What is tougher about Hard?
Hard puzzles keep the same five rules, but with only eight starting numbers. That is far fewer than any classic sudoku could survive on, and it works because the dots and the empty lines carry the rest. You will be reading lines from the very first move, and the squares far from any filled number will mostly be solved last.
Do not rush. Most Hard Kropki Sudoku puzzles take 15–35 minutes once you are used to them, and longer while you are still learning.
If you have not played the easier levels yet, do a few rounds at Medium Kropki Sudoku first to make sure the basic strategies — the staircase, clamping from both sides, the black-dot sweep — feel automatic. Hard relies on all of them at the same time, plus the empty-line read below.
Two real solver tricks
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Read the empty lines first
Every time you place a number, look at its four neighbours. Wherever there is NO dot, cross out one-above, one-below, double and half. It is the single most productive move on the board, and it is the one everybody forgets, because there is nothing there to look at.
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Naked pairs across a line
If two squares in the same row, column or box can only be 3 or 5, no other square in that region can be 3 or 5 either. This is even more powerful here, because the dots and the empty lines have already trimmed the candidate lists down to size.
- 1. There is no dot between the 4 and the ? — and on this board, that is a clue.
- 2. No dot means not one apart, so the ? is not 3 or 5 — and not double or half, so it is not 2 or 8 either.
- 3. Four numbers gone from a line with nothing drawn on it! With the 4 itself ruled out by the row, the ? can only be 1, 6, 7 or 9.
Hard puzzle game plan
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Step 1
Tap Auto Notes to fill in every candidate number.
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Step 2
Walk every chain of dots end to end and erase candidates that no longer have a partner next door.
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Step 3
Solve squares next to filled numbers whenever you can — read the dot or the empty line, then the row, column and box.
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Step 4
Every time you place a digit, re-read the four lines around it straight away. On Hard that is where the next digit almost always comes from.
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Step 5
When you are stuck, tap Hint — then keep going!
Common Hard mistakes
- ⏩ Going too fast. Hard punishes a quick guess — one wrong number ripples through a row, a column, a box AND every line around it.
- 🔄 Mixing up the colours. White is "one apart", black is "double". Tap a square to light up its dots if you are unsure.
- 🕳️ Ignoring the empty lines. An empty line next to a filled number is a clue with four eliminations in it.
- 📝 Forgetting to update notes after a placement. Always erase the new digit from the notes of its neighbours — and re-read their lines.
- ✅ Skipping the Check button. If the puzzle feels stuck, the likeliest reason is a wrong digit from earlier.
Frequently asked questions
Is Hard too tough for kids?
Hard is really meant for kids who have already played a lot of regular sudoku and a few Easy and Medium Kropki puzzles. If it feels too tough, jump back to Medium — there is no rush.
Can I solve a Hard Kropki Sudoku without guessing?
Yes — every puzzle on this site has exactly one solution and is proved by computer before it appears. If you are stuck, the Hint button always reveals a correct square.
How can eight numbers be enough?
Because the dots and the empty lines carry the rest. A classic sudoku needs at least 17 givens to have one answer; a Kropki board can manage with eight, or even none, because every one of its 144 lines is a clue.
How does Hard compare to Hard classic 9×9?
They are different challenges. Hard 9×9 is all about pattern spotting; Hard Kropki Sudoku adds "one apart", "double" and "neither" thinking that classic sudoku does not have. Most kids who can do one can pick up the other within a few sessions.
Try other sudoku games
Different rules, same friendly board. Pick a sibling puzzle to play next.