A puzzle of placement, not arithmetic
Sudoku uses the digits 1 to 9, but you never add them. They could just as well be nine letters. You are not solving sums; you are tracking which of nine symbols is still missing from a row, a column or a box, and where it could still fit.
The puzzle took its name in Japan and spread worldwide in the 2000s. Every grid on this site is generated with exactly one solution, so every cell can be reached by reasoning alone.
Two ways to find a digit
There are two basic questions, and good solvers switch between them constantly.
- What can go in this cell? Pick an empty cell and list which digits its row, column and box already contain. If eight of the nine are used, the last one goes there. That is a naked single.
- Where can this digit go? Pick a digit and a box, row or column that still needs it. Look at every empty cell in that group and cross off the ones whose row, column or box already holds that digit. If only one cell survives, the digit belongs there. This is a hidden single.
The second question finds far more placements early on. Say the top-left box is missing a 7. A 7 already sits in row 1 further right, and another 7 sits in row 2 inside the middle box. That rules out both of the top rows of your box. If a 7 in column 1 lower down also rules out one cell of row 3, only two cells remain. If one of those is already filled, the 7 has exactly one home.
Selecting a cell also lights up every copy of its digit, so selecting a 7 shows at a glance which rows and columns are blocked.
Using notes without drowning in them
Notes mode, toggled with N or the notes button, lets you pencil small candidates into a cell. When you place a real digit, this board removes that digit from the notes of every cell in the same row, column and box, so your candidates stay current.
A few habits keep notes useful:
- Do not note every cell at the start. On Easy and Medium, singles alone usually carry you through.
- Note a whole box or row at once when you are stuck, then look for a cell with one candidate left.
- Look for pairs. If two cells in a row can only be 3 or 8, those two digits are spoken for, and you can remove 3 and 8 from every other cell in that row.
- Watch box and line overlaps. If every candidate for 5 in a box sits in one row, the 5 for that row must come from that box, so you can clear 5 from the rest of the row.
Mistakes that cost the most
The board checks each digit against the solution. A wrong entry is flagged and adds to the mistake counter, which never ends the game but is worth reading.
- Placing on a hunch. If you cannot say why a digit must go somewhere, it is a guess. Leave it as a note instead.
- Forgetting the box. Players often check the row and column and skip the 3×3 box. Make all three checks a single habit.
- Leaning on hints too early. A hint reveals the correct digit for the cell you have selected. Use it when you have truly run out of ideas, then ask yourself why that digit had to be there.
Moving up the levels
The four levels differ by how many cells start blank: 30 on Easy, 40 on Medium, 50 on Hard and 55 on Expert. Fewer givens means fewer easy singles and more need for notes.
A sensible path is to play Easy until you can finish without a single mistake, using only scanning. On Medium, practise hidden singles by sweeping digit by digit from 1 to 9. Move to Hard once pairs and box overlaps feel natural, because that is where they start to matter. Treat Expert as the place to practise patience: fill notes carefully, keep them tidy, and expect long stretches where one small elimination unlocks the next.
The timer is only for your own pacing. A better goal than speed is a clean grid with zero mistakes and zero hints.
