Every properly built Sudoku has exactly one solution and can be reached by pure deduction. If you are guessing, there is a logical step available that you have not found yet.
That is worth stating up front, because the most common way people get stuck is not a missing technique — it is not knowing which technique they should be using at that moment.
Technique one: scanning
Pick a digit. Look at every place it already appears. Each one eliminates its row, its column and its box. Then look for a box where only one square is left that could hold it.
This is how you solve easy grids, and it is fast because you are working with a single digit at a time rather than tracking nine candidates per square.
The efficiency trick is ordering. Scan the digit that already appears most often on the board, because it eliminates the most squares. If there are six 4s placed and one 9, scanning for 4s will produce placements and scanning for 9s will produce nothing.
When scanning dries up
On a medium or hard grid you will reach a point where a full pass over all nine digits produces zero placements. This is not a signal to guess. It is a signal to change technique.
Technique two: candidate counting
Pick a single unit — one row, one column, or one box. For each empty square in that unit, work out which digits are still legal there.
Now invert the question. Instead of asking “what can go in this square”, ask “where can this digit go in this unit”. If a digit is legal in only one square of the unit, it belongs there.
This is the step most self-taught players never make, and it is the difference between finishing hard grids and stalling on them. The crucial part is that it works even when that square has several candidates of its own. The square might allow 2, 5 or 7 — but if 5 cannot go anywhere else in the row, the square is a 5.
Technique three: pairs
When counting candidates, watch for two squares in the same unit that both allow exactly the same two digits — say both can only be 3 or 8.
You do not know which is which, but you know those two digits are used up by those two squares. So 3 and 8 can be removed from every other square in that unit. That often unlocks a placement elsewhere immediately.
The same logic works for three squares sharing three candidates, though triples are much harder to spot and are rarely needed below expert level.
A working order
- Scan the most-placed digits until placements stop.
- Find boxes, rows or columns with only two or three empty squares and count candidates there — they are cheapest.
- Count candidates unit by unit, looking for digits with only one home.
- Look for pairs and eliminate.
- Go back to step one. Every placement makes scanning viable again.
Steps four and five matter more than they look. Sudoku solving is cyclical: a single hard-won placement usually re-opens three easy ones, and players who keep grinding at candidate counting after a breakthrough are doing unnecessary work.
Difficulty is about clues, not tricks
On our Sudoku the four levels differ in exactly one thing: how many digits you start with — 42 on easy down to around 24 on expert.
More clues means more squares that fall to simple scanning. Fewer clues means longer stretches where nothing is immediately determined and you have to count. No level requires a technique the others do not; expert grids just require you to use the slower one more often.
Every grid is generated fresh and checked for uniqueness before it is shown: digits are removed one at a time and any removal that would allow a second solution is undone. So when you are stuck, the answer really is there.
A note on writing in candidates
Pencil-marking every square with every candidate is a legitimate method and some very fast solvers use it. But it converts the puzzle into bookkeeping, and for most people it makes the game less interesting rather than easier.
The middle path — counting candidates in your head for one unit at a time, and only writing anything down when a unit is genuinely tangled — is where most of the enjoyment lives.