THE THREE RULES
A standard Sudoku grid is 9 columns wide and 9 rows tall — 81 cells in total. The grid is also divided into nine 3×3 boxes. That gives you three types of region: rows, columns, and boxes.
The puzzle has three rules, one per region type:
- Every row must contain the digits 1–9 exactly once.
- Every column must contain the digits 1–9 exactly once.
- Every 3×3 box must contain the digits 1–9 exactly once.
That is the entire rule set. No arithmetic, no memorisation, no hidden rules. A solved Sudoku is simply a 9×9 grid where every row, every column, and every 3×3 box contains all nine digits without any repeating.
The puzzle starts with some cells already filled in — called given digits or clues. Your job is to fill the empty cells using only logic: each empty cell has exactly one digit that satisfies all three rules simultaneously.
HOW TO READ THE GRID
Before solving, practise identifying the three region types by looking at any given digit and asking three questions:
- Which row is this digit in? That digit cannot appear again anywhere in the same horizontal strip.
- Which column is this digit in? That digit cannot appear again anywhere in the same vertical strip.
- Which 3×3 box is this digit in? That digit cannot appear again in the same 3×3 section.
Practice this on the given digits for two minutes before making your first move. Once you can instantly identify which regions a cell belongs to, solving becomes mechanical rather than confusing.
THE FIRST TECHNIQUE: CROSS-HATCHING
Cross-hatching is the technique that solves most cells in a beginner puzzle. Choose a digit — say, the digit 5 — and mark every row and column that already contains a 5. Any 3×3 box that does not yet have a 5 can only place it in cells that are not in a marked row or column. If only one cell in a box is unmarked, that cell must contain a 5.
Work through all nine digits systematically: 1 through 9, one at a time. For each digit, scan the nine 3×3 boxes. Boxes with exactly one valid cell for that digit give you a free placement. Mark it, then re-scan because each new digit you place eliminates candidates from other boxes.
On a Tier 1 (beginner) puzzle, cross-hatching alone often fills 60–70% of the grid. Once you've worked through all nine digits twice, move on to the second technique for the remaining cells.
THE SECOND TECHNIQUE: CANDIDATE COUNTING
For cells that cross-hatching doesn't immediately solve, count candidates — the digits that could legally go in each cell based on what's already in its row, column, and box. Write small "pencil marks" in each unsolved cell listing its remaining candidates.
Any cell with only one candidate is a naked single — fill it immediately. Any empty cell in a region where only one cell can hold a particular digit is a hidden single — even if the cell has multiple candidates written down, that digit goes there because it has nowhere else to go in that region.
These two singles — naked and hidden — are sufficient to solve every Tier 1 Sudoku and most Tier 2. Write the candidate lists carefully rather than guessing, and update them every time you place a digit.
WHAT TO DO WHEN YOU'RE STUCK
Feeling stuck almost always means one of two things:
- A candidate list is out of date. When you place a digit, update every cell in the same row, column, and box — remove that digit from their candidate lists. Missing one update is the most common source of "impossible" grids in beginners. Re-check your pencil marks before assuming the puzzle is wrong.
- You need a harder technique. If all candidate lists are correct and no naked or hidden singles remain, the puzzle needs a more advanced move (pairs, triples, or X-wing patterns). This should not happen at Tier 1. If it does, it usually means a placement earlier in the puzzle was wrong.
The hint system in GridJoy highlights the next logical move without revealing the answer — useful when you're stuck and want to understand what you missed rather than just seeing a solution.
HOW LONG DOES IT TAKE TO GET GOOD?
Most beginners can complete a Tier 1 Sudoku in 15–30 minutes after reading this guide. Tier 2 typically comes within a week of regular play — not because the rules changed, but because candidate tracking becomes faster and less effortful with practice.
The jump from Tier 2 to Tier 3 usually takes a month of daily play. It is the first tier where naked and hidden singles are insufficient — you will need to identify pairs (two cells in a region that share the same two candidates, eliminating those candidates from other cells). Tier 3 is where most casual players stay and find years of satisfying puzzles.
Tier 4 and Tier 5 require techniques with names: X-wing, swordfish, forcing chains. These are for dedicated solvers and represent months of deliberate practice. They are not necessary to enjoy the puzzle — most players never need them and remain perfectly happy at Tier 3.