No, checking an incomplete grid does not use a mistake in Killer Sudoku. The game uses incomplete checks as harmless reminders: fill fewer than all nine blanks, press Enter or Check grid, and the message tells you the grid is not complete, but no completed-attempt error is recorded. You can return to your entries and keep solving. The different rule begins only when all nine editable cells contain a digit. A valid completed grid earns exactly 1,000 points and enters a terminal completion state. If the nine digits are wrong, that is the first incorrect completed check; the grid remains editable so you can inspect and repair it. A second incorrect completed check deadlocks the run until you use Restart. In both the completed and deadlocked terminal states, later number input, clearing, cursor movement, and extra checks do not change the score or result. To work with the checker directly, open Killer Sudoku, choose one of the nine empty cells, and enter 1 through 9 with the number controls or matching keys. The distinction is simple: partial means reminder and retry; full means submit and count the result.
When an incomplete check costs nothing
The nine player entries determine whether a check is incomplete. Until every editable cell contains a digit, selecting Check grid or pressing Enter produces a reminder rather than recording a completed-attempt error. The feedback does not consume a mistake, and it does not prevent you from selecting, changing, or clearing a cell.
That makes an early check safe to use as a progress prompt. It can tell you that one or more of the nine blanks still needs attention, but it is not a completed submission. The score and terminal state remain unchanged. A mistake becomes relevant only after the grid has nine player entries and Check grid evaluates that completed attempt.
| Submitted state | What happens | Mistake effect | Available response |
|---|---|---|---|
| Fewer than all nine blanks filled | A reminder says the grid is incomplete | No mistake consumed | Continue entering or editing digits |
| All nine blanks filled and correct | The grid is completed | No mistake | Receive exactly 1,000 points; the result is terminal |
| All nine blanks filled incorrectly on the first full check | The first incorrect completed check is recorded | One repairable error | Inspect and edit the grid, then check again |
| All nine blanks filled incorrectly after one prior incorrect completed check | The run enters a deadlocked state | Second incorrect completed check | Use Restart to begin the fixed puzzle again |
What counts as a completed check
The published puzzle is a fixed 9 by 9 Killer Sudoku containing 72 givens and nine empty cells. Only those nine cells are player-editable. Selecting a given is not part of solving; the player fills the blanks one at a time until all nine contain digits.
After the ninth entry, the next Check grid action is no longer an incomplete reminder. The checker evaluates all nine entries as a completed attempt. This is why it helps to count the filled blanks before pressing Enter, particularly after using Delete or Backspace to clear a selection.
Keyboard use does not change the rule. Number keys enter a digit in the selected editable cell, the arrow keys move among editable cells, and Backspace or Delete clears the current entry. A new digit overwrites the selected player entry. Completed and deadlocked states are terminal, so these controls cannot reopen or change either result.
Complete and check the nine blanks without spending a mistake
- Open the fixed Killer Sudoku board and locate the nine empty cells among the 72 givens. Treat the printed digits as fixed starting values and the outlined connected groups as cages with target sums.
- Select one editable cell. You can select it with a pointer or touch input, or use an arrow key to move among the editable cells. The selected cell has a visible focus outline, and each editable cell is a button with a row-and-column accessibility label.
- Compare that blank with its row, column, and bold 3 by 3 box. Remove from consideration any digit already present in one of those ordinary Sudoku units.
- Check the cell's cage. Its target must equal the sum of its digits, and a digit cannot repeat inside that cage. Use the cage as a second constraint rather than relying on the target alone.
- Enter the remaining candidate with an on-screen 1 through 9 control or the matching number key. If the selected cell is empty, this places the digit in that editable position.
- Move to another blank and repeat the row, column, box, and cage checks. The fixture has one blank in each row, so many placements can be compared directly, while the cage arithmetic provides an additional test.
- Count the player entries before checking. If one or more of the nine blanks is empty, press Enter or choose Check grid for a harmless reminder. If all nine blanks contain a digit, the same action submits the completed grid.
- Read the result. A correct full grid earns exactly 1,000 points and cannot be changed afterward. After the first incorrect completed check, repair the entries and check again. If the next full check is also wrong, use Restart because the run is deadlocked.
