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Mini sudoku

Fill the grid so every row, column and box holds each digit once — 4×4, then 6×6, then 9×9, with fewer givens as you climb.

  • Logic
  • Untimed
  • 12 levels

How it plays

Every row, every column and every box (the blocks between the gaps) must hold each digit exactly once. Tap a cell, then a digit, or type it.

Look for singles: a cell where only one digit is left, or a digit with only one place left in a row, column or box. Each one you fill opens the next.

Levels take givens away until the puzzle is as bare as it can be while its answer stays unique, then move to the next size. Every puzzle has one solution, checked by a solver.

Keys

Space starts and skips ahead · 1–9 pick · Enter submits · Esc stops. On a phone, tap.

What it trains

This practises elimination — keeping several constraints in mind and finding the one cell they decide. It is practice at these puzzles, not a measured gain in reasoning elsewhere.

Learn

How to get better at mini sudoku: the method, the techniques, what each new level asks of you, and the traps the wrong answers are made of.

Your approach to each question

  1. Find the boxes first. The blank gaps in the grid mark them: 2×2 boxes on a 4×4, boxes two rows high and three columns wide on a 6×6, and 3×3 boxes on a 9×9.
  2. Pick one digit and scan for it. For each box that lacks it, cross off the rows and columns that already hold it elsewhere; if one cell of the box is left, the digit goes there. Do this for every digit in turn.
  3. Then look at the fullest rows, columns and boxes. A unit with one or two gaps is quickly finished by listing what it is missing.
  4. Each digit you place can open the next. After each one, look along its row, its column and its box before you scan elsewhere.

Techniques and tricks

  • Cross-hatching

    To place a digit in a box, draw imaginary lines through every row and column outside it that already holds that digit. The cells of the box not crossed, and not filled, are where it can go.

    e.g. On a 4×4, if row 1 and column 1 each hold a 3 outside the top-left box, that box’s 3 is at row 2, column 2

  • The naked single

    For a stubborn cell, list every digit its row, column and box already hold. If all but one digit appears, that one is the answer.

    e.g. On a 6×6, a cell whose row has 1, 4 and 6, whose column has 2 and 4, and whose box has 5 can only be 3

  • The hidden single

    In one row, column or box, take a missing digit and check each empty cell. If only one cell can hold it, place it there, even if that cell could take other digits too.

  • The locked pair

    If two cells in one unit can only be the same two digits, those digits are used up there: no other cell in that unit can take either. It often turns a stuck cell into a single.

    e.g. Two cells in a 9×9 row that can each only be 4 or 7: every other cell in the row loses 4 and 7

  • Pointing

    If, inside a box, a digit can only go in cells of one row (or one column), it must sit in that row within that box. So the rest of that row, outside the box, cannot take it.

    e.g. If, in the top-left 3×3 box, the 5 can only go in row 2, then no cell of row 2 outside that box is a 5

  • The total check

    Every row, column and box adds up to the same total: 10 on a 4×4, 21 on a 6×6, 45 on a 9×9. Use it on a unit with one gap, or to check a finished board.

    e.g. A 9×9 row with 1, 2, 3, 4, 6, 7, 8 and 9 adds to 40, so the gap is 5

As the levels climb

  • The ladder starts on a 4×4 with about ten of its sixteen cells given, then fewer, then as few as one answer allows. Cross-hatching and singles do everything here.
  • Next is the 6×6, from about 60% of the cells given down through 50% and 42% to the fewest. The boxes are wider than they are tall, so scan rows and columns separately — the box crosses three columns but only two rows.
  • The 9×9 comes last, from about 56% given through 48%, 42% and 36% to the fewest. On the barest boards singles can run out; that is where the locked pair and pointing come in.
  • The ladder ends there. If Time pressure is on, a board gets 60 seconds plus 10 for each empty cell.

Traps to avoid

  • Checking only the row and column and forgetting the box. A digit that fits both lines can still repeat inside its box — look at all three before placing.
  • Mixing up the 6×6 boxes. Each is two rows by three columns; treating them as three rows by two puts the box lines in the wrong place.
  • Guessing and carrying on. When singles run out, look for a locked pair or pointing first; if you must try a digit, follow it only until it breaks a rule, then undo every cell it led to.
  • Placing a digit and moving on without updating. A new digit removes itself from its row, column and box at once; missing that is how a later cell ends up with the same digit twice.

How to practise

  • Set “Largest board” to 4×4 or 6×6 while you learn cross-hatching, and move up to 9×9 once the fewest-givens 6×6 falls without a guess.
  • Turn “Symmetric givens” off to see boards whose givens no longer come in half-turn pairs. The patterns look different, so you cannot lean on where printed puzzles usually leave their clues.
  • Practise scanning by digit, 1 to 9 in order, on every new board. A fixed order stops you skipping a digit that would have been easy.
  • When you stall, read the first step in the working and find it on your board. Ask which of the moves above would have found it.

Levels

It is untimed unless you turn the clock on. There are 12 levels with the default settings, and the counter stops at the real top. Your level is judged on your last ten answers, every ten: 90% moves you up, below 80% moves you down, which holds most people near four in five right.

All of it is in the settings beside Start — how many answers a judgement looks at and how often it recalibrates, the thresholds, the time, which questions, how many options, and whether you see each verdict at once or review them together at the judgement. You can also jump to any level.

More logic

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