About PencilGrid
PencilGrid is a free puzzle site built for people who just want to solve puzzles. No popup asking you to create an account. No paywall blocking the grid mid-solve. No notification asking for push permissions. Just the puzzle, a timer that pauses when you switch tabs, and a clean interface that gets out of your way.
We serve daily Sudoku, Nonogram, Killer Sudoku, Sudoku X, Binairo, and Suguru puzzles you can solve in your browser. Every puzzle has exactly one solution, guaranteed. Your progress saves locally using IndexedDB—no account needed, no server tracking your solve times. Come back anytime and your puzzle state is exactly where you left it.
We built this because we got tired of puzzle sites that felt like slot machines. Flashing banners, artificial achievement systems, dark patterns nudging you toward premium tiers. Traditional logic puzzles don't need that. They're satisfying on their own—the moment you fill the last cell and the grid clicks into place is enough.
The design is editorial: warm paper colors, tabular numerals, keyboard shortcuts, and printable PDFs for Nonogram. It's meant to feel like a printed puzzle book you'd find at a newsstand, not a gamified app trying to maximize your screen time.
Why Free?
Sudoku, Nonogram, and their variants are traditional logic puzzles. The algorithms to generate them have been public knowledge for decades. We're not inventing anything—we're just packaging well-understood logic in a fast, clean interface.
Charging money for access to Sudoku feels like charging for access to crosswords or chess puzzles. These are cultural artifacts, not proprietary products. The value we add is convenience: a reliable daily puzzle at midnight UTC, a generator that guarantees unique solutions, and an interface that respects your time.
Ads appear above or below the puzzle, never mid-solve. If you use an ad blocker, the site still works—we don't detect it or nag you about it. Your browser handles persistence through IndexedDB, so we don't run a backend for user data. No database means no data to leak, sell, or subpoena.
This is sustainable because the site is static. Puzzles are pre-generated offline and deployed as JSON files to a CDN. Hosting costs are measured in dollars per month, not thousands. We're not venture-backed, we're not trying to grow to a billion users. We just want a place to solve puzzles without friction.
How Puzzles Are Made
Every puzzle on PencilGrid is generated by an algorithm, not hand-crafted. Sudoku uses a backtracking solver that fills a grid, removes cells, and verifies that exactly one solution remains. Nonogram uses a constraint-based generator that ensures the row and column clues lead to a unique picture. Killer Sudoku, Sudoku X, Binairo, and Suguru each have their own specialized generators.
We generate puzzles in batch offline—thousands at a time—then deploy them as static JSON files. This means the site loads instantly. There's no server spinning up a generator when you click "New Puzzle." The files are already sitting on a CDN, ready to serve. It also means puzzles are deterministic: puzzle #147 for Sudoku will always be the same grid, so you can reference it or share it.
Daily puzzles update at midnight UTC and are uniquely numbered from the day we launched that game. Practice mode cycles through a pool of pre-generated puzzles, shuffled so you won't see the same one twice in a session. When you run out, the pool reshuffles. This gives you variety without requiring a database to track which puzzles you've seen.
Difficulty ratings are based on measurable properties: clue count, solving techniques required, and symmetry. We don't use subjective human ratings. A "Hard" Sudoku has fewer givens and requires advanced techniques like X-Wing or Swordfish. An "Expert" Nonogram has a larger grid and clues that don't overlap much. The generator tests every puzzle to confirm it's solvable through logic alone.
Contact
Questions or feedback? Email us at contact@pencilgrid.com