Frequently Asked Questions
Questions that come up often, about the puzzle itself and about how this site works.
Does every Sudoku puzzle have exactly one solution?
A properly formed Sudoku has exactly one solution, and every puzzle on this site is verified to meet that standard before it is shown to you. Grids with two or more solutions do circulate online, usually because they were produced by removing clues without re-checking uniqueness.
The Sudoku solver will tell you which kind you are holding: enter the clues from any grid and it reports one solution, none, or more than one.
Do I ever need to guess?
No. Every puzzle here can be finished by deduction alone using standard techniques. If you feel stuck enough to guess, there is a logical step available that has not been spotted yet — most often a hidden single or a pencil mark that was never erased after an earlier placement.
The techniques guide covers the deductions in the order they usually become necessary.
What makes a Sudoku puzzle hard?
Not the number of clues, which is the common assumption. Difficulty comes from which techniques the puzzle forces you to use. An easy puzzle can be solved by scanning alone; a hard one reaches a state where no single square can be resolved on its own and you must reason about pairs of squares or intersections between a box and a line.
Concretely, that means naked pairs and pointing pairs at hard, and the X-Wing at expert.
Is there a minimum number of starting clues?
Yes — 17. It was proven in 2012, by exhaustive computer search, that no Sudoku with 16 or fewer clues can have a unique solution. Puzzles at that limit are rare and extremely difficult; most published puzzles start with somewhere between 22 and 35 clues.
How is my score calculated?
You earn 10 points for each square you correctly fill in yourself. On completion a difficulty bonus is added, plus a speed bonus for every second under the par time for that level. Each mistake costs 50 points and each hint costs 75. Squares revealed by a hint earn no points.
Do I need an account to use the leaderboard?
No. When you finish a puzzle you can post your result with just a nickname. There is no sign-up, no email address and no password anywhere on this site. Your nickname is stored in your browser so you do not have to retype it.
See the current boards, or the privacy policy for exactly what a submission stores.
When do the leaderboards reset?
The daily board covers one UTC calendar day and resets at midnight UTC. The weekly board runs Monday to Sunday in UTC and resets at midnight UTC on Monday. Using UTC for both means every player competes on the same board regardless of time zone.
Does pausing stop the clock?
Yes. Pausing stops the timer and blurs the board so the puzzle is not visible while you are away. The game also pauses itself automatically when you switch to another browser tab, which keeps the recorded time honest.
Is my game saved if I close the tab?
An in-progress game is saved in your browser's local storage and restored when you come back. It never leaves your device. Clearing site data or using a private window will lose it.
Is this site free?
Yes, entirely. Running costs are covered by advertising. There is nothing to buy, and no feature is held back behind a payment.
Playing with a child? Try the smaller 4×4 and 6×6 boards. Prefer paper? Every level is available as a printable Sudoku sheet. Something not covered here? Get in touch — we read everything.