DKM Sudoku Online Help

How To Play

  • Click the "Menu" button, then select the "New Puzzle" menu item and select a difficulty level from the dropdown menu.
  • Fill in blanks with the Pen (select a cell, then click on the number you want to enter).
  • If you're not 100% sure of a number, you can make a note of it in the cell using the Pencil. To do this, click the "Pencil" icon to be placed in "Pencil mode".
  • You may enter as many pencil marks as you like in each cell.
  • To change back to "Pen" mode, click the "Pen" icon. You can also switch between pen and pencil modes quickly by pressing the "P" key or by using the mouse wheel. (See tips below.)

Input Tips


If you prefer to use the keyboard rather than the mouse, you may:
  • Use the arrow keys to navigate around the grid.
  • Use the number keys to enter a number in the highlighted cell.
  • Press the space or Delete key to remove any numbers from the highlighted cell.
  • Press the "P" key to toggle between Pen and Pencil or use the mouse wheel (if not in Active Number mode - see below).
  • Press the "U" key to undo last input.
  • Press the "C" key to change all cells with one possible (pencil mark) to pen.
  • Hold Shift key down to always enter in pencil (not operable with the number pad).

Other Tips

  • Click the Undo icon (yellow arrow) to undo last input. You can undo all the way back to the beginning.
  • Holding the Shift key when clicking on a number in the input panel will pencil that number in. You may find this more convenient than toggling between the Pen and Pencil icons.
  • To remove just one pencilled number in a cell, enter that number again.
  • To remove all pencil numbers in a cell, either click the eraser button or simply "pen" the final choice in.
  • To remove all pencil marks from all cells, use the menu option under "Edit".
  • There is an alternate mode of number input via the mouse called "Active Number Mode" (see below).

Active Number Mode

If you select this mode (via Preferences menu option), then the number you click from the Number Pad becomes the "active number". The mouse cursor will change to this number and clicking on any cell will enter it in that cell. This is the reverse of the "cell select" mode, where you highlight a cell and then select the number to input. Those that prefer to solve by focusing on one specific number at a time will find this mode much more convenient.

To change the active number, select a new one from the Number Pad or scroll the mouse wheel down/up to increase/decrease the number. (If not in Active Number Mode, the mouse wheel will toggle between pen and pencil.)

In Active Number mode, all pencil functionality remains the same as does the ability to use the keyboard.


The scoring system attempts to provide a uniform measure of your performance across all puzzles, including the ones you input manually from other sources. The score is calculated when you complete the puzzle (unless ineligible - see section below) according to the following formula:

Score = Rating + TimeBonus - Accuracy - SolveAids

  • Rating = the rating index assigned to each puzzle. This index attempts to measure the difficulty of the puzzle by examining at each stage of the solving process how many cells can be solved by logic relative to the number of unsolved cells. It also looks at what techniques are required to solve those cells. Those that rely on more sophisticated techniques greatly increase the rating index. The current difficulty level (Easy, Medium, Hard, Tough) is only based on the techniques required to solve and you may now find that some Easy puzzles have a higher rating than other Medium ones or similarly for Medium/Hard/Tough.
  • TimeBonus = extra points for completing the puzzle quickly. In fact, time is still the most important factor determining your score. The time bonus is computed by taking the average time per difficulty level divided by your time and then multiplied by the rating index. For example, the average time for Medium puzzles is currently set at 11 minutes (660 seconds) so if you complete a puzzle with a rating of 1100 in 6 minutes, you will score a time bonus of (660/360 * 1100), ie 2017. If you completed the puzzle in 20 minutes (1200 seconds), your time bonus would be (660/1200 * 1100) ie 605.
  • Accuracy = the number of errors you make. Errors are only recorded when you enter an incorrect value and then enter a value in another cell. So, if you accidentally enter an incorrect number and immediately correct it, no error will be recorded. For the first error you lose 30 points, for each subsequent error the points lost increases by 10. So you would lose 40 points for the second error and so on. Note that pencil marks do not affect the scoring in any way.
  • SolveAids = each time you use a hint or clue or check your input. You lose 40 points for each hint, 50 for each clue and 20 each time you check your input. Warning! If you set "Auto Show Errors" preference, it will give you a check penalty after each input.

Scoring Ineligibility

If any of the following are true, you will not receive a score for that puzzle.
  • You use the "Solve", "Solve One Cell" or "Possibles .." options from the Solve menu.
  • You save the puzzle and then reload.
  • You print the puzzle.
  • You do not complete the puzzle.


Statistics are now recorded for those users who have registered and logged in. Roughly the last 100 scores will be recorded and the stats will show your average score, highest score and your Rank Index. This index is the average of the last twelve scores discarding the highest and lowest. The stats will be displayed whenever you complete a puzzle eligible to be scored (see above).

Description Of Menu Options

New Puzzle Menu

Main options to do with generating new puzzles.

