Almost every explanation of Sudoku starts by telling you the rules and then abandons you in front of a grid. The rules take fifteen seconds; knowing what to actually do takes longer, and it is the part most guides skip. This article is the missing part. By the end you will be able to open any easy or medium puzzle and finish it without guessing, using two techniques and one habit.
The rules of Sudoku
Start with the rules, because “Sudoku rules for beginners” is usually all a newcomer needs to get moving. A standard grid is nine rows by nine columns, divided into nine three-by-three boxes. A handful of cells arrive already filled — these are the givens, and you never change them. Your task is to fill every remaining cell so that three conditions hold at the same time:
- Each row contains the digits 1 through 9, each exactly once.
- Each column contains the digits 1 through 9, each exactly once.
- Each three-by-three box contains the digits 1 through 9, each exactly once.
That is the entire rulebook. There is no arithmetic — the nine digits are just nine distinguishable symbols and could as easily be nine colours — and a correctly made puzzle is always solvable by logic alone, so you never have to guess. Everything below turns those three constraints into a method you can actually run at the grid.
What the rule really means
The rule has a consequence more useful than the rule itself. Because a digit appears exactly once per unit, placing a digit anywhere forbids it in twenty other cells: the other eight in its row, the other eight in its column, and the four remaining cells of its box that the row and column have not already covered. Sudoku is not a game of finding where digits go. It is a game of finding where they cannot go, until only one place is left. Everything below is a way of organising that search.
One more guarantee worth knowing: a properly constructed puzzle has exactly one solution, reachable by pure logic. If you ever find yourself flipping a mental coin between two options, you have not hit a hard puzzle — you have missed something, or the puzzle is faulty. Every puzzle on this site is machine-verified for a unique solution before it is published.
Technique one: the Naked Single
Pick any empty cell. Look along its row and note every digit already there. Do the same for its column, then its box. Cross all of those off the list 1–9. Whatever survives is what could legally go in that cell. When exactly one digit survives, you have a Naked Single, and you can write it in with complete confidence — not because it looks right, but because the other eight digits are provably impossible.
Naked Singles are most common in crowded areas of the grid. A cell sitting in a row with six digits already placed, inside a box with five, has very little room left. So do not start at cell one and read left to right. Start wherever the grid is most full.
The habit that doubles your speed here is simple: after writing any digit, immediately look at the cells around it, in the same row, column and box. You have just eliminated a possibility from twenty cells; if any of them was down to two candidates, it is now down to one. Beginners tend to place a digit and then restart their scan from the top-left corner, which discards exactly the information they just created. Follow the chain instead. On easy puzzles a single placement can cascade into fifteen more without you ever needing another technique.
Technique two: the Hidden Single
Eventually the cascade stops. No cell is down to one candidate, and the puzzle looks stuck. It is not — you have just been asking the wrong question. The Naked Single asks what can go in this cell? The Hidden Single asks the reverse: where can this digit go in this unit?
Take a digit, say 7. Look at a box that does not contain a 7 yet. Now look at the rows and columns passing through that box: any of them that already contain a 7 forbid it throughout their length. Mentally strike those lines through the box. Sometimes only one empty cell in the box survives — and that cell must be the 7, even if three or four other digits could also legally fit there. The 7 has nowhere else to go in that box, and the box needs one.
This is the technique that trips people up, because the target cell does not look special. It is not down to one option; it may have five. The single is hidden in the sense that the evidence lives outside the cell, in the absence of alternatives across the rest of the unit. Getting comfortable with this shift — reasoning about a digit's options rather than a cell's — is the single biggest step in a beginner's development.
The systematic version of this scan is called cross-hatching, and it is worth doing in order: take digit 1, sweep every box, then digit 2, and so on. Digits that already appear six or seven times on the grid are the most productive, because they project the most lines. A full pass of cross-hatching, done properly, will finish a large share of newspaper puzzles by itself. The scanning guide covers the mechanics in more detail.
Where to start on a blank grid
Faced with a fresh puzzle, the most common beginner question is simply where to put the first digit. The answer is to let the givens decide: start where the grid is already most crowded, because that is where a digit has the fewest places left to hide. There is no particular cell you are meant to begin from and no left-to-right reading order — constraint, not position, is what opens a puzzle up. Here is how to start, step by step:
- See which digit already appears most often across the whole grid. A digit placed six or seven times projects the most row and column lines, so it is the easiest one to pin down next.
