Play Thermo Sudoku Online

A thermometer turns a run of Sudoku cells into an ordering puzzle. Digits must rise strictly from the round bulb toward the tip. This board marks four horizontal thermometers, so short runs give useful limits before you fill any digits. Start at a bulb: a four-cell thermometer cannot begin above 6.

1. Read the given digits; these dark numbers stay fixed.
2. Select an empty square and use the number buttons to enter a candidate from 1 to 9.
3. Put each digit exactly once in every row and column.
4. Put each digit once in every standard three by three box.
5. Digits rise strictly along each of the four marked thermometers from bulb to tip. Equal digits never fit on a single thermometer.
6. Use Clear to erase an editable cell. Complete the board and check the variant rules to win.
7. The marked bulbs and tips provide useful ranges even before you know an exact digit. In a four-cell thermometer, each successive cell must exceed the previous one, so a 7 in its first cell leaves too few larger digits for the remaining three. Scan the four runs individually, then return to the usual row and box restrictions.
8. The bulb and tip markers are small labels in the cell corners. Follow each thermometer in the direction of increasing values before making a normal Sudoku placement there. If a path already holds two digits, compare their gap with the number of empty steps between them. When the gap is too small, one of those entries must change before the grid can finish.

• Every row contains the digits 1 through 9 once.
• Every column contains the digits 1 through 9 once.
• Every standard three by three box contains 1 through 9 once.
• Digits rise strictly along each of the four marked thermometers from bulb to tip. Equal digits never fit on a single thermometer.
• A thermometer is strict: 3, 4, 4, 7 is illegal because the two middle digits do not increase. Likewise, a valid sequence within the thermometer can still break a column or box. The board checks both sets of rules after all cells are filled, so inspect the entire path when a completed grid does not register.

• Start with rows that have several given digits and list their missing values.
• For a four-cell thermometer, the bulb can hold at most 6 and the tip at least 4. Cross those ranges with each row and box.
• Check columns crossing a marked constraint before placing a digit.
• When a region has just one empty cell, fill its missing value and revisit the variant clue.
• A complete standard Sudoku grid can still fail a variant rule. Review every marked constraint before the final move.

• Sudoku: Practice the standard number-placement rules before trying a variant.
• Arrow Sudoku: Compare how a related puzzle uses regions, paths or tile placement.
• Hyper Sudoku: Compare how a related puzzle uses regions, paths or tile placement.

• Each extra constraint adds a second way to narrow candidates.
• The number pad works with touch, mouse and keyboard focus.
• The variant rules reward scanning more than one region at a time.
• Thermo rules add direction to a familiar grid. Instead of scanning only for missing numbers, you can ask whether a candidate leaves enough room for the rest of a path. That question often removes a high digit from a bulb or a low digit from a tip before any full row is solved.

Play Thermo Sudoku free in your browser. Start the interactive board above, use its on-screen controls and check the rules below when a deduction gets stuck. No download or account is needed.

Try another round of Thermo Sudoku on the board above. Start with the most constrained part of the grid, test each move against the stated rules, and use the reset control to try a new approach.

• Variant Sudoku reference