Common Sudoku Mistakes and How to Stop Making Them
By the Sudoku Sagax editors · · 7 min read
Most Sudoku mistakes are not arithmetic errors. They are habits, and habits can be changed once you can name them. Five of them cost almost every player time, and four can be dropped within a week.
Mistake 1: Guessing when you get stuck
The most expensive mistake in Sudoku is a digit you enter because it might fit. The problem is not the entry itself but what follows. You build twenty more digits on top of a guess, and when the contradiction shows up, you no longer know where it started. A puzzle you had to guess your way through was not solved, it was survived.
A properly built Sudoku never requires guessing. Every puzzle our generator produces has exactly one solution, and that solution can be derived step by step. When nothing moves, a technique is missing, not luck. The missing technique is rarely something exotic. Nine times out of ten it is reading the boxes systematically, which is mistake three below.
What helps instead: when you stall, stop looking for a digit and start looking for the cell with the fewest candidates. It is the same work, but it ends in a reason rather than an attempt.
Mistake 2: Starting your notes too late
Many players only start writing candidates once they are stuck. That is the most expensive moment to start. You have to fill in half the grid at once, you make errors doing it, and the notes are unreliable from the first entry onwards. Note from the beginning and the information is already there when you need it.
The second half of the same mistake is not maintaining the notes. A candidate list you forget to clean up after placing a digit elsewhere is worse than no list at all. It looks like knowledge and is not. In the app the notes update themselves when you place a digit; on paper you have to do it yourself.
In the example below one cell has a single candidate left. That is called a naked single and it is the simplest technique there is. Step through it:
Mistake 3: Reading only rows and columns
Sudoku has three kinds of unit: nine rows, nine columns and nine boxes. Players who get stuck have almost always neglected the boxes. That happens because rows and columns are visually obvious while a box takes a moment of deliberate attention.
The fix is an order you follow stubbornly. Scan all nine boxes for a single digit first, then the rows, then the columns. Do that three times and most easy and medium puzzles fall without a single note. And if it still stalls, the next step is a named technique rather than a shot in the dark.
The two techniques that pay off most after that are the naked pair and the hidden pair. Both clean up a unit far enough for the next single to appear by itself. An overview of all of them is on the solving techniques page.
Mistake 4: Moving up to hard too early
We have four difficulty levels, and they differ not just in the number of givens but in the techniques they require. The times we allow for three stars show the gap plainly: five minutes on Easy, ten on Medium, twenty on Hard and thirty five on Extreme. That is a sevenfold jump from the first level to the last, and it is not because anyone types more slowly.
Start on Hard and you will spend most of your time hunting for techniques you have not learned yet. It feels like thinking and it is groping. It works better to stay on one level until you get through it without stalling, and only then move up. Easy Sudoku is not a step backwards for that; it is the practice ground for exactly the scanning order from mistake three.
Mistake 5: Not analysing your own errors
When a digit turns out to be wrong, most players delete it and carry on. The error is gone, the cause is not. Almost every wrong entry belongs to one of three families: the digit was already in the box, it was already in the column, or the row only looked free because a note had not been cleaned up.
It pays to name which of the three it was, every time. After ten puzzles you know which unit you systematically overlook, and that is the only piece of information that actually changes anything. With us it costs stars as well, and faster than most people expect: a single mistake costs the third star, on every level. The second star comes with a budget, and that budget grows with difficulty: two mistakes on Easy, three on Medium, four on Hard and six on Extreme. One more than that and you are left with one star.
What to do when a contradiction appears
Sooner or later a row holds two sevens, or a cell has no candidates left. The first reflex is to delete the digit you just entered. That is almost always wrong, because the error usually sits several steps earlier. The last digit was merely the one that made it visible.
A better approach is to check backwards by unit rather than by time. Take the row, the column and the box in which the contradiction appears and check every digit in them against the givens. Nine times out of ten the wrong entry is inside those three units, because a mistake can only take effect where it was written.
What helps here is the habit from the previous section. Maintain your notes and you see the contradiction earlier: a cell with no candidates stands out long before a digit appears twice in the grid. And if you want to avoid one wrong entry costing you the whole puzzle, play a round one level down until the scanning order is automatic. Our eleven puzzle types share that property: solve Kakuro or binary puzzles cleanly and the checking routine transfers straight back to Sudoku.
Frequently asked questions about Sudoku mistakes
Why do I keep making the same mistakes in Sudoku?
Because a Sudoku mistake is rarely a slip of arithmetic. It is a skipped check. If you do not scan the boxes systematically, you miss the same kind of collision again and again. Name the unit behind each mistake for your next ten puzzles and the pattern shows up.
Do you ever have to guess in Sudoku?
Not in a correctly built puzzle. Any puzzle with exactly one solution can be derived logically. When nothing moves, a technique is missing, and usually one of the simple ones.
When should I start writing candidates?
As soon as one full pass through boxes, rows and columns produces no new digit. Starting later costs more time than you saved by skipping it.
Do mistakes matter if I only play for fun?
Not for the fun. For progress they do, because an unnoticed wrong entry makes the rest of the puzzle unsolvable, and you spend your time hunting the contradiction instead of solving.