- Take that digit and cross-hatch it — the Hidden Single scan above — through every box that still lacks it, writing it wherever only one cell survives.
- After each placement, glance at the surrounding row, column and box for a Naked Single, and follow any chain of forced cells to its end before moving on.
- When that digit is used up, move to the next most common one and repeat. Most of an easy grid falls out of this single habit.
That is the orientation. Below is the fuller loop an experienced solver runs once the first few digits are down.
A worked sequence
Rather than abstract advice, here is the loop an experienced solver actually runs, in order, on an easy grid:
- Cross-hatch each digit 1–9 through every box, placing any Hidden Singles you find. This is usually where the first ten or fifteen placements come from.
- Every time you place a digit, check its row, column and box for a Naked Single. Follow any cascade to exhaustion before returning to the cross-hatch.
- When cross-hatching by box stops producing, cross-hatch by row and by column instead. The same digit that had three homes in a box may have only one in a row.
- When that stops producing too, and only then, start writing pencil marks.
Step four is deliberately last. Filling in candidate marks everywhere is slow, and on an easy puzzle it is wasted work — you will place most of the grid before you would have finished annotating it. The pencil-marks guide explains when marking up genuinely pays for itself and how to keep it from becoming a chore.
When you get properly stuck
On a medium or hard puzzle, singles will run out while the grid is still half empty. That is by design: the puzzle has been built to require something more. At that point the next rung is almost always one of two things.
The first is Omission, also called locked candidates. If within one box a digit can only appear in cells that all lie in the same row, then that digit is somewhere in that row — so it can be eliminated from the rest of that row outside the box, even though you have not placed anything. This is the highest-value technique per unit of effort in all of Sudoku, and it is the rung most self-taught solvers are missing.
The second is the Naked Pair: two cells in the same unit showing the same two candidates. Those two digits must fill those two cells in some order, so both can be struck from every other cell in the unit. This is the first technique that gives you eliminations without a placement, which feels unproductive the first few times and is in fact how all harder Sudoku works.
Both are covered properly on their own pages, with live demonstrations and practice puzzles. The learning-order guide sets out the full ladder beyond them, up through X-Wing and Swordfish.
Four habits worth building early
- Never guess. A guess that happens to be right is worse than a guess that is wrong, because you cannot tell which one it was, and twenty placements later a contradiction appears with no way to trace it. If you cannot justify a digit, do not write it. The board on this site is built around this principle — you eliminate candidates you can defend, and cells resolve themselves once one candidate remains.
- Work the fullest units first. Constraint concentrates. A row with seven givens will yield before a row with two, every single time.
- Re-scan after every placement. Almost every beginner stall is not a hard puzzle but an un-followed cascade.
- When stuck, change the question. If “what fits in this cell?” has stopped working, switch to “where does this digit go in this unit?” The two questions surface different information from the same grid, and alternating between them is most of practical solving.
Where to go next
The fastest way to consolidate all of this is deliberate repetition on puzzles that are guaranteed to contain the pattern you are practising, rather than random puzzles that might. Start with the curated Naked Single practice set, then Hidden Singles, and stay there until an easy grid falls without pencil marks at all. Fluency in singles is not a beginner phase you graduate out of — it is the substrate every advanced technique cashes out into, and time invested there pays off at every level above.
When singles are automatic, work through the worked walkthroughs, which show real boards before and after each technique fires, and pick up the glossary for any term you meet that this article did not cover.
Frequently asked questions
What is the first thing to do in a Sudoku?
Find the digit that is already placed most often and work on that one first. Because it projects the most row and column lines, it is the easiest digit to pin into new boxes by cross-hatching. There is no magic opening cell; the first move is whichever placement the givens have already made forced, which is almost always in the fullest part of the grid.
Do you have to guess in Sudoku?
No. A properly constructed puzzle has exactly one solution reachable by pure logic, so every digit can be justified before you write it. If you feel forced to guess, you have missed an available deduction rather than hit a wall — step back and switch the question from “what fits in this cell?” to “where does this digit go in this unit?” Every puzzle on this site is machine-verified for a unique solution, so a guess is never required.
What order should a beginner solve in?
Cross-hatch each digit 1–9 for Hidden Singles first, chase any Naked Single cascade each placement sets off, and only reach for pencil marks when both have genuinely stopped producing. The quickest way to make that loop automatic is deliberate repetition — the Naked Single practice set drills the cascade, and a sheet of easy printable puzzles lets you rehearse the whole opening away from the screen.