Gridmark: a free sudoku where "Hard" means something specific

01  What it is

Gridmark is sudoku in a browser tab: one 9×9 grid, nine rows, nine columns and nine 3×3 boxes, each of which has to contain the digits 1 to 9 exactly once. A few cells are filled in when you start. Your job is to finish the rest with logic alone.

It is built for a phone rather than adapted to one. The board fits the screen without pinching, you tap a cell and then tap a digit on the pad under it, and there is nothing between you and the puzzle — no account, no install, no payment, no energy meter. The app is ad-supported.

Each puzzle is generated on your own device at the moment you press play, from a fresh seed, rather than pulled from a fixed library — so you will not spend a month working through a set that everyone else has already solved and posted.

02  What the four tiers actually mean

Most sudoku apps decide difficulty by counting how many cells are filled in at the start. That is a rough proxy: a grid with 26 clues can fall apart in ten minutes of straightforward scanning, and another with 30 can stall an experienced solver cold. Gridmark grades the puzzle instead. Each candidate grid is run through an eight-rung technique ladder — 1 naked single, 2 hidden single, 3 naked pair, 4 hidden pair, 5 pointing and claiming (box-line reduction), 6 naked or hidden triples, 7 fish such as X-wing and swordfish, 8 forcing chain — and the puzzle is accepted only when the hardest rung it needs lands inside the tier’s band.

To check what that produces in practice, we generated twenty puzzles per tier with the shipped engine. Easy arrived with exactly 42 givens every time, and nineteen of the twenty needed nothing beyond naked and hidden singles. Medium ran 30 to 33 givens and every puzzle needed a pair — eighteen at the naked pair rung, two at hidden pairs. Expert ran 23 to 27 givens and all twenty were graded at the chain rung, which is expected: the Expert band starts at a fish, so an Expert grid can never be one you finish with box-line reduction alone.

Hard is the interesting one. Its band spans 24 to 29 givens and accepts anything from box-line reduction up to a chain, and in our sample only three of the twenty could be finished below the chain rung. The other seventeen stalled there. So Hard is often already an Expert-grade puzzle that happens to have a few more clues: the floor is what differs, not the ceiling.

The daily puzzle is always Medium, deliberately. It makes your daily times comparable from one day to the next, which is the only way a streak means anything.

03  Playing a grid: what the board does

Select a cell and the board answers two questions at once. The cell’s row, column and box dim, so you can read its neighbourhood at a glance, and every other cell holding the same digit lights up, so a duplicate announces itself the moment you create one. The timer starts on your first digit — or on your first hint — and stops when the last correct digit lands.

Notes turn any empty cell into pencil marks: pick a digit while Notes is on and it goes down as a small candidate instead of committing. Filling a cell clears its marks. Hint is the more direct help: it writes the correct digit into one cell. It takes the cell you currently have selected if that cell is still empty, and otherwise the first empty cell in reading order, so if you are stuck on one particular square, select it before you press Hint — that is the cell it will fill.

Check keeps the game honest without solving it for you: it marks every filled cell that disagrees with the solution and says nothing about what should be there instead. The mistake counter in the header is stricter and simpler. A digit is counted only when it repeats a digit already sitting in the same row, column or box — a clash you could have seen yourself from the rules alone. A digit that is legal but wrong is not counted at all; Check is how you find it. Erase clears a cell, and does not un-count a mistake you have already made.

On a keyboard the whole game is reachable without the mouse: 1 to 9 to fill, Backspace or Delete to erase, the arrow keys to move the selection (they wrap around the edges of the grid), and N, H and C for Notes, Hint and Check.

04  The daily puzzle, the streak, and what you get to share

The daily is keyed to your local calendar date and generated from it, so everyone playing on the same day gets the same Medium grid without any server deciding who plays what. Leave halfway through and it comes back exactly where you left it — cells, notes, elapsed time and mistake count — and that saved state is cleared the moment you finish. Your streak counts daily completions and nothing else: consecutive days extend it, a missed day sends it back to 1, and finishing a day you have already completed does not add a second day. Free-play best times are tracked separately, one per tier, on the home screen.

Finishing a puzzle produces a share card rather than a leaderboard entry: a 1080×1350 PNG drawn in a printed-paper palette, with the tier, your time as the large number, your mistake count — or the word "flawless" if it is zero — a streak badge when the puzzle you finished was the daily, and a thumbnail of the grid you solved. The thumbnail shows the givens only, so posting it never spoils the puzzle for anyone. The line that goes with it reads like a brag because that is what it is: something like "I solved a Hard sudoku in 4:12 · flawless 🧩".

Nothing is tied to an account, which is also why your numbers live in one browser on one device: clear your browsing data and the streak starts over at zero.

05  Habits that actually make you faster

Speed at sudoku is rarely about seeing further ahead. It is about scanning in the right order, which is why a few habits beat staring.

• Look for hidden singles before naked ones. Asking "where can the 7 go in this box?" cuts a candidate list faster than examining every empty cell in turn.
• When you stall, change the unit you are scanning — the same elimination looks different along a row than inside a box, and a grid rarely stalls on all three at once.
• Use Notes for two or three real candidates per cell, not to dump every digit that might fit. A wall of pencil marks costs more to read than it saves.
• Select the cell you are least sure about before you press Hint. That is the cell it fills, so a hint spent on a square you had already worked out is a wasted one.
• On Expert, expect the endgame to stall. The way through is a short chain: assume one of the two candidates in a two-candidate cell, follow the consequences, and if both assumptions force the same digit into the same cell, that digit is settled — a deduction, because you never had to commit to either branch.
• Do not trade mistakes for seconds. Your best time is saved with the mistake count from that run, so a rushed solve with three clashes is the one sitting on your home screen as a personal best.

06  Questions people ask

Is Gridmark free, and do I need an account?

Free, and no account. It runs in your browser, is supported by ads, and has no sign-up, no install and no paywall on any part of the game. Your times and streak are kept on your own device.

What is the difference between Hard and Expert sudoku here?

The technique each tier is guaranteed to require, not the number of clues. Expert starts at a fish (an X-wing or swordfish) and may need a forcing chain. Hard accepts anything from box-line reduction upward, and in a sample of twenty generated Hard puzzles, seventeen needed the chain level anyway — so Hard is often Expert-grade logic with a few more clues.

Does every puzzle have exactly one solution?

Yes, and it is proved rather than assumed. The generator strips cells one at a time from a completed grid and puts a cell back unless the grid still has exactly one solution, so an ambiguous puzzle never reaches the screen.

Does everyone get the same daily puzzle?

Yes. The daily is generated from your local calendar date, so anyone playing on the same day gets the same Medium grid — and if you close the tab mid-solve, it resumes where you left it.

Do hints and Check count against me?

No. The mistake counter only counts a digit that repeats something already in its row, column or box — a clash visible from the rules alone. A legal but wrong digit is never counted, and Check exists to point those out. Hints and Check do start the clock if you had not made a move yet.

Where are my times and my streak stored?

In your browser’s local storage, on the device you played on, which is why clearing your browsing data resets them. The site also collects basic usage counts through Google Analytics, such as puzzles started; the privacy policy sets out exactly what that covers.

07  More from the shelf

• Lumesweep — Never lose to a 50/50.
• Tilewright — Draw seven tiles. Outword the AI.
• Yearmark — The year you were born, as a poster.
• Chess — Beat the engine. Post the win.

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