What is Thermo Sudoku?
Thermo Sudoku is normal sudoku with a few thermometer shapes drawn on top of the grid. The board is the same 9×9 you already know, with the same nine 3×3 boxes, the same rows and the same columns. The fresh twist is the thermometers: each one has a round BULB at one end and a thin LINE leading to a pointy TIP. The numbers along a thermometer must keep going up as you walk from the bulb to the tip — just like the temperature climbing up a real thermometer.
You can play Thermo Sudoku as soon as you’re comfortable with our classic 9×9 sudoku. The number rules are exactly the same: each row, each column and each 3×3 box still has to contain the digits 1–9 with no repeats. The thermometers just add one extra rule on top of the puzzle you already know.
Some books call this game "Thermometer Sudoku" or simply "Thermo". Whatever the name, the idea is the same — every thermometer is a strict-up-the-line clue that helps you place the next number.
How is Thermo Sudoku different from regular sudoku?
In a normal sudoku you have three rules: every row, every column and every 3×3 box must contain the digits 1–9 with no repeats. Thermo Sudoku keeps all three of those rules and adds one more — the numbers along each thermometer must STRICTLY INCREASE as you go from the bulb to the tip.
"Strictly increase" is a fancy way of saying every step is bigger than the one before. It can NEVER stay the same, and it can NEVER go down. So if a thermometer has three cells, the bulb might be 2, the middle cell might be 5 and the tip might be 8 — but never 2, 5, 5 (same number) or 5, 5, 4 (going down).
- 🟪 Every row, column and 3×3 box must contain 1–9 (same as classic sudoku)
- 🌡️ The numbers on each thermometer must keep growing from bulb to tip
- 🔵 The round bulb is the cold end — the smallest number on the line
- 🎯 The tip is the hot end — the biggest number on the line
- ✨ The thermometers stay drawn on the grid all game so you can always see them
The 4 simple rules
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One of each in every row
Every row needs the digits 1 through 9 — no repeats.
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One of each in every column
Every column needs the digits 1 through 9 — no repeats.
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One of each in every 3×3 box
Each of the nine 3×3 boxes needs the digits 1 through 9 — no repeats.
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Numbers climb up every thermometer
Along each thermometer, the number at the bulb must be smaller than the next, which must be smaller than the next, all the way to the tip. They strictly increase.
The thermometer trick that unlocks the puzzle
The biggest shift going from classic sudoku to Thermo Sudoku is learning to "read" each thermometer like a clue. Any time you’re about to place a digit on a thermo cell, ask yourself: how does this fit with the strict-up rule?
Cells at the BULB of a thermometer can never be too big. If a thermo has 4 cells, the bulb has to be small enough that three more numbers (each one bigger) can still fit before reaching 9 — so the bulb of a 4-cell thermo can be at most 6.
Cells at the TIP can never be too small. If a thermo has 4 cells, the tip has to be big enough that three smaller numbers fit before it — so the tip of a 4-cell thermo is at least 4.
A great way to start every puzzle is to look at the longest thermometer. The bulb candidates are very narrow and the tip candidates are very narrow, which usually unlocks one or two cells right away.
Easy strategies for kids
- 🌡️ Look at the bulb and tip of each thermometer first — those cells are the most constrained
- ➕ Walk along the thermometer step by step: each cell must be at LEAST one more than the cell before
- ➖ Then walk back: each cell must be at LEAST one less than the cell after
- ✏️ Tap Notes to pencil in candidate numbers
- 💡 Auto Notes works out the thermometer rule for you, so you don’t need to do the maths in your head
Most kids pick up Thermo Sudoku in about five puzzles. If you’ve never played sudoku of any kind, a couple of rounds at Easy 9×9 sudoku first will get the basic pattern into your head. Once rows, columns and boxes feel automatic, the thermometer rule is a fun new layer on top.
Why kids love Thermo Sudoku
- 🧠 Trains step-by-step thinking — you literally walk along each line
- 🎨 The blue thermometers make every puzzle look bright and fun
- 🕵️ Detective brain — every thermometer is a chain of clues
- ⏱️ Quick wins on Easy, longer challenges on Expert
- 📱 Plays great on phones, tablets, Chromebooks and laptops
Ready for more?
Once Easy Thermo Sudoku feels comfortable, try Medium Thermo Sudoku. From there you can graduate to Hard Thermo Sudoku, and finally take on Expert Thermo Sudoku — our toughest thermo puzzle.
Want a totally different brain workout? Try our Hyper Sudoku (sudoku with four extra mini-boxes), our X-Sudoku (sudoku with two big diagonals), our Killer Sudoku (sudoku with cage sums) or our Jigsaw Sudoku (classic rules, but the boxes are wiggly shapes). Younger players can also try our 4×4 sudoku and 6×6 sudoku starter grids.
Frequently asked questions
What age is Thermo Sudoku good for?
Most kids enjoy Thermo Sudoku from about 8 years old, once they’re comfortable with regular sudoku. Younger kids should start with our 4×4 or 6×6 starter grids, then move to classic 9×9, and add the thermometer rule when rows, columns and boxes feel automatic.
Is Thermo Sudoku harder than normal sudoku?
Not always! The thermometer adds one extra rule, but it also gives you extra clues — the bulb is always small and the tip is always big, which often pins a cell down right away. Many kids find Easy Thermo Sudoku about the same difficulty as Easy classic sudoku.
Do thermometers ever go diagonally?
Not in our kid-friendly version. Every thermometer here is a straight horizontal or vertical line of 3 or 4 cells, so they’re easy to spot and easy to follow.
Which end is the bulb?
The bulb is the round circle at one end of the thermometer. The other end (the pointy tip) is just where the line stops. Numbers go UP as you walk from the bulb toward the tip.
Can two thermometers cross?
On this site, no — every thermometer is on its own cells. That keeps the puzzles friendly for kids and beginners.
Can I play without making an account?
Yes! Every game on this site is free, with no signup, no email, and no ads in the way of the puzzle.
Does it save my progress?
Yes — your puzzle, notes and timer save automatically in your browser. Close the tab and come back later, it’ll still be there.
Try other sudoku games
Different rules, same friendly board. Pick a sibling puzzle to play next.