How to play
First, create a new puzzle. You can do this by pressing 'Random Puzzle' or 'Quick Puzzle', or you can type the starting numbers in from another puzzle, e.g. one from a book or newspaper. Follow the instructions in the right-hand column for typing in your own numbers.

To solve the puzzle, type numbers into the empty squares so that each horizontal row and vertical column contains the numbers 1 to 9 (i.e. with no numbers repeated). The difficult bit is that each 9-square rectangle (the areas surrounded by darker lines) must also contain the numbers 1 to 9, with no repetitions. To see an example of a completed puzzle, press 'Solve Puzzle'. 

If 'Show errors' is checked and you type an incorrect number, it will be shown in red (note - this only works with single-solution puzzles).
Tip - after typing a number, press Tab, or click elsewhere on the page, to make sure the number is checked.

You can also press 'Check my answer'. It'll tell you whether your answers so far are correct, without revealing the rest of the puzzle.

Basic tips for solving have now been moved to the new (and expanded!) How to solve sudoku page. Also see the 'How to solve' section at http://www.sudoku.com/.

Using candidate lists. A key technique in sudkou solving is writing the candidate (possible) numbers for each empty square in the corner of the square. You can do this in this page, by clicking in the top-left corner and typing the numbers in.

In manual mode it's up to you to maintain these lists, just like a printed puzzle. In automatic mode, the page calculates the lists for you, updating them whenever you solve a square. That's not 'proper' solving, of course, but is useful for practising pattern recognition.

Tip - Clicking 'automatic' on a new puzzle does the equivalent of crosshatching the entire puzzle. Fill in any single-candidate squares, click 'Manual', and maintain your lists from there. (Click the link to 'How to solve sudoku', above, for details of crosshatching, single-candidate squares etc.)

Generating puzzles.
This page features the all-new MKII puzzle generator, which actually works a bit like the serious ones (i.e. it doesn't depend on the 'infinite monkeys' theory of random content generation). It's still 'random' though, in that it uses random numbers to fill the starting squares.

Puzzles generated by the 'Random Puzzle' button have just one solution, and a symmetrical layout. You can choose from 36 down to 22 starting squares. The difficulty rating of a puzzle is unknown until it's been generated, but choosing fewer squares will tend  to create harder puzzles.

The 'Quick puzzle' button generates a single-solution puzzle with between 29 and 36 starting numbers, in a non-symmetrical layout (some people consider non-symmetrical puzzles not 'proper' sudoku, while others think they're a bit of a walk on the wild side!). Quick puzzles are almost all rated 'easy'.

If you type your own puzzle in, or import it via the Import/Export button, then you can find out if it has more than one solution by clicking 'Check'. 

To be solvable entirely by logic (no guesses), a puzzle must have only one possible solution. Please note - puzzles rated 'outlaw' by this page may still require a guess, even though they have just one solution. All other puzzles generated by this page's 'Random Puzzle' button are guaranteed to be solvable by logic alone.

Have fun!


 

Tip: Use Firefox to view the solver - it shows the numbers as they're being calculated.

Tip #2: Press Tab after you type each number (or just click somewhere else on the page) - this will check your data, and update auto candidate lists if you're using them.

Tip #3: When importing and solving really tough puzzles, your browser may show 'Script running slowly' messages. Choose 'No' (IE) or 'Cancel' (Firefox) to let the solver keep on running. It'll get there in the end!

Links:
Sudoku.com. The 'official' sudoku site - puzzles, tips, links.

The Daily Sudoku. A new puzzle every day, plus lots of links.

Daily Telegraph Sudoku . Puzzles and links, plus an excellent guide to solving techniques.

Sudoku Solver. This Excel-based solver gave me the idea for the solver in this page - thanks!

Mark Huckvale's Sudoku Workpad. Puzzle generator and solver, and the inspiration for the Seed Puzzle generator on this page.

How the solver in this page works for anyone who's interested.

 

 

Switching symbols?

 

The big secret about sudoku is that it isn't really a number puzzle at all (it's about logic). The characters 1 to 9 are a convenient set of symbols that we all recognise, but they have no mathematical significance in this context.

 

Sudoku works just as well using shapes, colours - even vegetables ('each row must contain a carrot, onion, potato...."). Any set of nine distinct entities will do.

 

We can't stretch to vegetables here at paulspages, but we can manage a couple of alternative symbol sets. The alphabetical ones are fairly easy, but the punctuation symbols are harder, mainly because we don't have a pre-conceived order for them (unlike 'ABC..'), so it's harder to work methodically through them.

 

To switch symbol sets, just select a different set from the drop-down list. You can do this at any time, without losing the current puzzle. (Note: manual candidate lists won't be converted - sorry!)

 

Alternative symbol sets also work on the printer page, and import/export.

 

Go on - try it if you're tough enough!