What this is
Fill a 9×9 grid with the digits 1 to 9. There is one rule:
No digit may repeat within a row, a column, or a 3×3 box.
The digits printed at the start are fixed. Fill every remaining square correctly and you are done. Your score is the time taken.
There is always exactly one answer. Squares are removed one at a time while checking that the puzzle still has a single solution, and any removal that would allow a second answer is undone. You never have to guess.
Controls
| Select a square | Tap / click, or arrow keys |
| Enter a digit | Tap the keypad below, or press 1–9 |
| Erase | ⌫ on the keypad, or Backspace |
Selecting a square highlights every matching digit on the board. You can see at a glance where else that digit can go, which cuts out most of the scanning.
A square that breaks the rule turns red immediately. Being told at the end that something is wrong leaves you no way to work out where it started.
How to solve
Only one digit fits (naked single)
Pick a square and look along its row, its column and its box. Eliminate every digit already used; if exactly one remains, that is the answer. Easy boards need nothing else.
The digit has only one home (hidden single)
Work the other way round. Pick a box and ask “where can the 7 go in here?” If there is only one square that can take it, that square is a 7 — even if that square could also hold several other digits.
This is what beginners miss most. Looking only square-by-square hides it.
Scan along the lines
Pick one digit and find every instance of it on the board. Trace the rows and columns it occupies; in the boxes those lines cross, that digit is excluded from the covered squares. When a box has only one square left, it is settled.
Running this for 1 through 9 in order usually fills several squares per pass.
Where a box meets a line
If, inside one box, a digit can only go in squares that share a single row, then that digit can be eliminated from the rest of that row outside the box. You can use this before knowing which square it actually is.
Change digits when stuck
Grinding on one square costs the most time. If the 3s are stuck, look at the 4s. Filling other digits shrinks the candidates where you were stuck.
Getting faster
Skip pencil marks. Writing every candidate is accurate but much slower. These boards leave 45 squares empty and are solvable with hidden singles and scanning alone.
Use the matching-digit highlight. Selecting a square lights up its digit everywhere. When you work digit by digit, this removes the visual search entirely.
Never ignore a red square. Fix it the moment it appears. Leaving it invalidates everything you fill afterwards.
Daily challenge
A new board opens every midnight (KST) and everyone solves the same one. Only uniquely-solvable puzzles are served, so luck never enters into it.