Summary: 3D sudoku, also called cube sudoku, is sudoku wrapped around a cube. Every line of cells runs across one face, turns at an edge, and carries on down the next — so a line is twice as long as a face is wide, and a 4 × 4 cube runs on the digits 1 to 8. This guide covers the rules, how to read a line that bends, and a worked example on a real board.
3D sudoku puts a sudoku on the surface of a cube. Three faces are visible — the top, the front-left and the front-right — and the rows and columns do not stop at the edges. Each one runs to the end of its face, folds over the corner, and continues on the neighbouring face. That single change gives the puzzle a new shape, a new digit range, and a habit you have to unlearn: on a flat grid your eye follows a straight line, and here it has to turn a corner.
Every puzzle comes with its answer key, so you can check yourself at any point.
The Goal
Fill every visible cell so that each line, and each outlined region — a block of eight cells fenced off by the heavy borders you can see in the picture above — holds every digit exactly once.
On the standard 4 × 4 cube that means the digits 1 to 8, in 48 cells across the three faces. Every line holds eight cells, every region holds eight cells, and every digit appears exactly six times on the whole cube.
Why the Digits Run 1 to 8
This is the part that surprises people, and it falls straight out of the shape.
A face is 4 cells wide. A line does not stop at the end of a face — it continues onto the next one — so it is 4 + 4 = 8 cells long. A line has to hold each digit once and has no repeats, so there must be exactly 8 digits to go round. A cube with 4 × 4 faces is therefore a 1-to-8 puzzle, not a 1-to-4 one.
The same arithmetic sets every size. A 6 × 6 cube has 12-cell lines and uses the digits 1 to 12; an 8 × 8 cube has 16-cell lines and uses 1 to 16. The name always describes the face, never the digit range.
The Lines Wrap Around the Cube
There are three ways a line can run, and each bends exactly once, at a cube edge. Below is the same solved cube three times, with one line shaded in each copy.
- Over the top and down the left face (left panel). The line crosses the top face towards you, reaches the top-left edge, and drops straight down the front-left face.
- Over the top and down the right face (middle panel). Same idea, mirrored: across the top face the other way, over the top-right edge, and down the front-right face.
- Across both front faces (right panel). This one never touches the top. It runs across the front-left face, bends at the vertical corner nearest you, and carries on across the front-right face, making a shallow V.
Count the digits in any shaded band and you will find 1 to 8, once each.
On a single cube, every cell sits on exactly two lines — its row and its column, exactly as on a flat grid. That is the one thing hardest to see on paper, so here it is: one line shaded amber, the other blue, and in orange the single cell they share.
Two lines running in different directions always cross in exactly one cell, just as a row and a column do on a flat grid. So although the board is folded, the logic underneath it is ordinary sudoku logic.
The Rules of 3D Sudoku
- Every line holds each digit once. A line is one face’s row or column continued onto the neighbouring face, so it is twice as long as a face is wide.
- Every region holds each digit once. Regions are the outlined blocks, and each holds as many cells as a line does — eight on a 4 × 4 cube.
- Every region is one connected piece. You can walk from any cell of a region to any other by stepping between cells that share an edge — including across a fold in the cube, in the jigsaw styles. Regular regions stay on a single face; see Region Styles below.
- Each face is clean on its own. No digit repeats within a face’s own row or its own column — those are just the near halves of full lines — so you can scan one face at a time and still be following the real rule.
- There is exactly one solution. Every board is checked before it is printed, so any filling that satisfies the rules is the answer.
- You never have to guess. Each puzzle is confirmed solvable by human technique alone, at exactly the level it claims. A board that feels stuck is hiding a deduction, not a coin flip.
A Worked Example
One real deduction is worth ten rules, so here is a cell being solved on an actual puzzle. Below is an unsolved 4 × 4 cube with two crossing lines shaded, and the cell they share in orange.
Take the amber line first. It runs across the top face and turns down the front-left face. Read off what it already holds: 4, 5, 1 on the top, then 8 and 3 below the fold. That is five of its eight digits, so its three empty cells — the orange one among them — must be 2, 6 and 7 in some order.
Now use the blue line, which crosses the amber one at exactly the orange cell. It already holds 5, 6, 1 and 8. There is a 6 in it, so the orange cell cannot be the 6. Two candidates left: 2 and 7.
Now use the region. The orange cell sits in the upper strip of the front-left face, marked off by the heavy border. That strip already holds a 2. So the orange cell cannot be the 2 either.
Only 7 survives. Every one of those three steps used a different house through the same cell — a line, its crossing line, and the region — which is exactly the shape of every deduction you will make on a cube. The only thing that made it feel unfamiliar is that two of the three houses bent around an edge on the way.
How to Solve a 3D Sudoku
- Trace a few lines before you write anything. Put a finger on a cell and follow both of its lines to their ends, letting your finger go over the edges. Thirty seconds of this rewires the habit that makes 3D sudoku feel hard, which is stopping at a fold. The mistake beginners make most often is reading a line as four cells instead of eight.
- Work one face at a time, but check the whole line. Because no digit repeats in a face’s own rows and columns, a single face behaves like a small grid you can scan quickly. Use it to find candidates fast — then confirm against the other half of the line before you commit.
- Scan for singles exactly as you would on a flat grid. A single is either a cell with only one digit still open to it, or a digit with only one legal cell left in a line or a region. These are most of the moves in most puzzles.
- Start on the top face. Every one of its lines is half visible right in front of you, which makes it the cheapest place to count. A line already holding five or six of its eight digits — like the amber one in the worked example — is where the next placement usually is.
