Ninebark Sudoku

What is sudoku?

Sudoku is a number placement puzzle on a 9 by 9 grid. The grid is split into nine 3 by 3 boxes, and some cells already hold a number. Your job is to fill every empty cell so that each row, each column and each box holds the numbers 1 to 9 exactly once. There's no math involved. The numbers could just as well be colors or letters; it's all about where things can and can't go.

Every puzzle in Ninebark Sudoku has exactly one answer, and you can always reach it with logic. You never have to guess. We check that for every puzzle before you see it. It's free, plays in your browser on phone, tablet or computer, and there's nothing to download.

How to play

• Tap a cell, then tap a number on the pad. On a computer you can click, or just type the number.
• The small count under each number on the pad tells you how many of that number are still left to place. When it reaches zero the key fades.
• Tap Notes to pencil in small candidate numbers instead of answers. You can also press and hold a number key to add it as a note without switching modes.
• Fill notes writes every number each empty cell could still be. Undo takes them back if you'd rather write your own.
• When you place a number, we tidy it out of the notes in the same row, column and box. You can turn that off in Settings.
• Undo and Redo go back and forward as far as you like, and your history survives a reload. Erase clears a cell.

Tap a filled cell and every matching number lights up, notes included, along with the row, column and box around your cell. That makes it quick to see where a number still fits. Tap a number on the pad while a given cell is selected and it highlights that number everywhere.

Controls on a keyboard

The five levels

Most sudoku apps sort puzzles by how many numbers are given. We don't. A puzzle with few clues can be easy and one with many can be tough. Instead, our solver works through each puzzle the way a person would and notes the hardest technique it needs. That technique sets the level.

Easy puzzles start with at least 36 numbers given, so they move quickly. From Hard up the grids are lean, usually 25 to 27 clues.

Hints that teach

Tap Hint and we don't just fill in a number. We point to the cell, shade the row, column or box that matters, name the technique and explain it in a sentence or two. If the step needs some notes cleared first, the hint walks you through it one step at a time and shows the crossed out notes on the grid. Tap Hint a second time and the number goes in.

If you've placed a number that doesn't belong, the hint shows you that first, since nothing else will make sense until it's fixed. Hints look at the real numbers on the grid, not your notes, so a wrong note can't send them the wrong way.

Mistakes, the timer and pausing

There's no mistake limit. By default a number that doesn't fit shows up right away, underlined and striped so it's clear without relying on color. If you'd rather find your own errors, set Show mistakes to On check and a Check button appears.

Want some pressure? Turn on the Three mistake challenge in Settings. Your next puzzle ends after three wrong numbers, and you can try again or keep going without the limit.

The timer can be hidden if it makes you rush. It still runs quietly for your stats. Pausing hides the grid, and we pause for you when you switch tabs or apps.

Daily Sudoku and leaderboards

The Daily Sudoku is one puzzle for everyone, changing at midnight UTC. The level follows a gentle week: Easy on Monday, Medium on Tuesday and Thursday, Hard on Wednesday, Friday and Sunday, and Expert on Saturday.

Your first solve of the day posts to the daily board, ranked by board time: your solve time plus 30 seconds for each hint and 10 seconds for each wrong number. Replays, a second tab and reloads never post twice. If you start just before midnight and finish after, your time counts for the day you started.

Solve on back to back days to build a streak. The calendar shows every daily you've solved, and you can tap any day from the last few months to play it for fun. Those past puzzles stay off the board.

The all-time board counts points from every puzzle you solve: Easy 10, Medium 20, Hard 35, Expert 60 and Master 100, plus half again when you finish with no hints and no mistakes.

Tips and strategy

• Start with the number that's already placed the most times. With six or seven of it on the grid, its last spots often fall out right away.
• Scan box by box before cell by cell. Asking where can the 4 go in this box is usually faster than asking what can go in this cell.
• Pencil in notes only when singles dry up. Fill notes is fine to use, but writing them yourself in one box at a time teaches you to see patterns.
• Look for pairs. When two cells in a row can only be 3 and 8, those numbers are taken, and every other cell in that row loses them.
• When a box has a number's spots in a single line, that line is spoken for. This is pointing, and it unlocks a lot of Medium and Hard puzzles.
• On Expert, check each number for an X-wing: two rows where it has exactly two spots, sitting in the same two columns.
• Stuck? Take a hint and read it. Each one names a real technique, and after a few you'll spot them on your own.

Look and comfort

Choose a light or dark look, or follow your device. Settings also has larger numbers, a high contrast mode, and switches for the row and column shading, matching number shading, sound and vibration. Highlights use blue and amber tones that stay distinct for most kinds of color blindness, and every key is at least 40 pixels tall.

Your progress saves on your device as you play, so you can close the page and pick up where you left off. Sign in on Airscape Works and your stats, streak and puzzle in progress follow you to other devices. Like sudoku? Try Mosaic Logic for picture logic puzzles or Cipher Cafe for cryptograms.

Questions