Thermo Sudoku

Thermo Sudoku. Along each thermometer shape, digits must increase from the bulb to the tip.

How to play

Fill the 9×9 grid so every row, every column, and every 3×3 box contains the digits 1 to 9 with no repeats — the classic Sudoku rule. Thermo Sudoku adds one twist: Along each thermometer shape, digits must increase from the bulb to the tip.

Thermo Sudoku fills the standard 9x9 grid but overlays thermometer shapes — each a line with a rounded bulb at one end and a tip at the other. Along every thermometer the digits must strictly increase as you travel from the bulb to the tip, so the bulb always holds the smallest digit on its line and the tip the largest. Because the ordering in Thermo Sudoku is strict, equal digits are banned even where the ordinary row, column and box rules would happily allow them, and the length of a thermometer alone pins a range on every cell it covers.

How to solve Thermo Sudoku

Start by reading each thermometer's length, because that alone bounds every cell on it: a cell sitting k steps from the bulb (counting the bulb as step 1) can be no lower than k and no higher than 9 - L + k on a thermometer of length L. So a 5-cell thermometer forces its bulb to 5 or below and its tip to 5 or above, while a full 9-cell thermometer is completely determined as 1-2-3-4-5-6-7-8-9. Once any digit lands on a line, propagate the strict inequality both ways — every cell nearer the bulb must be smaller, every cell nearer the tip larger — and cross out the candidates that would break the run. Layer these bounds on top of normal row, column and box eliminations, since a thermo cell whose box has already removed the small digits often collapses the entire line. On thermometers that bend or branch, the increase follows the drawn path cell by cell from the bulb rather than a straight line, so trace the route with your eye before committing a digit near a fork.

Every Thermo Sudoku puzzle is free to play in your browser — no ads, no sign-up, and it works offline.

Frequently asked questions

What is Thermo Sudoku?

Thermo Sudoku is a variant played on the normal 9x9 grid with extra thermometer shapes drawn over the cells, each starting at a rounded bulb and running to a tip. The single added rule is that digits must strictly increase from the bulb to the tip of every thermometer; everything else — 1 to 9 in each row, column and 3x3 box — works exactly as in classic Sudoku.

What are the rules of Thermo Sudoku?

Fill the grid so every row, column and 3x3 box contains 1-9, and additionally make the digits along each thermometer increase strictly from the bulb (the round end) to the tip. 'Strictly' means every cell must be larger than the one before it, so a thermometer can never repeat a digit or hold two equal values — the bulb is always the smallest number on its line and the tip always the largest.

Do the digits on a thermometer have to be consecutive?

No — they only have to increase. A four-cell thermometer could read 1-2-3-4, but 2-5-6-9 or 1-4-7-8 are equally valid because each digit merely has to exceed the one before, not be exactly one greater. The only spacing that is forced sits at the ends: a length-L thermometer needs at least L-1 steps up to fit, which is why its bulb cannot exceed 10-L and its tip cannot fall below L.