Nineward guide: sudoku and killer sudoku rules and strategy
The rules of classic and killer sudoku, the cage arithmetic that makes Killer work, the solving techniques Nineward's hints teach, and how the Daily, Arcade Sprint and Pro fit together.
How to play sudoku
Fill the grid so every row, every column and every box holds each digit exactly once. On a classic 9×9 board that means 1 to 9 in each row, column and 3×3 box. A well-made puzzle has exactly one solution, so every digit can be reached by logic.
Three board sizes
Classic 9×9, plus Mini 4 (a 4×4 grid with 2×2 boxes) and Mini 6 (a 6×6 grid with boxes two rows tall and three columns wide). The Minis are a quick way to practise the same logic.
Four difficulties
Easy, Medium, Hard and Expert. Harder boards start with fewer given digits and lean on the deeper techniques below.
Calm or Challenge
Calm counts mistakes as slips and never ends your game. Challenge ends the game at three mistakes. Every entry is checked against the solution as you place it.
Pencil marks
Pencil mode adds candidate notes to a cell. When you place a correct digit, Nineward clears that digit from the notes in its row, column and box. Undo goes back up to 100 steps.
Killer sudoku rules and cage math
Killer sudoku adds cages: groups of cells outlined with a dashed border, with a small sum printed in the corner. The digits in a cage must add up to that sum, and a digit can't repeat inside a cage. The normal row, column and box rules still apply.
Cage arithmetic is where Killer gets its extra logic. Three ideas carry most solves:
- 01Learn the fixed combinations.Some sums can only be made one way. Two cells totalling 3 must be 1 and 2; two cells totalling 17 must be 8 and 9. Three cells totalling 6 are 1, 2 and 3, and three totalling 24 are 7, 8 and 9. Four cells totalling 10 are 1 to 4, and four totalling 30 are 6 to 9.
- 02Use the 45 rule.Every row, column and box on a 9×9 board adds up to 45. If the cages inside a box cover all but one cell, subtract their sums from 45 to get that cell. The same works for a cell poking out of a box: add the cages, subtract 45, and the leftover is the outlier.
- 03Cross cages with ordinary singles.A two-cell cage of 4 can only be 1 and 3. If the row already has a 3, the cage cell in that row is the 1. Cage limits and sudoku limits narrow each other down.
How Nineward does Killer. Nineward's Killer Sudoku draws cages over a 9×9 puzzle that keeps its given digits, so the cage sums add information to a board you could already solve. That makes it a gentle way into Killer. Cages get larger as difficulty rises: one to three cells on Easy, two to three on Medium, two to four on Hard, and three to four on Expert.
Sudoku strategy tips: the techniques that matter
These are the techniques Nineward's hint engine names, from the first ones you'll use to the last.
| Technique | What to look for |
|---|---|
| Naked single | A cell with only one candidate left. Place it. |
| Hidden single | A digit that fits in only one cell of a row, column or box, even if that cell has other candidates. |
| Pointing pair | Inside a box, a digit's candidates all sit in one row or column. Remove that digit from the rest of that line. |
| Naked pair | Two cells in a unit share the same two candidates. Those digits go there, so remove them from the unit's other cells. |
| Hidden pair | Two digits appear in only the same two cells of a unit. Clear every other candidate from those two cells. |
| Naked triple | Three cells in a unit hold only three digits between them. Remove those digits from the rest of the unit. |
| X-Wing | A digit appears exactly twice in each of two rows, in the same two columns. Remove it from the rest of those columns. It works with rows and columns swapped, too. |
| Swordfish | The X-Wing idea across three rows and three columns. |
Work in that order. Sweep for singles after every placement, because each new digit opens more. Reach for pairs and triples when the singles dry up, and look for X-Wing and Swordfish only on Hard and Expert boards where the simpler moves run out.
The Technique Trainer drills these as packs. Naked Singles and Hidden Singles are free; the pair, triple, X-Wing, Swordfish and Clean Solving packs come with Pro.
Hints that teach, not solve
A Nineward hint doesn't fill in the answer. It highlights the cell, names the technique that applies, and tells you whether to place a digit or rule one out, so you still make the move yourself. When no named technique applies, it falls back to the best candidate.
Free players get one hint per puzzle; Pro raises that to 99. Hints are switched off on today's ranked Daily, in Compete mode and in friend challenges so rankings stay fair. The hints teach the classic techniques above, so in Killer the cage arithmetic is yours to work out.
Daily sudoku, Arcade Sprint and custom puzzles
The Daily
Everyone gets the same Daily, and it is always a Hard 9×9 puzzle. A new one arrives at 00:00 UTC. Your first finish of the day is your ranked attempt, and only a clean win ranks: no hints and no mistakes. The leaderboard is free and needs no sign-in.
The free weekly league runs on the same Daily: each ranked Daily you solve earns a point, and ties go to the lower total solve time.
Winning today's Daily builds your streak; archive wins don't. The Daily Archive holds today plus the last three days for free players, and today plus 730 past days with Pro.
Arcade Sprint
A Medium 9×9 puzzle against a five-minute clock, played under Challenge rules, so three mistakes also end it. Sweep for singles first; there's rarely time to pencil-mark the whole board.
Custom Puzzle
Paste any classic sudoku as 81 characters, using digits for givens and 0 or a full stop for blanks. Nineward checks it has exactly one solution before you start.
Nineward Free vs Pro
There are no ads on either tier, and the ranked Daily is the same for everyone.
| Feature | Free | Pro |
|---|---|---|
| Daily, Arcade Sprint, Custom Puzzle | Included | Included |
| Difficulties (including Killer) | Easy, Medium | Adds Hard, Expert |
| New Game boards | 3 a day | No daily cap |
| Hints per puzzle | 1 | 99 |
| Daily Archive | Today + 3 days | Today + 730 days |
| Game history | 20 games | 500 games |
| Solve replays, full stats | Not included | Included |
| Smart Notes, dot notes, Auto-Complete | Not included | Included |
| Friend challenges, cloud sync | Not included | Included |
| Themes | Atelier | Adds Midnight, Concrete |
The three-a-day limit counts only New Game boards and resets at midnight UTC; the Daily, Killer, Arcade Sprint and custom puzzles don't use it up. Classic New Game boards come from a catalogue of 500 puzzles, 125 per difficulty. Friend challenges and cloud sync need a signed-in account.
Pro comes with the one-time Nineward Full Game Unlock, or with the StackForge Play Pass, monthly or annual, which includes Pro across StackForge games.
FAQ
What are the rules of killer sudoku?
Normal sudoku rules, plus cages: the digits in each dashed cage must add up to the number in its corner, and no digit repeats within a cage.
What is the 45 rule in killer sudoku?
Every row, column and 3×3 box sums to 45. Subtract the cage totals inside a unit from 45 to find the cells the cages don't cover.
Do Nineward's hints give away the answer?
No. A hint highlights a cell and names the technique that applies, and you make the move. Free players get one hint per puzzle; Pro gets 99.
When does the Nineward Daily reset?
At 00:00 UTC. Everyone plays the same Hard puzzle, and a clean win with no hints or mistakes is what ranks.
Can I play my own sudoku puzzles?
Yes. Paste an 81-character puzzle into Custom Puzzle. It's free, and Nineward checks it has one solution first.