Sudoku Solving Techniques for Beginners Who Get Stuck

You breeze through the first 30 numbers of a medium sudoku, then hit a wall where every empty square seems to allow three or four digits. Most people start guessing at that point, and one wrong guess wrecks the grid ten moves later. A small set of sudoku solving techniques removes almost all of that guessing, and you can learn them in an evening.

I play a daily sudoku on my phone and have tested most of the popular apps for this site. The techniques below are the ones that actually come up in easy through hard puzzles. Expert-only tricks with names like "swordfish" exist, but you will not need them for a long time.

How a sudoku grid works

A standard grid has 81 cells in 9 rows, 9 columns and 9 boxes of 3 by 3. Each row, column and box must contain the digits 1 to 9 exactly once. A well-made puzzle has one solution and can be solved by logic alone. If an app's puzzle needs a guess, it is badly generated, and that is a good reason to switch apps.

Every technique here is a way of answering one question: where can this digit go, or what can this cell be?

Five sudoku solving techniques in order of difficulty

Learn these in order. Each one builds on the last, and the first two alone solve most easy and medium puzzles.

1. Scanning for a digit

Pick a digit, say 7. Look at each box that does not have a 7 yet. Check which rows and columns already contain a 7 elsewhere, and cross them out in your head. If only one cell in that box is left, the 7 goes there. Run through digits 1 to 9 this way, starting with whichever digit already appears most often on the grid.

2. Last free cell and naked singles

Look at a single cell and list every digit already in its row, column and box. If eight different digits are taken, the cell must be the ninth. This is slower than scanning, so use it on rows or boxes that are nearly full, with two or three cells left.

3. Pencil marks

When scanning stops working, write candidates. In each empty cell, note the small digits that are still possible. Most apps have a notes mode and many can auto-fill candidates. Once marks are in, look for any cell with a single candidate, or any digit that appears as a candidate in only one cell of a row, column or box. That second case is called a hidden single.

4. Naked pairs

If two cells in the same row each have exactly the same two candidates, for example 3 and 8, then those two digits must go in those two cells. You can erase 3 and 8 from every other cell in that row. The same works in columns and boxes, and with three cells sharing three candidates.

5. Pointing pairs

Inside one box, if a digit's candidates all sit in the same row, that digit must go in that row within this box. So you can remove it from that row in the other two boxes. This one feels abstract until you see it once, then you start spotting it everywhere in hard puzzles.

What each difficulty level actually needs

App labels are not standardized, but this is what I have found across the apps we test.

Notice that the number of givens is only a rough guide. A puzzle with 28 givens can be easier than one with 32 if the clues are well spread.

The pencil marks advice most beginners get wrong

A lot of guides tell beginners to fill in every candidate in every cell before placing a single number. I disagree. On an easy or medium puzzle, marking all 50 empty cells takes ten minutes and buries the obvious moves under a wall of tiny digits. You stop seeing the grid.

Scan first. Place every digit you can find by scanning, which usually fills 15 to 25 more cells. Only then add pencil marks, and only in the boxes where you are stuck. The marks will be fewer and the patterns easier to see.

The other habit I would drop is relying on the auto-candidate button from day one. It is convenient, but filling candidates yourself is how you learn to see them. Use it after a month, once the technique is in your hands.

Choosing a sudoku app that respects your time

Many free sudoku apps run an ad after every puzzle and some after every mistake. That breaks concentration and stretches a ten minute puzzle into fifteen. When we compare apps, we check four things: whether puzzles have unique solutions, whether notes mode is quick to toggle with one thumb, whether there is a no-penalty mistake setting, and how ads are placed. Our approach is explained in how we review casual games without the hype.

A good test is to play five medium puzzles in one sitting. If the app makes you watch more than two ads or push a subscription screen more than once, keep looking. Paying a one-time fee of $3 to $5 to remove ads is usually worth it if you play daily.

If you enjoy the logic side of sudoku, the same way of thinking carries over to other grid puzzles. Our guide to Minesweeper patterns that remove most of the guessing uses the same idea of ruling out options before committing.

A practice plan for the first two weeks

• Days 1 to 3: Easy puzzles only, using just scanning. No notes.
• Days 4 to 7: Medium puzzles. Scan first, then add pencil marks by hand in stuck boxes.
• Days 8 to 14: One hard puzzle a day. Each time you stall, search the grid for a naked pair before anything else.

Keep a note of your times. Most people cut their medium solve time roughly in half over these two weeks.

Frequently asked questions

Is it cheating to use pencil marks in sudoku?

No. Pencil marks are a standard part of solving, and competitive solvers use them on hard puzzles. They are simply a way of keeping track of what logic has already told you.

Can every sudoku be solved without guessing?

A properly made sudoku has one solution that can be reached by logic. Very hard puzzles may need advanced techniques, but not guessing. If an app's puzzles regularly force a guess, the puzzle generator is poor.

What is the fastest way to get better at sudoku?

Play one puzzle a day slightly above your comfort level and learn one new technique at a time. Scanning well is the biggest speed gain for most players, so practice it on easy grids until it feels automatic.