and tested by Brasslight Games. It runs in your browser; nothing you type is sent to us.