Thermo Sudoku keeps every Classic rule: each row, column and 3×3 box holds 1 to 9 once. On top of that, grey thermometers run through the grid, and on each one every digit is higher than the one before it, counting from the round bulb. The thermometers give a lot away, which is why a Thermo puzzle can start with as few as 12 clues.
The rules
Fill the 9×9 grid so that every row, every column and every 3×3 box holds the digits 1 to 9 exactly once, as in Classic Sudoku.
Each thermometer starts at a round bulb. Moving from the bulb to the tip, every digit must be higher than the one before it. The digits do not have to be consecutive: 2, 5, 9 is fine.
Because the digits rise, two cells on the same thermometer never hold the same digit, even when they are in different rows, columns and boxes.
What the thermometers change
A thermometer limits every cell on it before a single digit is placed. On a thermometer of five cells the bulb can only be 1 to 5, the tip only 5 to 9, and the third cell is at least 3.
Every placed digit squeezes the rest of its thermometer. A 6 halfway along means every cell after it is 7 or more and every cell before it is 5 or less.
On this site every puzzle has 5 to 9 thermometers of 3 to 6 cells, drawn for that puzzle alone. They turn freely, and about a third of their steps go corner to corner.
How to solve it
Start with the ranges. Count from the bulb for each cell’s lowest digit and back from the tip for its highest. The candidates the board shows already follow this.
Carry the ranges along. If the cell before cannot be below 4, this one is at least 5, even when both are still empty. Next Step calls this a thermometer step and lights the thermometer it means.
Then solve as in Classic: singles, locked candidates, pairs. The thermometers keep feeding them, so look at them again after every digit you place.
Worked examples
Three positions from real puzzles on this site, walked through by the same Next Step teacher you get in a game. Step through each one.
A digit squeezes its thermometer
Digits rise along this thermometer, from the bulb at R1C6 to its tip at R3C8. Each cell must be higher than the one before it and lower than the one after it.
A range carried along empty cells
Digits rise along this thermometer, from the bulb at R3C4 to its tip at R1C6. Each cell must be higher than the one before it and lower than the one after it.
A thermometer gives a single
Look at R9C3, on a thermometer. Only one candidate is left here.
The four levels
Thermo levels use the same technique scale as Classic, with the thermometer step counted as an Easy technique. Every puzzle is solvable by logic alone, which is why there is no Diabolical level.
Classic techniques, unchanged
The houses are the Classic rows, columns and boxes, so every Classic technique works as it is. These lessons teach them:
Frequently Asked Questions
Is Thermo Sudoku harder than Classic?
Not by its levels: they use the same technique scale as Classic. The thermometers give extra information, so the puzzles start with fewer clues.
Do the digits on a thermometer have to be consecutive?
No. They only have to rise: 1, 4, 8 is fine.
Which end is the bulb?
The round end. The digits rise from it to the other end, the tip.
Can a thermometer turn or go diagonally?
Yes. Each cell touches the next one along a side or at a corner, and a thermometer can turn as often as it needs. Two thermometers never cross.
Can two thermometers share a cell?
Not on this site. Each cell is on at most one thermometer.
Are the clues symmetric?
Yes. Turn the grid half a turn and the clues land on clues. The thermometers are drawn freely for each puzzle.
Why is there no Diabolical level?
Every Thermo puzzle on this site can be solved by logic, up to Extreme techniques. Diabolical puzzles need trial and error, so they are left out.
Which tools can I use?
All of them: pencil marks in corner or centre notation, candidates, Auto Check, Check Cell and Check Puzzle, and Next Step, which lights the thermometer whenever it is the reason for a move.
Try one now. Easy is a good place to start.