Sudoku-Layer strategies

Solving Sudoku-Layer means bringing together information from several groups. A cell may allow a digit in its circle but rule it out in its layer; a center cell also crosses a row and column. These strategies help organize that information and justify a move before entering it.

Start with what is missing

Look for nearly complete layers, rows or columns. If a nine-cell group contains 1, 2, 3, 4, 6, 7, 8 and 9, its remaining cell must be 5. Then check how that 5 affects its circle. Do not only scan left to right: following a layer of one color can reveal an easy gap that is hard to see in an isolated circle.

When several gaps remain, list the digits missing from the group. If 3 and 6 are missing, each gap can only be one of those two. A restriction from another layer or from the sum may determine which goes where. A local possibility remains provisional until you check all of the cell’s groups.

Layers Pending: start with the fewest gaps

The Layers indicator mirrors the board’s five positions and colors. Each number tells you how many correct cells remain in that layer, not which digit to enter. Starting clues already count as filled; Pencil notes do not. A ✓ means all nine cells are complete.

If a layer shows 1 remaining, find its only gap and compare the eight visible digits with 1–9: the missing digit is a direct candidate. With 2 remaining, find the missing pair and use circles, rows, columns or sums to place those two digits. Starting with the layers with the fewest gaps is a useful way to find short advances and then connect them to less complete areas.

After confirming a digit or using Hint, check the indicator again: another layer may now have only one gap. The rows-and-columns grid marks complete groups with lines; cells in those completed rows or columns can still provide information to other layers.

Use the sum as another constraint

Subtract the digits already placed in the circle from Σ. If the target is 25 and you have 8, 5 and 2, its two gaps must add up to 10. Possible distinct pairs are 1 and 9, 2 and 8, 3 and 7, or 4 and 6. The pair 5 and 5 fails because it repeats a digit. The pair 2 and 8 also fails: both digits are already in the circle.

A pair does not yet tell you the order. To decide which digit goes at the top and which goes on the left, check each position’s layers and groups. If only one gap remains, subtraction gives a direct candidate, but it must also satisfy the other rules.

Settings: make the game comfortable for you

Colors help you recognize a layer across the nine circles. Change them for visual comfort, personal preference or if some shades are hard to distinguish, including with color vision differences. This is more than an accessibility option: a palette you recognize quickly also makes the board’s connections easier to follow.

Open Settings, select one of the five positions and choose its background color. The change applies to that position in all nine circles and in the Layers indicator. Rules and digits stay the same. The game rejects very similar colors and adjusts text to black or white according to the background; still, choose a combination that is clear on your screen. Restore colors brings back the initial palette. Your chosen colors are saved in this browser.

Circle sum: an optional helper

Hover the mouse over Σ to see the sum of the digits currently placed in that circle, including its starting clues. Small Pencil notes do not count. The number shown beside Σ remains the fixed target sum, not the current total.

In Settings, turn off “Show current circle sum in tooltip” to calculate it yourself and increase the challenge. This preference is saved. The Σ target and rule checking still work. The helper uses the browser’s hover tooltip; touchscreens may not offer that same hover interaction.

Music, sound and silence

Every page load begins with Auto-mute on and Music and Sound unchecked. Turn off Auto-mute to unlock the controls, then choose Music, Sound or both. Music plays the electronic loop; Sound enables selection and error effects. Volume affects both music and effects.

Turning Auto-mute back on switches off and clears both channels. Hiding the tab silences audio. If the browser requires an interaction to play it, a click or tap lets it retry. Your colors persist between visits; audio starts muted on each page load.

Expand requests full screen when the browser supports it. Choosing colors or changing levels before PLAY does not start the timer. Restart discards the current game and prepares another at the same level, with the timer at 00:00 until PLAY. A started game requires confirmation.

Cross-check candidates and update notes

For a left cell, rule out digits in its circle, left layer and row. For a top cell, check its circle, top layer and column. A center cell requires all four groups. Then filter candidates using the circle’s remaining sum.

Use Pencil to record candidates you have already narrowed down. Since each cell holds up to four, there is no need to note every digit before reasoning. After committing a move, review related gaps: a candidate may no longer be possible. The game removes a correctly entered digit from related notes; further eliminations from sums require your reasoning.

The single-note check button checks whether those proposed digits are correct. It can help you practice, but a successful check does not explain the deduction. To learn, first try stating the reason: “this is the only digit that fits the layer and the sum.”

Worked example: four connected cells

This partial board comes from a valid game, with one extra clue in the top cell of the bottom-left circle to focus the example. Dots are gaps. We will solve the two empty cells in each of the first two circles.

Fixed guide example; new games have different numbers and sums.

  1. Calculate what is missing in the first circle

    In the top-left circle, 25 − (8 + 5 + 2) = 10. Its top and left positions must add up to 10. After removing repetitions within the circle, the pairs {1, 9}, {3, 7} and {4, 6} remain.

  2. Eliminate two pairs using a layer and a row

    The top layer already contains 9 and 7. With {1, 9}, the 9 would have to go on the left, but the first row already has 9 in the second circle’s center. With {3, 7}, the 7 would have to go on the left, but that row already has 7 in the third circle’s left cell. Neither pair can be placed.

  3. Place 4 at the top and 6 on the left

    Only {4, 6} remains. The left layer already has 4 in the circle at the middle row and right column. So 4 cannot go on the left: it goes at the top, and 6 goes on the left. The circle totals 4 + 8 + 6 + 5 + 2 = 25. Both digits fit their groups.

  4. Continue in the second circle

    In the top-middle circle, the missing sum is 30 − (7 + 1 + 9) = 13. Since 7 and 9 cannot repeat, the only available distinct pair is {5, 8}. The first row already has 5 in the first circle’s right cell, so the second circle’s left cell cannot be 5. Place 8 on the left and 5 at the top. The sum is 5 + 7 + 8 + 1 + 9 = 30.

The reusable idea is simple: calculate pairs, check which positions they fit and revisit the groups after every move. Not every game advances through these same four deductions, but the rules that justify them always stay the same.

HINT: a boost when you get stuck

One revealed cell can unlock several deductions. If you have been stuck for a while, Hint lets you discover the correct digit in an empty cell you choose, then keep solving the rest yourself. It is optional: you can complete a game without using it.

Once the game has started, select an empty cell and press Hint. The offer shows which cell will be revealed. You can cancel without changing the board. In rewarded-video mode, the cell is revealed only after the reward is received for completing the video; if unavailable or the sequence is canceled, the hint is not applied.

The website currently uses a demonstration of this sequence; it does not yet show real rewarded ads. The revealed cell is marked on the board, updates Layers and rows/columns, and removes that digit from related notes. Undo can reverse the hint.

To make the most of it, first consider where it gives you the most information. A center cell participates in four groups; another cell may complete a layer with only one digit remaining. After the reveal, revisit its sum and candidates: the benefit can be a chain of moves, not just one more digit.

Choose a level that lets you practice

Easy has three or four clues per circle. Intermediate keeps at least 24 clues overall; Expert leaves exactly 18, two per circle. MASTER leaves 13 clues: five circles have one and four have two, distributed at random. It offers less starting information; Hint can help unlock a chain of deductions. Generated games have a unique solution. Clue count is a difficulty guide, but does not by itself measure the complexity of the deductions required. If you get stuck, switch groups, review notes and look for a constraint you have not yet used.

Try these strategies Review the rules