Sudoku Made and maintained by Brian Hamilton and Karan Hamilton.
Brian builds the playable puzzle tools, while Karan helps test clarity, difficulty feel, and the player experience.
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Summer sudoku for the long school holidays
Summer Sudoku takes classic sudoku to the beach, with nine pictures in place of the digits: the sun, sunglasses, beach umbrella, ice cream, watermelon, palm tree, ocean wave, seashell and cold drink. Each takes one square in every row, column and box, and the rule does not take August off. For the views, sizes and levels, see the Themed Sudoku page.
Six weeks of freedom, or ten, depending on where you live, and the first “I’m bored” arrives in week one either way. This board is for the flat stretches: the back seat of a long drive, the airport gate, the hour after lunch when it is too hot to be outside. A 4×4 Easy board leaves six squares to fill and a 9×9 Medium 49, and the browser holds on to a board in progress, so a puzzle started at the gate is still there at the hotel, on the same phone.
Summer sudoku for kids: from the packing list to the beach
Start them on the 4×4. The pictures are dealt like the trip itself: the 4×4 is the packing list, with the sun, the sunglasses, the beach umbrella and the ice cream; at 6×6 you have arrived, because the watermelon is cut and there is a palm tree; and the ocean wave, the seashell and the cold drink wait for the full 9×9, so the sea has to be earned. Strictly, it makes one early cameo: on Windows the Beach Umbrella stands in sand with a strip of blue water behind it.
The nearest thing to a look-alike is on that first board: the Sun and the Sunglasses, two orange discs the same size, since the Sunglasses picture is a whole smiling face. Even on a 9×9 square the dark glasses show in our Windows screenshots, light and dark, and the other seven differ in shape and colour. Your phone’s own emoji may look different.
Teaching the rules from the start is what Sudoku for Kids is for. A child who outgrows the pictures can carry on in digits with 4x4 Sudoku and 6x6 Sudoku.
For young solvers: start a 4×4 Easy at the same time as an actual ice cream. The puzzle should win. Rematches are traditional.
Can you play sudoku on a phone in bright sun?
You can, but glare is the one real hazard here. Light on a screen adds the same glow to a picture and to the square behind it, so the pictures nearest their square in brightness fade first. We measured each one on a phone-width board, and the two colour themes differ a lot:
| Picture | First on | Light | Dark |
|---|---|---|---|
| ☀️ 1 Sun | 4×4 | 1.8:1 | 6.7:1 |
| 😎 2 Sunglasses | 4×4 | 2.7:1 | 4.4:1 |
| 🏖️ 3 Beach Umbrella | 4×4 | 1.7:1 | 6.7:1 |
| 🍦 4 Ice Cream | 4×4 | 1.4:1 | 8.4:1 |
| 🍉 5 Watermelon | 6×6 | 2.5:1 | 4.3:1 |
| 🌴 6 Palm Tree | 6×6 | 2.8:1 | 3.9:1 |
| 🌊 7 Ocean Wave | 9×9 | 2.1:1 | 5.6:1 |
| 🐚 8 Seashell | 9×9 | 1.8:1 | 6.5:1 |
| 🥤 9 Cold Drink | 9×9 | 1.8:1 | 7.0:1 |
Each picture’s average colour against its square in the light and dark themes, as a contrast ratio (1:1 would be invisible), averaged over its nine squares on a solved 9×9 board in Pictures view, in Chrome on Windows at 390 pixels wide. Given squares carry a light tint and score a little lower than the ones you fill. Names and numbers are the key’s; other phones draw other pictures.
In the light theme the squares are cream, and so are several pictures: all nine come in under 3:1, and nearly two-thirds of the Ice Cream is within 1.5:1 of the cream behind it. In the dark theme the Ice Cream is the clearest of all, and the weakest picture, the Palm Tree at 3.9:1, beats the strongest in the light theme.
So outdoors, turn the brightness up and try the dark theme: the moon button at the top of the page, in the menu on a phone. Until you press it, the page follows your phone’s own setting. If the pictures still wash out, switch to Numbers and save the beach scenery for the shade.
In the Numbers view each square shows a bold digit: dark teal on cream at 8.2:1 in the light theme, near-white on dark teal at 14.8:1 in the dark. That beats every picture in either theme, so it is the view to pick when the screen is losing to the sun.
What makes a Medium summer sudoku harder than Easy?
The sea comes with the 9×9, and so does a harder kind of looking. The easiest move is the naked single: a square whose row, column and box already show eight different pictures takes the ninth. Medium boards deal fewer pictures than Easy, and have far fewer of these. Of 40 boards per level from the game’s own generator, the Easy ones opened with 3 to 13 such squares, the Medium ones with none to 7, and 28 of those 40 with three or fewer. This Medium board has exactly one, and its answer is the Ocean Wave.
Asking “where can the Wave go?” gets nowhere here, because the board deals only one Wave. That picture-first sweep is how you find a hidden single, drawn on the Themed Sudoku page. What works is the square-first question: name what an empty square’s row, column and box hold, and when the list reaches eight, the square is done. Medium boards are dug so that singles alone finish them; the game’s own code finishes this one that way, and an independent counter finds exactly one answer.
On a board this bare, pencil marks keep track of the counting. The notes in the diagram are what Auto Notes draws there, and it fills in every empty square at once.
Which other games deal the sun, the sunglasses or the ice cream?
Six of the nine are in no other game here: the Beach Umbrella, the Watermelon, the Palm Tree and all three 9×9 pictures. The Sunglasses and the Ice Cream also go to the cookout in 4th of July Sudoku, as pictures 7 and 8, so only on its 9×9. The Sun shines in Space Sudoku and in Spring Sudoku, which calls it Sunshine. Once the holidays are over, Emoji Sudoku deals nine emoji that do not change with the season.