What "Hard" really means
A Hard sudoku starts with about 29 numbers filled in. That’s a LOT of empty squares — over half the grid is blank when you start.
You can rarely solve a square at Hard just by looking at one row or column. Usually you need clues from a row, a column AND a 3×3 box all together — sometimes from two different rows and a column. The good news: every Hard puzzle on this site has exactly one solution and you can always find it without guessing. The not-so-good news: you’ll need a few new techniques to get there.
If the techniques in this article are new to you, that’s normal. Most kids who are comfortable with Medium sudoku spend a few weeks practising before Hard puzzles start to feel routine. Take it one puzzle at a time.
Three techniques that unlock Hard puzzles
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Hidden singles
Look at a row, column or box. Is there a number that can only fit in ONE square (even if that square has lots of other candidates too)? That’s a hidden single — fill it in. Hidden singles are the single most common move at Hard.
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Naked pairs
See two squares in the same row, column or box that BOTH have the same two pencil-marks (say 4 and 7)? Those two numbers belong to those two squares — you don’t know which goes where yet, but no other square in that row/column/box can be a 4 or a 7. Cross them off.
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Pointing pairs
Inside one 3×3 box, the only places for the number 5 are all in the same row? Then no other square in that row can be a 5 — even outside the box. Erase 5 from those squares’ notes. This is a classic "removing candidates" move that often unlocks a chain of new answers.
How to handle getting stuck
It happens to everyone, even adults. You’ll be cruising along, then suddenly there’s nothing obvious to do anywhere. Don’t panic. Take a breath, look away for a minute, then come back. Sudoku puzzles often "click" the second time you look at them — your brain keeps working on them in the background.
Still stuck? Run through this checklist. Did you tap Auto Notes recently? Did you remember to update your notes after your last few placements? Have you tried scanning each number 1–9 in turn? Sometimes the missing trick is just one of those.
And if all else fails, tap Hint. Even one hint can unstick the whole puzzle, because it usually completes a row or box and reveals new hidden singles.
How to spot a "naked pair" in real life
Naked pairs sound complicated but they’re actually really simple once you’ve spotted one. Here’s a worked example. Imagine a row where the empty squares have the following pencil-marks: 4-7, 1-2-9, 4-7, 3-5, 1-9.
Look at the two squares with just "4-7" in them. They’re in the same row. Between them they MUST contain a 4 and a 7 (in some order). That means no other square in that row can possibly be a 4 or a 7.
So you can erase the 4 and the 7 from any other square’s notes in that row. Suddenly "4-7" disappears from the other squares, and they collapse to smaller candidate lists — which often reveals a hidden single.
Coming from Medium?
The biggest jump from Medium to Hard isn’t the difficulty — it’s the patience. Medium rewards quick scanning. Hard rewards the kid who is willing to try a few approaches before finding the right one.
Stick with it. Every Hard puzzle you solve makes the next one easier, because the techniques become muscle memory. After about ten Hard puzzles, you’ll start spotting hidden singles and naked pairs without even thinking about it.
Already breezing through Hard? You’re ready for Expert sudoku. Or try our Killer Sudoku variant for a totally different brain workout.
Frequently asked questions
Is Hard sudoku too hard for kids?
Most kids who can solve Medium comfortably can also solve Hard with the help of Notes and the occasional Hint. Try one, take your time, and use the helpers without guilt.
Why does my puzzle have so many empty squares?
That’s normal for Hard — there are only about 29 starting numbers. Use Auto Notes to see your options for each square. The empty look is part of what makes Hard feel different.
How many hints is too many?
There’s no hard rule. If you’re using more than 5 hints to finish a Hard puzzle, you might enjoy Medium more for a while. If you’re using zero, you’re ready to try Expert.
My child gets frustrated. Should they keep going?
It depends on the child. Sudoku is meant to be fun. If a Hard puzzle is causing real frustration, drop back to Medium for a week or two and come back later. The puzzles will still be here.
What’s the difference between Hard and Expert?
Expert puzzles have even fewer starting numbers (under 25) and need you to combine 2–3 techniques in the same move. They typically take 30+ minutes and are aimed at confident kids age 10+.
Try a different size
New to sudoku? Start with a smaller grid — you’ll learn the rules in a few minutes.