- Work the near vertical corner. Three cells meet there, one on each face. Place a digit in one of them and you have constrained a line running up over the top and a line running away across the other front face at the same time, so check both before you move on.
- Squeeze regions against lines. If every remaining place for a 7 inside one region sits on a single line, then that region’s 7 is on that line — so the 7 can be removed from the rest of that line, including the half on the other face. This is the same box-line rule you may know from flat sudoku, where a digit confined to one row of a box is eliminated from the rest of that row. It reaches further here, because a line passes through parts of the cube a region never sees.
Region Styles
The outlined regions come in three styles. They all obey the same rule — each region holds every digit once — so what changes is the look and how much of the solve is region work.
Regular regions (left panel) are neat strips, and each stays on a single face: on a 4 × 4 cube each face carries two strips of eight cells, six regions in all. This is the cleanest look and the friendliest introduction, because the regions never fold and only the lines do.
Jigsaw regions (middle panel) are free-form shapes that may wrap around an edge, so a single region can occupy parts of two faces. That makes the region constraint as three-dimensional as the lines, and it is the harder, more interesting board of the two.
Symmetric Jigsaw (right panel) keeps the free-form shapes but arranges them so the whole layout is unchanged when you rotate the cube a third of a turn about its front corner — spin it so the top face becomes the front-left, and the pattern lands back on itself. It plays like Jigsaw and looks like a designed object. It is built on the single-cube board.
Cube Sizes and Difficulty
The 4 × 4 cube is the standard board: 48 cells, digits 1 to 8, and a difficulty ladder of seven levels — Very Easy, Easy, Normal, Hard, Very Hard, Tough and Extreme. The level names the hardest technique the board actually requires, not how few digits it starts with.
The 6 × 6 cube (108 cells, digits 1 to 12) and the 8 × 8 cube (192 cells, digits 1 to 16) are the long-haul boards, and they run from Very Easy to Normal. A bigger cube is a much longer solve at the same logical level, because every deduction means scanning twelve or sixteen symbols instead of eight — so a Normal 8 × 8 is already a substantial sitting.
New to it? Start on a 4 × 4 at Easy, and move up once your finger stops stopping at the edges.
More Than One Cube
A 3D sudoku can also be built from two or three 4 × 4 cubes standing together. More cubes do not change the digit range — the lines are still eight cells long, so a twin or a triple is still a 1-to-8 puzzle, just on 96 or 144 cells instead of 48.
Where two cubes meet at a shared vertical corner, the front faces that face each other are joined by lines that run out of one cube and into the other — shaded above, and holding 1 to 8 once across the pair, exactly like any other line. That makes the two cubes one puzzle rather than two printed side by side: a digit placed on the left cube settles cells on the right.
Those two joined faces are the one place the two-lines-per-cell rule grows: their cells keep both of their own cube’s lines and pick up the join line, so each of them sits on three. They are the richest cells on the board and the best place to start a twin.
The three-cube arrangement stands a third cube in the valley behind the other two. The two lower cubes are joined by a shared corner just as a twin is. The raised cube touches them along its lower edges, and in the jigsaw styles a region can spill across that join onto its neighbours — which is what ties the third cube into the rest of the puzzle.
Try It Online
3D sudoku is far easier to read on screen than on paper, because the browser can show you the fold. Click any cell and every cell that shares a line or a region with it lights up, wrapping around the corners of the cube as it goes — then type the digit, or tap it on the on-screen keypad. Play a 3D sudoku in your browser — no download, no sign-up. The general player controls, including checking your answer, resetting and the timer, are covered once in How to Play Puzzles Online.
You Are Ready to Play
You now know what a 3D sudoku asks for — every digit once in every line and every region — why a 4 × 4 cube runs on the digits 1 to 8, and the one habit the puzzle is really testing: following a line over an edge instead of stopping at it. Trace first, scan a face for the quick candidates, then do what the worked example did and squeeze a cell between its two lines and its region. Ready to make your own? See How to Create 3D Sudoku Puzzles in Puzzle Maker Pro.
FAQ
Is 3D sudoku harder than ordinary sudoku?
The logic is the same and the levels are graded on the same ladder, so a Normal cube asks about as much reasoning as a Normal flat grid. What is genuinely harder is the reading: your eye has to follow lines around corners, and that costs beginners far more time than the deductions do. It gets much easier after the first two or three puzzles.
Why does a 4 × 4 cube use the digits 1 to 8?
Because a line is not four cells long. It runs across one face and continues onto the next, making it eight cells, and eight cells that each hold a different digit need eight digits. The size in the name describes a face, so a 6 × 6 cube uses 1 to 12 and an 8 × 8 uses 1 to 16.
Do I have to imagine the hidden faces?
No. Only the three visible faces are part of the puzzle, and everything you need is printed in front of you. The cube is the shape the lines wrap around, not a solid you have to picture in the round.
Where do I write my pencil marks?
The cells are diamonds rather than squares, so there is less room than on a flat grid — most solvers write candidates small, tucked into the corner of a cell that points away from the digit. On a 4 × 4 cube most people manage without pencil marks at all; on a 6 × 6 or an 8 × 8, print at the largest size you comfortably can and give yourself the room.
Can a 3D sudoku have more than one solution?
No. Every board is verified to have exactly one solution before it is printed, and it is also confirmed solvable by human technique at the level it claims — so there is never a point where guessing is the only move left.
Do the regions ever cross from one face to another?
In the Jigsaw and Symmetric Jigsaw styles they can, and that is much of the appeal — a region that folds over an edge ties two faces together the way a line does. Regular regions stay on one face each, which is why they make the gentler board.