Sudoku Techniques: From Naked Singles to Smart Hints
How Sudoku Logic Actually Works
Every Sudoku puzzle, regardless of difficulty, is solvable by logic alone — no guessing required. The puzzle sets up a constraint system: each digit 1–9 must appear exactly once in every row, every column, and every 3×3 box. Once you internalize that those three constraints apply simultaneously, solving becomes a process of elimination rather than intuition.
The techniques below are ordered from simplest to more involved. Easy puzzles on this site are designed to need only the first two. Medium puzzles will require you to reach for the third. Hard puzzles demand that you use all three fluently, sometimes in combination.
Naked Singles: The Foundation
A naked single is a cell where only one digit is possible. You find it by looking at the digits already placed in the cell's row, column, and box, eliminating all of those, and seeing what's left. If exactly one digit remains, write it in.
The scan for naked singles is the first thing to do when you open a puzzle. Move systematically — row by row, or box by box — and look for cells that have eight of the nine digits already accounted for between their row, column, and box. These are the cells where the answer is immediate.
A clean puzzle-opening habit: start with the most densely filled rows, columns, or boxes. A row that already has seven digits placed gives you almost nothing to figure out. A box with six filled cells is a prime hunting ground. Concentrate your initial scan where the constraints are already tightest.
Hidden Singles: The Most Useful Intermediate Step
A hidden single is a cell that doesn't look obvious, but is the only cell in a row, column, or box where a particular digit can legally go. The cell might have several possible digits at first glance, but one of those digits can't fit anywhere else in the unit — which forces it here.
The key word is "unit." For each of the nine digits, ask: in this row (or column, or box), where could this digit possibly go? If only one cell is available for it, that's your hidden single, regardless of how many candidates the cell appears to have.
In practice, scan each 3×3 box for digits that are missing, then look at which cells in the box aren't eliminated by their row or column constraints. If a digit can only land in one of the nine box cells, you've found a hidden single. This technique feels slower than naked singles at first, but it becomes fast once the mental pattern clicks.
A useful shortcut: if you've found all the naked singles and the puzzle still has gaps, look for digits that appear seven or eight times across the grid. Those near-complete digits almost always produce hidden singles somewhere.
Locked Candidates (Pointing Pairs): The First Real Technique
Locked candidates, also called pointing pairs or pointing triples, are where Sudoku starts to feel like genuine deduction rather than scanning.
The pattern: within a 3×3 box, if a particular digit can only go in cells that all share the same row or column, then that digit can be eliminated from the rest of that row or column outside the box. The digit is "locked" to that line within the box, so anything else on the line can't hold it.
Example: suppose in the top-left box, the digit 7 can only fit in two cells, and both of those cells happen to be in row 2. You don't know which of those two cells holds the 7, but you know it's in row 2. So any other cells in row 2 (outside that box) cannot be 7 — you can eliminate 7 as a candidate from all of them.
This technique is powerful because the eliminations it produces often create naked or hidden singles elsewhere. One locked candidate can cascade into three or four resolved cells.
Using Notes Mode Without Drowning in Pencil Marks
Notes mode lets you pencil candidate digits into cells before committing to an answer. It's enormously useful, but only if you use it selectively. The mistake most people make is penciling every possible candidate into every empty cell at the start — turning the grid into a wall of numbers that's harder to read than a blank puzzle.
A better rule: use notes only in cells where you've narrowed the candidates to two or three. Cells with four or five candidates aren't ready to be notated yet — work the simpler techniques first to reduce them. When you add a note to a cell, also ask yourself whether placing it eliminates any notes from cells in the same row, column, or box. Actively maintain your notes; stale candidates lead to bad placements.
In practice, notes mode is most valuable when you're working through hard puzzles and need to track where a digit can go across an entire row or box. Write the candidates, then look for where each digit appears only once — that's your hidden single.
When to Spend Your Three Hints
Each puzzle gives you three hints. A hint reveals one cell's answer. Three sounds generous until you hit a hard puzzle and realize three hints cover maybe 3% of the grid.
The worst use of a hint is impatience — spending one early when you haven't fully worked the logical techniques. Always exhaust naked singles, hidden singles, and locked candidates before reaching for a hint. Most "stuck" moments on medium difficulty resolve within two minutes of focused scanning.
The best use of a hint is a strategic unlock. On hard puzzles, there's often one cell that, once filled, cascades into a chain of singles and resolves a large section of the board. If you've genuinely stalled and one region looks particularly constrained — a box where you've eliminated most candidates, a row that's nearly full — use the hint there. A single revealed cell in a bottleneck is worth more than one at the edge of the board.
The check button tells you whether any of your current placements are wrong without revealing the solution. Use it before spending a hint — if you have an error somewhere, fixing it may be all you need to move forward.
Timing Benchmarks by Difficulty
The timer runs from the first keystroke, giving you an honest measurement. Rough benchmarks for consistent solvers:
- Easy: Under 5 minutes is solid; under 3 is excellent. These puzzles are largely naked singles with a few hidden singles. If you're taking longer, practice the row-by-row scan until it becomes automatic.
- Medium: 8–15 minutes is typical. Hidden singles become frequent, and you'll need notes mode for at least a few cells. If you're stalling past 20 minutes without using the check button, there's likely a misplaced digit early on.
- Hard: 20–40 minutes for methodical players. Locked candidates are necessary, and you'll often need to chain multiple techniques together. Under 20 minutes means the techniques are genuinely automatic.
Don't optimize for time at the expense of accuracy. A wrong digit placed confidently costs more time than slow, correct scanning. The fastest solvers aren't rushing — they're not backtracking.