SudokuBoku

Medium sudoku

One technique separates the casual solver from the regular one, and this is the level where you learn it.

Easy Medium Hard Expert Evil Extreme

00:00 Mistakes 0
Puzzle —
Rules
  • Fill every row, every column and every 3×3 box with the digits 1 to 9.
  • A digit can appear once in each of those — never twice.
  • There is exactly one solution, and logic alone will reach it.
  • Rows and columns each take the digits 1 to 9, once apiece.
  • The boxes are irregular shapes, drawn with the heavy outlines.
  • However a region bends, no digit repeats inside it.
  • The colour patches are cages; the small number is what a cage's digits add up to.
  • No digit repeats within a cage — nor in any row, column or 3×3 box.
  • There are no given digits: the sums alone pin down the whole grid.
  • Everything from classic applies: rows, columns and 3×3 boxes take 1 to 9.
  • The two tinted diagonals must each hold 1 to 9 exactly once as well.
  • All the classic constraints hold — rows, columns and 3×3 boxes take 1 to 9.
  • Each of the four shaded windows must also contain 1 to 9 exactly once.
  • The windows sit a step in from the corners, overlapping four boxes apiece.
  • Rows, columns and 3×3 boxes work exactly as they always have.
  • On top of that, two squares a chess knight's move apart may not hold the same digit.
  • Select a square and its knight partners are ringed with a dashed outline.
  • Every row, column and 3×3 box holds 1 to 9, exactly as in classic.
  • Along each grey path, digits strictly climb from the round bulb to the far end.
  • They don't have to be consecutive — each square just beats the one before it.
  • Rows, columns and 3×3 boxes take 1 to 9, exactly as in classic.
  • The digits along each arrow add up to the digit in the circle it comes from.
  • Digits may repeat along an arrow — as long as no row, column or box objects.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • Between squares in the same box you'll find a sign; its point aims at the smaller digit.
  • Every comparison inside every box is shown — a hundred and eight of them.
  • The higher levels hand you fewer digits, not fewer signs. Expert gives you four.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • A white dot between two squares means their digits differ by one.
  • A black dot means one digit is exactly double the other. 1 and 2 are both, and always show black.
  • A boundary with no dot is a clue too — those two digits are neither one apart nor double and half. Every boundary is marked or deliberately bare.
  • Expert hands you no digits at all. The dots and the gaps are the whole puzzle.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • A square with a ring behind it holds an odd digit — 1, 3, 5, 7 or 9.
  • A square with a frame behind it holds an even one — 2, 4, 6 or 8.
  • Most squares are marked with neither, and those are genuinely unrestricted.
  • Every square switches to the fully marked board — one level, gentler than it looks, and two puzzles sharing a grid rather than one.
  • Every row, column and box holds five odd digits and four even ones — so once five squares of a house are known odd, the rest of it is even, marked or not.
  • Solve the 9×9 sudoku as usual — every row, column and box takes 1 to 9.
  • Ten ships are hidden on the same squares: one of four, two of three, three of two and four single squares.
  • Ships are straight, and no two of them touch — not even at a corner.
  • The numbers beside and below say how many ship squares each row and column holds.
  • A ship listed with digits sits on exactly those digits, in order — so the grid is your map.
  • Five 9×9 grids interlock: four corners and a centre, each corner sharing one 3×3 box with it.
  • Every grid obeys ordinary sudoku on its own — rows, columns and boxes each take 1 to 9 once.
  • A tinted box belongs to two grids at once, and only together do the five have one solution.
  • Fill every row, every column and every box with the digits 1 to 6.
  • The boxes are three squares wide and two tall.
  • Nothing repeats in any row, column or box.
  • Put 1, 2, 3 and 4 into each row, each column and each 2×2 box.
  • No digit may appear twice in any of them.
  • Every square can be worked out — no guessing needed.
Keyboard shortcuts
1–9
Place a digit, or toggle a pencil mark
Arrows
Move around the grid
N
Notes mode on or off
Backspace
Clear the square
Ctrl + Z
Undo · Ctrl + Y redo

Medium adds exactly one thing

A medium puzzle here is verified to need hidden singles — and verified not to need anything beyond them. No pairs, no locked candidates, no wings. If you can find hidden singles reliably, every medium grid on this site will fall to you, every time.

That narrowness is deliberate. Rather than a vague step up in "difficulty", medium is a place to drill one specific skill until it becomes automatic.

Turning the question around

On an easy grid you point at a square and ask what can go here? That's a cell-first question, and it works while there are squares with only one survivor.

A hidden single needs the opposite question. You point at a digit and a unit — a row, a column, or a box — and ask where can this digit go? If there is only one square in that unit able to take it, that's where it goes. It doesn't matter that the square could also have held four other digits; nowhere else in the unit can host this one, so the matter is settled.

This is the single most useful habit in sudoku, and it is genuinely harder than it sounds, because you have to hold a digit in your head while your eyes move — rather than holding a square while you run through digits.

The scan that finds them

Take one digit, say 4. Now go box by box. In each box, look at the rows and columns crossing it that already contain a 4 — every square they touch is eliminated. Look at the squares that survive. One left? That's your placement.

Scanning the first box for 7: the four 7s already on the board rule out its top two rows and its left two columns. One square survives, so the 7 lives there.

Then do 5. Then 6. Working one digit at a time across the whole grid feels mechanical, and that's the point: it's exhaustive, so you stop missing things. Wandering the grid looking for "something obvious" is how hidden singles get overlooked for ten minutes at a stretch.

Why they hide so well

A naked single announces itself — the square looks starved. A hidden single looks like an ordinary empty square with plenty of options, and nothing about it draws the eye. The information isn't in the square at all; it's in the absence of anywhere else for the digit to go. That's why you have to go looking digit by digit rather than waiting to notice one.

Using the hint button to train

If you get stuck, the hint here won't just fill a square — it names the technique and shows you the unit. When it says "in row 4, 7 can only go in R4C3", look at row 4 and confirm it for yourself before applying it. Doing that a dozen times teaches the pattern far faster than being handed digits.

What comes next

When medium grids stop stopping you, the change at hard is not another single technique but a change of method: you'll start writing candidates down, because the deductions stop fitting in your head. That's a real shift, and it's where sudoku starts to feel like a craft.