Frequently Asked Questions
Everything players ask about solving Sudoku, Kakuro, and Killer Sudoku — plus how Sukuro itself works. Each answer starts with the short version.
Sudoku
- What are the rules of Sudoku?
- Fill the 9×9 grid so that every row, every column, and every 3×3 box contains each digit from 1 to 9 exactly once. The starting clues can never be changed, and a proper Sudoku has exactly one solution. No arithmetic is needed — Sudoku is pure logic.
- What is a naked single in Sudoku?
- A naked single is a cell where only one digit remains possible, because the other eight digits already appear in its row, column, or box. It is the most basic solving technique: find the cell, place the digit, repeat. Scanning for naked singles is usually the first thing to do on any board.
- What is a hidden single in Sudoku?
- A hidden single is a digit that can go in only one cell of a row, column, or box — even though that cell might allow other candidates too. For example, if 7 is blocked everywhere in a box except one cell, that cell must be 7. Hidden singles are found by scanning where each digit can still fit, rather than looking at one cell at a time.
- What is an X-Wing in Sudoku?
- An X-Wing is an advanced technique for very hard Sudoku. If a digit is restricted to the same two columns in exactly two rows, those four cells form a rectangle, and the digit must occupy opposite corners — so it can be eliminated from those two columns everywhere else. The same logic works with rows and columns swapped.
Kakuro
- What is the 45-rule in Kakuro?
- Every complete row, column, or 3×3 region of nine cells containing the digits 1–9 sums to 45, because 1+2+…+9 = 45. In Kakuro you use it on overlapping runs: if a group of runs covers nine cells except one, the missing cell equals 45 minus the sum of the surrounding clues. It is the single most powerful shortcut in Kakuro.
- How do Kakuro sum combinations work?
- Each Kakuro run must reach its clue sum using digits 1–9 with no repeats inside the run. Short runs with extreme sums have very few options: a 2-cell run summing to 3 must be {1,2}, to 4 must be {1,3}, to 16 must be {7,9}, and to 17 must be {8,9}. Starting from these forced combinations and intersecting runs is how every Kakuro is cracked.
- Which digits can a 2-cell Kakuro run summing to N use?
- Any pair of different digits a and N−a, both between 1 and 9. So the possible sums range from 3 (1+2) to 17 (8+9), and a sum can never be twice one digit — 10 can be {1,9}, {2,8}, {3,7}, or {4,6}, but never {5,5}. The fewer valid pairs a sum has, the more useful the clue.
Killer
- What are cages in Killer Sudoku?
- Cages are the dotted-line regions overlaid on the grid, each with a small target sum in its corner. The digits inside a cage must add up to that sum, on top of the normal Sudoku rules for rows, columns, and boxes. There are no given digits in Killer Sudoku — the cage sums are the only clues.
- Can a digit repeat inside a Killer Sudoku cage?
- No. In standard Killer Sudoku a digit may appear at most once within a cage, even when the cage spans more than one row, column, or box. This is a real constraint you should exploit: it eliminates many combinations that would otherwise fit the sum.
- How do I find cage combinations, e.g. 23 in 3 cells?
- List the sets of distinct digits 1–9 that reach the sum. A 3-cell cage totalling 23 has only one option: 6+8+9. Likewise 24 in 3 cells must be 7+8+9, and 6 in 3 cells must be 1+2+3. Cages with a single combination are the best starting points, and the 45-rule on rows and boxes narrows the rest.
About Sukuro
- Is Sukuro free, and do I need an account to play?
- Yes — playing Sudoku, Kakuro, and Killer Sudoku on Sukuro is free, and you do not need an account to start solving. A free account adds extras like saved progress across devices and PDF downloads. The paid Professional plan is optional.
- Does Sukuro work offline?
- Yes. After the first load, Sukuro keeps working without an internet connection, so you can solve on a plane or on the subway. Online-only features, such as syncing progress to your account, catch up the next time you are connected.
- Is there a large-print mode for seniors?
- Yes, and it is free. Large Print mode enlarges the grid, digits, and clues for comfortable reading, and the setting is remembered between sessions. It works for all three puzzle types and pairs well with the printable PDFs.
- What does the AI coach do — and does it give away answers?
- The AI coach looks at your exact board and explains the next logical step: which cell to focus on, which technique applies, and why the deduction holds. It never reveals the full solution — the point is to teach you to see the move yourself, so the next puzzle gets easier. You stay in control of every digit you place.
- Can I print puzzles from Sukuro?
- Yes. Any board can be downloaded as a clean, printer-ready PDF — including the digits you have already entered, printed in blue and distinct from the black givens. Downloading a PDF requires a free account, and the file never spoils the solution.
- What does the Professional plan add?
- Professional is Sukuro’s paid tier at $3.99 per month or $23.99 per year. It unlocks the full experience on top of the free game — including unrestricted use of the AI coach. The puzzles themselves stay free to play either way.
- What languages does Sukuro support?
- Eight languages: English, Japanese, German, French, Spanish, Hebrew, Simplified Chinese, and Traditional Chinese. The whole interface, including the AI coach’s explanations, is available in each of them. Hebrew is fully right-to-left.
- Is there a Sukuro mobile app?
- Mobile apps are coming — a native iOS app is currently in TestFlight testing. In the meantime, the website works well on phones and tablets, including offline after the first load. Your account will carry your progress into the apps.