Getting better at Sudoku
Almost everyone who plays Sudoku for a while hits the same wall: the easy digits go in quickly, and then the grid just sits there. The good news is that getting past it is not about being cleverer or faster — it is about having two or three specific things to look for. Most people only ever discover the first one by themselves.
Scan by digit, not by cell
The instinctive approach is to stare at an empty square and ask “what could go here?” It works, but it is slow, because a single empty cell usually has four or five candidates.
The faster habit is to pick a digit and hunt for where it has to go. Take the sevens. Look at each box in turn and ask where a seven could possibly fit — often the rows and columns already containing sevens rule out every square but one. This finds far more than cell-by-cell searching, and it is what experienced solvers spend most of their time doing.
The two kinds of “only one”
These sound similar and are genuinely different, and knowing both is the single biggest step up available.
A naked single is a cell with only one candidate left, because the other eight digits already appear in its row, column or box. Everybody finds these.
A hidden single is the other way round: a digit that has only one possible home in a row, column or box — even though that cell could hold other digits too. Suppose a row has four empty cells, and three of them sit in columns that already contain a 4. The fourth cell must be the 4, whatever else it could also have been.
Hidden singles are where most stalled grids unlock, and they are invisible unless you go looking digit by digit. If you only take one thing from this page, take this one.
Locked candidates
Slightly more advanced and worth the effort. If the only places a 7 can go inside one box all sit in the same row, then wherever that 7 ends up, it is somewhere along that row — so the 7 can be crossed off every square in that row outside the box.
This one does not place a digit directly. It clears candidates away, which almost always makes a single appear somewhere else a moment later.
Pencil marks, used sparingly
Filling every candidate into all fifty-odd empty cells takes ten minutes and is rarely necessary. It is usually better to scan first and only mark up the region where you are stuck.
Note that marks here are yours to manage — writing a 7 into a cell does not rub 7 out of the marks around it. That is deliberate: doing that automatically would be performing the deductions on your behalf, which is most of the puzzle.
When you are stuck, change the question
Almost every stall is one of two views being tried and the other not. If you have been asking “what goes in this cell”, switch to “where does this digit go in this box”, or the reverse. It is remarkable how often the answer was sitting there in the other view.
The other reliable trick is to look at the most crowded box, row or column on the grid. Wherever there are most clues, there are fewest possibilities left.
You will never need to guess here
This is worth saying plainly, because it is not true everywhere. Plenty of Sudoku labelled “expert” cannot actually be finished by reasoning — past a point the only way on is to pick a cell, guess, and find out twenty moves later whether it worked.
Every puzzle here is built so that never happens. Digits are removed from a completed grid only while what remains can still be solved by the three techniques above, and a grid is only dealt if a checking solver can finish it using nothing else. If you are stuck, there is something findable on the board — not a coin to flip.
That also guarantees exactly one solution, since every digit the checking solver placed was forced rather than chosen.
Common questions
Do I need to be good at maths? Not at all. The digits are just nine different symbols — the puzzle would work identically with nine colours. There is no arithmetic anywhere in it.
How many clues does a Sudoku need? It was proved in 2012 that no puzzle with a unique solution can have fewer than 17 clues. Puzzles here sit in the low-to-mid twenties, which is comfortably enough to be solvable by reasoning while leaving real work to do.
Is there always only one answer? Yes, always — see above for why that comes for free from the way the grids are made.
Does everyone get the same grid? In the daily, yes. Free play generates a fresh puzzle whenever you want one.