For a separate illustration of the cage method, imagine a three-cell cage with a target of 15, two distinct known digits of 6 and 4, and one blank. The arithmetic route is 15 - 6 - 4 = 5, so the remaining candidate is 5. Before entering 5, confirm that it does not repeat a digit already in the cage and that it fits the cell's row, column, and box. This 15, 6, and 4 example demonstrates the calculation only; it is not a target or solution claim about the product's fixed board.
Use every clue to protect a completed check
A useful placement has to survive two different constraint systems. First, it must follow ordinary Sudoku placement: every row, every column, and every bold 3 by 3 box contains each digit from 1 through 9 exactly once. Second, every connected cage must reach its displayed target, and no digit may appear twice within that cage. A number can therefore look plausible from one row while still being invalid elsewhere.
The familiar row, column, and box model is also described in Nikoli's Sudoku guide. The public Killer Sudoku Online rules page provides additional background on combining those placement rules with cage sums. Those links explain the public rule concepts; this site's board, givens, cages, solution, and checker are product-authored and independently validated.
For a cage with two known digits and one blank, subtraction gives the remaining sum. For example, a small target can expose an arithmetic inconsistency even when the proposed number would not duplicate anything in the same row. After the arithmetic passes, check the ordinary Sudoku units again. A candidate that makes the cage total work but repeats a digit in its row, column, or box is still wrong.
Checking only one unit is especially risky near the end. Because the puzzle has nine editable cells, most mistakes can involve a small number of entries, but every completed check evaluates the combined full grid. Treat each row, column, box, and cage as a necessary cross-check rather than an optional hint.
Recover from a wrong completed submission
The first incorrect completed check does not end the game. The grid stays editable, allowing you to inspect the rows, columns, boxes, and cages and correct the entries. Clear a doubtful cell with Backspace or Delete, reselect it, and enter a different digit with the number controls or matching key. You can also move among editable cells with the arrow keys while reviewing the full board.
A successful repair can lead to a valid completed grid. Once the nine entries pass the full constraints, the correct completion awards exactly 1,000 points. The completion state is terminal, so later clearing, cursor changes, number input, or additional checks do not alter that result.
The repair window is limited. If your first incorrect completed check is followed by another incorrect completed check, the run deadlocks. At that point, further input and checks cannot change the mistake count, score, feedback, or terminal state. Choose the shared Restart control to reset the run.
Restart clears all nine player entries, the mistake count, feedback, score, and terminal state. It preserves the same published givens rather than replacing them with a random puzzle. The fixed presentation gives you a clean way to apply the same mistake policy again: solve the nine blanks, use incomplete checks freely, and reserve completed-attempt consequences for full-grid checks.
Why the published board is consistent
This Killer Sudoku uses one stable fixture instead of randomly generated boards. The puzzle divides all 81 cells into 27 non-overlapping, orthogonally connected three-cell cages. Each cage's target matches the sum of the corresponding solution digits, and the solution digits within a cage do not repeat.
The completed Sudoku solution is checked independently, and the cage layout is tested as a full partition rather than assumed from array order. Orthogonal connectivity is verified through neighboring cells. This validation matters because a valid-looking target can still be attached to the wrong set of cells, and overlapping or disconnected cages would not provide a reliable deduction.
Uniqueness is also verified explicitly. A separate backtracking count starts from the published 72 givens and applies the Sudoku rules together with partial and complete cage checks while testing candidate digits. The released fixture must produce exactly one complete solution. It is not enough to look at the visible givens and assume that the remaining entries are unambiguous.
That fixed and verified route gives the mistake policy a clear boundary. Incomplete checks remain reminders, the first incorrect completed check permits repair, and the second incorrect completed check locks the run. The checker is not judging a random batch of puzzles; it is evaluating this one reproducible nine-cell completion.
What the 1,000-point result measures
A correct completed grid earns exactly 1,000 points. Incomplete checks do not award that completion score. The first wrong full submission does not remove the score earned elsewhere, because the game is scored when a valid full grid is completed; its function is to identify one repairable completed-attempt error. A second wrong full submission deadlocks the run until Restart.
The game is intended as casual entertainment and transparent logic practice. Its score shows only that this fixed fixture was completed under the stated two-error route. It is not a standardized intelligence, educational, aptitude, psychological, or clinical assessment and should not be used to compare people or make decisions about learning, employment, health, or ability.