How to Play Sudoku: A Gentle Guide

What is the one rule of sudoku?

Sudoku has exactly one rule: every row, every column and every three-by-three box must contain the numbers 1 to 9, each exactly once. Some numbers are printed for you at the start; your job is simply to work out the rest. There is no adding up and no arithmetic of any kind — the numbers could just as well be nine kinds of flower.

That is genuinely the whole rulebook. Everything else in this guide is simply about how to find the next number pleasantly, without ever having to guess. Every grid on this site — today's easy sudoku included — follows this one rule and no other, whatever the difficulty.

What is the 'single candidate' technique?

The single candidate technique — sometimes called a naked single — means picking an empty square, looking along its row, down its column and around its box, and noting every number you can already see. If eight different numbers are visible from that square, only one number is left to be its candidate, and that's your answer.

On a genuinely easy puzzle there is always at least one square where the single candidate technique gives the answer straight away. Fill it in, and that new number unlocks another square, then another, like a row of dominoes. Our Easy grids are built — and machine-checked — so that this one method alone carries you from the first square to the last. You will never need anything cleverer, and you will never need to guess.

What is the 'single position' technique?

The single position technique works the other way round: instead of asking what fits in one square, pick a number and a region — a row, column or box — and ask where that number could possibly go. If every square but one is already ruled out by that number appearing in a crossing row or column, the number belongs in the square that's left, even if that square alone still looks like it could hold several different numbers.

Say a box is missing four numbers, including a 7, and three of those empty squares already share a row or column with another 7 elsewhere on the grid. Only one square is left that a 7 could occupy — so in it goes, no guesswork required.

How do you pencil in candidates without making a mess?

Write your candidate numbers small, in the same corner of every square, in the same order every time — say, always starting top-left and reading like a tiny row of print. A consistent layout means you can tell candidates from confirmed answers at a glance, without needing different colours or symbols.

Only pencil in a square once you've narrowed it down to two or three candidates, not all nine, and rub each one out the moment a neighbouring square rules it in or out. A page of pencil marks that never gets tidied up becomes its own puzzle, and not the enjoyable kind. On our website there's no need for pencil marks at all: tap a filled square to highlight every other square holding that number, which does the same job with nothing to rub out.

What if I'm stuck?

Move, don't stare. Being stuck in one corner of a sudoku almost never means the puzzle has stopped — it means the next easy square is somewhere else. Slide your attention to the row, column or box with the most numbers already filled in; the fuller a region is, the closer its remaining squares are to solving themselves.

Cycle through the single candidate and single position questions on whichever region looks fullest, rather than sticking with the square that's beaten you. Between those two techniques, every easy puzzle, and most medium ones, will open like a tin with a good key.

Why do you never need to guess in a well-made sudoku?

You never need to guess — and on a well-made puzzle you never should. Every proper sudoku has exactly one solution, reachable by logic alone. If you find yourself wanting to guess, it's a sign to go hunting for the square you've missed, not a sign the puzzle requires it. Guessing is also how one small slip quietly poisons half the grid and turns a nice sit-down into a rubbing-out session.

Kindly habits for happier solving

• Start with the fullest rows, columns or boxes — they're closest to done and give the quickest wins
• Say the missing numbers quietly to yourself: 'this row needs 2, 5 and 8' sticks better than scanning
• Keep pencil marks small, consistent and tidied up, or skip them and use the on-screen highlight instead
• Tap a filled square on our website to softly highlight every other square with that number — lovely for the single position question
• Use 'Check my progress' whenever you like; it marks slips without judgement and takes nothing away
• One puzzle a day keeps it a pleasure; there's always a fresh grid tomorrow morning

How can you tell easy from medium?

Easy gives you around 40 starting numbers out of 81, and every Easy puzzle is checked so that the single candidate and single position techniques alone will always get you to the end — no guessing required. Medium gives you fewer starting numbers, around 32, so the same two questions still do all the work, but you'll lean on single position more often and need a little more patience.

If you're new, spend a week on Easy before peeking at Medium. And if a Medium ever feels like homework, abandon it without a flicker of guilt — both grids are saved separately on our site, so switching loses nothing. Whichever you choose, every grid we generate has exactly one solution — never two, and never none — so a finished puzzle is always a correctly finished one.

How do you check your work?

The simplest check is your own eye: run along any finished row, column or box and confirm all nine numbers are there once each, with no repeats and no gaps. If a number's missing or doubled, you'll usually spot roughly where the mistake crept in, because everything nearby will suddenly look crowded or short.

On our website, 'Check my progress' does the same job in an instant — it marks any square that doesn't match the solution without giving the answer away, so you keep everything you got right and simply try again on the rest. Use it as often as you like; there's no penalty and no score being kept. A finished, unmarked grid is its own quiet reward — the numbers simply sit there, all nine of them, exactly where they belong.