The first four options will generate a puzzle at four different difficulty levels.
The first two levels - Easy and Medium - will generate a new puzzle on demand.
The third level - Hard - will generate a new puzzle every hour and will require more advanced strategies to solve.
The fourth level - Tough - will generate a new puzzle every day and provide even more challenging puzzles.

Note: All puzzles are solvable by pure logic - no guessing required!

Archived Tough
This will give you a list of recent Tough puzzles in case you wish to play more.
Input External Puzzle
This option allows you to manually enter a puzzle that you may have got via a newspaper or elsewhere. Enter all the fixed numbers, then click the Done button. The puzzle will then be validated to see that it is solvable, and if so, you can then solve in the same manner as for generated puzzles. All the solve options will be available to help you.

Edit Menu

Undoes the last input. You can undo right back to the beginning of the puzzle (unless you loaded a partially solved one). There is an Undo button which performs the same task, also the "U" key is a shortcut to this option.
Clear Possibles
This will clear all possibles displayed in the grid.
Clear All Input
This will clear all input (pen and pencil marks) displayed in the grid.
Pencil Marks to Pen
This option will convert all cells that have one pencil mark to a pen mark. The "C" key is a shortcut key for this option.

Solve Menu

Main options to aid in solving puzzles.
Check My Solution
Checks to see if you have solved the puzzle correctly. Any errors made will be shown in red.
Suggests which cell may be the next easiest to solve. It will not fill in the value - for that you need to select the "Solve One Cell" option.
If a cell is solvable via basic solving techniques, then the clue will suggest you look where a particular number is located in a block, row or column and will highlight that area of the grid. This helps you more than the "Hint" option and is the easiest aid you can ask for, save getting the actual answer.

If the only way to solve a cell is via more advanced solving techniques (as found in Hard and Tough puzzles), then the clue will point you to the cell and say what technique(s) is/are required. This is not as helpful and you may need to resort to the "Solve One Cell" if still stuck.

Solve One Cell
This is a very powerful option, especially as a learning tool. It will fill in one of the cells and displays the reason why that number is the answer to the cell. In the case of advanced techniques, it will walk through the steps to solve the cell. Sometimes there may be additional steps shown which are not necessary for the solving of that cell but they offer a useful practice tool.

It is recommended you do the Sudoku Tutorials first to understand the terminology used and an explanation of the solving strategies.

Possibles Basic 1
This option will show you the possible numbers for each unsolved cell using the most basic logic technique ie it looks at what numbers are already present in the same block, row and column.
(NOTE: In the non-Flash based version, there is only one Possibles menu option. It will show you the possibles relative to the puzzle difficulty selected initially.)
Possibles Basic 2
This option, besides basic elimination, also looks at each possible number in each cell and sees if this number can be placed anywhere else on the same row/column/block. If not, the possibles for that cell are reduced to just this number (ie it is the solution for that cell). This strategy is referred to as "OnePlace".
Possibles Hard
This option goes further by using more advanced solving strategies such as "Naked Pairs" and "Box-line Pointing" in determining possible numbers for each unsolved cell. These strategies are required to solve "Hard" puzzles.

For an explanation of these strategies, please consult the online tutorials.

Possibles Tough
This option goes one step further using more complex strategies such as "X-Wing" and "Y-Wing".
Solve the puzzle and shows the completed solution.

File Menu

Loads the last saved puzzle on this computer.
Saves the current state of the puzzle to temporary user data on this computer. This is useful if you wish to come back another time to continue solving it, or if you are guessing which may necessitate you to backtrack later on.
This will print the puzzle at the state it is currenly displayed in the grid.


Here you may set various preferences. They are remembered and will reload from what was last set on that computer.

Active Number Mode: This toggle Active Number Mode, described above.

Show Timer: Toggles display of timer.

Show Pen/Pencil Cursor: Toggles whether the pen/pencil cursor is displayed over the grid or just the standard finger cursor. (Does not apply if Active Number Mode set as the cursor changes to the active number and will be gray if pencil mode.)

Show Possibles In Grid: If set, the possible numbers for each cell will be displayed in a 9 point grid format, else will be displayed next to each other. (Only for HTML5 version)

Auto Resize Grid: If checked, the grid will resize according to your browser height. Press F11 for full screen to achieve maximum size.

Auto Show Errors: If checked, as you enter a number in a cell, it will display in red if incorrect.

Auto Remove Pencil marks: If checked, when you enter a final (pen) number in a cell, that number will be automatically removed from the possibles in all other cells in same row, column or block.

Sound Effects: Toggles sound on/off.

When Select Num Show All Its Cells: If checked, when you click on a number on the Number Panel, it will highlight all the cells that contain that number. (Does not include pencil numbers.)

De-emphasize Completed Numbers: If checked, when all 9 of a number have been entered correctly, that number will be de-emphasized on the Number Panel. You can still select the number, though.

You can also customize the color of the grid and the numbers.