Play Hard Sudoku Online
For solvers who read the whole board: a hard Sudoku is set up and waiting – free in your browser, without ads, sign-up, or time pressure.
Hard Sudoku asks you to read the whole board: box-line interactions and triples are required by design, yet every puzzle stays solvable with pure, nameable logic – no guessing needed. Each grid has exactly one solution, checked by machine. Play free in your browser, without ads or sign-up, and bring your full notes discipline.
Rules: nothing new — that’s the point
The rules never change: digits 1–9, once per row, column, and 3×3 box. Hard changes the depth, not the game. Grids start from 29 givens, and each one carries our standing guarantee: exactly one solution, verified by machine. Notes and the subtle error hint remain available, and there’s still no clock.
Strategy: reading across boxes
The jump from medium to hard is the jump from units to interactions between units:
Box-line reduction
When all of a digit’s candidates within one box line up on a single row or column, the digit is locked to that line inside the box — which means it cannot appear on the same line in the neighboring boxes. This one technique (pointing/claiming) is required somewhere in every hard puzzle we serve.
Triples
Three cells of a unit sharing three candidates between them work just like pairs, one size up. Naked triples are easier to spot when your notes are tidy; hidden triples reward a second, slower look.
Keep the chain honest
Hard rewards discipline: after every elimination, re-scan for fresh singles before hunting the next big deduction. The grid falls in waves — patience between waves is the actual skill.
Below this tier sits Medium for rebuilding fundamentals; above it, Expert deliberately leaves the standard ladder behind. The Sudoku overview compares all four.
Frequently asked questions
Is hard Sudoku still solvable without guessing?
Yes – on this tier that is a checked property, not a promise. Every hard puzzle is generated and machine-verified to fall to the standard technique ladder: singles, pairs, box-line interactions, and triples. If you're stuck, a nameable step exists; it just hides well.
Which techniques do hard puzzles actually require?
By construction, at least one deduction from the third rung of the ladder: a box-line reduction (pointing or claiming) or a triple. Singles and pairs remain your bread and butter in between.
How many givens does a hard puzzle start with?
29 – noticeably sparser than medium's 33. The real difference, though, is the depth of reasoning the generator demands, which we grade per puzzle rather than merely counting clues.
What's the fastest way to spot box-line reductions?
Inside one box, note where a digit's candidates cluster. If they all sit on a single row or column, that digit must occupy that line inside the box – so erase it from the rest of the line outside the box. Train that one glance and hard opens up.