Thermo Sudoku

Create Printable Thermo Sudoku — Digits Climb from Bulb to Tip Along Each Thermometer

Quickly create free printable Thermo Sudoku puzzles, a playful variant of classic Sudoku in which every thermometer shape overlaid on the grid forces the digits inside it to increase strictly from bulb to tip.

Generate Thermo Sudoku in A4 or US Letter PDF format for variant-puzzle fans, club meetings, and a change from the standard 9x9. Two puzzles fit on a page so the thermometer overlays remain crisp and readable, with an optional full solutions key.

Thermo Sudoku adds a spatial order constraint to the usual row, column, and box rules. Learning to read that ordering turns a familiar puzzle into a delightful new one.

Why use this Thermo Sudoku generator?

Thermo Sudoku is still relatively rare in print. This generator gives you a fresh batch whenever you want. Use it for:

• variant-puzzle clubs
• experienced solvers seeking novelty
• enrichment activities on ordering and constraints
• travel puzzles that reward careful pencil work
• quiet evenings with a pencil and a cup of tea

The thermometer overlay printed on a light grey gives the puzzle a distinctive look without obscuring the digits.

What you can customise

• Puzzle count: two Thermo Sudoku per page to keep overlays clear
• Solutions page: add or omit a completed-grid answer key
• Paper size: download as A4 or US Letter PDF
• Seed: reproduce the same Thermo set later

Two puzzles per page is the default because thermometers need space — squashing more onto one sheet makes the bulbs and lines harder to read.

Notes and limitations

• Rules are standard Sudoku (1-9 once per row, column, and 3x3 box) plus the thermometer rule.
• Each thermometer has a round bulb at one end and a line of cells extending to a tip; the digits must strictly increase from bulb to tip.
• In v1 the thermometers are short (3 cells) and run along rows or columns.
• Pencil marks are highly recommended — the extra constraint rewards careful candidate tracking.

Who Thermo Sudoku is for

Beginners

If you are new to Sudoku, start with a classic 9x9 Easy or Medium first. Thermo Sudoku assumes the standard rules are second nature.

Puzzle enthusiasts

Variant fans adore Thermo Sudoku. The ordering constraint forces you to consider which digits could even fit at each end of a thermometer, often eliminating candidates that a plain Sudoku would leave dangling.

Classroom teachers

Secondary school mathematics clubs benefit from Thermo Sudoku as a gentle introduction to constraint-based reasoning — a topic that underlies computer science and optimisation.

Parents

Parents with older children use Thermo Sudoku as a quiet shared project, working through a pair of puzzles over the course of an evening.

How Thermo Sudoku works

Alongside the usual Sudoku rules, each thermometer adds a local constraint. Start at the bulb (drawn as a filled circle) and read along the connected cells to the tip. The digits must increase strictly — if the bulb is 2 the next cell is at least 3, and so on.

How to use the tool

1. Keep the puzzle count at two per page or adjust if needed.
2. Turn the solutions page on or off.
3. Select your paper size: A4 or US Letter.
4. Click Generate.
5. Preview the sheet.
6. Download the PDF.

Worked example

Consider a 3-cell thermometer. The digits must be three different values increasing strictly. That means the bulb cannot be 8 or 9, and the tip cannot be 1 or 2. Even before you look at the rest of the grid, those candidates can be pruned.

For a club pack, generate two Thermo Sudoku on US Letter with solutions off, and let members swap grids after they finish the first.

Methodology

The engine generates a valid 9x9 Sudoku solution, places a handful of thermometers along rows or columns where the existing digits already satisfy the strict-increase rule, then removes cells to create the puzzle. The thermometer overlay is drawn on the printed grid in a light tone so the digits remain clearly readable. The optional answer page reprints the fully solved grid with thermometers still visible.

Helpful preset ideas

• 2 puzzles on A4 with solutions for classroom or club use
• 2 puzzles on US Letter with solutions off for travel
• Seeded Thermo set for a club that wants identical puzzles
• Pair with a standard Hard Sudoku for a varied evening

Tips for solving Thermo Sudoku

• Check each thermometer first and prune candidates based on its length.
• Remember that a 3-cell thermometer forces the bulb to be at most 7.
• Use pencil marks for every empty cell on a thermometer.
• Combine the ordering constraint with the normal Sudoku rules to eliminate more candidates than either alone.

Designed for A4 and US Letter Printing

The Thermo Sudoku worksheet supports both A4 and US Letter. Grid sizes, thermometer thickness, and pencil-mark space are tuned for both paper sizes so the overlay remains clear and the digits stay crisp.

Related logic puzzle tools

You may also enjoy these related printable logic puzzles:

• Samurai Sudoku
• Sudoku — 9x9 Hard
• Sudoku Generator
• Number Pyramid Generator