Fill every cell in the grid so that each row, each column, and each bold-bordered box contains every digit exactly once. In a 9×9 grid that means the digits 1–9 appear once in each of the 27 regions.
Some cells are pre-filled — these are your starting clues and cannot be changed. The puzzle is designed so that exactly one valid solution exists. Every deduction you make follows from them.
Click any empty cell to select it, then tap a number on the numpad below the board — or press a key on your keyboard. Arrow keys move the selection between cells.
If you place a digit that already appears in the same row, column, or box, the conflicting cells turn red immediately. Remove the offending entry and try again.
Pick a digit — say, 5. Scan each row, column, and box to see where 5 already appears. Any cell that shares a row, column, or box with an existing 5 cannot hold another. The remaining cells with only one legal position must be that digit.
Scan each empty cell and list which digits are still possible for it. If only one digit fits — because all others already appear in its row, column, or box — fill it in immediately. Repeat until solved.
The 4×4 uses digits 1–4 with 2×2 boxes — ideal for beginners. The 6×6 uses 1–6 with 2×3 boxes. The classic 9×9 uses 1–9 with 3×3 boxes. The 16×16 uses 1–9 and A–G with 4×4 boxes.
Check — verifies your completed grid. Reset Cell — clears the selected cell. Clear Mine — removes all your entries. Reveal Solution — shows the full answer if you are stuck.
Never guess. Every digit in a well-formed Sudoku can be placed through pure logic. If you feel the urge to guess, it means there is a constraint you have not yet seen — slow down, and look again.