Play Suko Online

The compact arithmetic puzzle behind the colours

Classic Suko fits into nine squares, yet every square belongs to two overlapping systems. The digits 1 to 9 are used once each. Four dark circles sit where groups of four squares meet, and each circle states the sum of those four neighbours. The coloured areas cut across the same grid and provide a separate total for each colour. A placement has to satisfy both systems at the same time.

That overlap is the point of the puzzle. A circle can narrow a set of four values without deciding their order; a colour clue can then separate those values because its cells usually extend into other circles. Suko is therefore less about performing long calculations and more about choosing the subtraction that exposes one useful difference.

Reading the two clue layers cleanly

Treat a dark circle as a local clue. On the 3×3 grid it covers one of the four possible 2×2 blocks. A colour total is a distributed clue: its cells may bend around the board, but the small letter in each square makes membership unambiguous. The letter remains useful when two colours look similar or the page is viewed without full colour.

The number bank is the third source of information. A greyed-out value is already on the board, so it cannot be spent elsewhere. Fixed starter clues have a darker border and a dot in the lower corner. Difficulty only changes the number of those starters; it does not change the sum rules.

A first pass that creates useful remainders

1. Add the fixed values already present in each colour. Subtract that subtotal from the colour target and note how many empty cells must make the remainder.
2. Do the same for every circle that contains a starter. A circle with two known neighbours turns a four-number clue into a two-number remainder.
3. Compare those remainders with the unused number bank. Reject pairs that repeat a used value or require a number outside the board’s range.
4. Revisit the crossing clue after every firm placement. One new value changes both its circle and its colour group, which often creates the next move immediately.

Overlapping circles turn totals into differences

Adjacent circle clues share two cells. Subtracting one total from the other cancels the shared pair, leaving a relationship between the two outside pairs. Label the upper-left circle total L and the upper-right total R. The cells in the middle column appear in both equations, so they disappear on subtraction.

If R is 5 larger than L, the right-hand outside pair must total 5 more than the left-hand outside pair. That is already a strong filter because all values are distinct. When one outside pair also forms most of a colour group, the colour remainder can determine its exact total, and the difference gives the other pair for free.

Colour totals become strongest near completion

A three-cell colour with target 18 is broad when all three cells are empty. Once a 7 has been placed, the remaining two must total 11. If 5 is already used elsewhere, the pair 5+6 vanishes; if one of those cells sits in a circle requiring a low remainder, 3+8 may vanish too. This is how separate clues tighten one another without trial and error.

High and low targets also set a useful tone. A small target cannot absorb several large unused values, while a large target cannot be completed entirely from the bottom of the bank. Use that as a candidate filter, not as permission to place the largest or smallest number on instinct.

The 45 invariant and the central square

The digits 1 to 9 always total 45. This gives the classic board a checksum: the three colour totals must also add to 45. It also explains why the centre square carries so much information. A corner belongs to one circle, an edge square belongs to two, and the centre belongs to all four.

Add the four circle totals together, then subtract 45. The result equals the sum of the four edge squares plus three times the centre. This identity rarely names the centre by itself, but it links the most influential square to the edges. A partly solved colour group can supply the missing edge total and turn the identity into a direct centre calculation.

Classic Suko and the expanded boards here

Printed Suko is conventionally a 3×3 puzzle using 1 to 9. This online version keeps that format as the Classic option and extends the same machinery to 4×4 and 5×5. Larger boards add a circle at every internal grid intersection and keep one colour group per row of the board, with the group shapes rotated and reflected from puzzle to puzzle.

Feedback and recovery in this version

Select a square and choose an unused value from the bank. On a keyboard, arrow keys move between squares, digits 1 to 9 enter directly, and Delete clears an editable square. Values above 9 remain one tap away in the bank.

Check evaluates the rules rather than merely comparing the board with one hidden arrangement. It flags duplicate values, completed clues with the wrong total, and partial clues whose target has become unreachable with the numbers left. Undo restores the previous board. Hint clears one conflicting entry first; if the current path is consistent, it places one value. Reset returns to the starters, while Solution ends the timer and reveals the generated arrangement.

False dead ends and their usual causes

• Solving a circle in isolation. Several four-number combinations can share a total. Carry the candidates into the colour clue before ordering them.
• Forgetting global uniqueness. A pair may fit arithmetically but still be impossible because one of its values is already committed elsewhere.
• Reading colour as decoration. The group letter and total are equal in authority to the dark circles; neither clue layer is secondary.
• Spending an extreme value too early. The largest and smallest numbers have outsized effects on totals. Keep them available until a remainder or difference requires them.
• Continuing after a contradiction. Undo to the last justified placement. Rechecking one local remainder is faster than repairing a speculative chain.

Suko beside Sujiko, Sudoku and Kakuro

The closest relative is Sujiko because it uses the same classic grid and circle geometry. The colour sums are the clean dividing line: removing them leaves the Sujiko clue system; adding them creates the cross-checking structure that defines Suko.

A steady solving rhythm

Strong Suko solving alternates scale. Start globally with the unused-number bank and colour remainders, move locally to one circle, then compare that circle with its neighbour. Place only a value supported by both views. After the placement, recalculate the affected colour and each touching circle before looking elsewhere. On 3×3, that loop is short enough to keep mentally; on the expanded boards, Undo and the visible number bank take over the bookkeeping.

Rules were checked against the Puzzler Suko sample sheet and Ian Benson’s technical Suko description and solver. The expanded 4×4 and 5×5 formats are Glorified Calculator variants of the classic rules.

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