Cross Sums
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Guide

How to Play Cross Sums

Everything you need to solve cross sums puzzles: how clue cells work, the no-repeat rule, how intersections drive the solve, the controls, and the three grid sizes.

Play Cross Sums

The basics

Four steps to a solved grid

  1. 1

    Read the clue cells

    Dark cells with a diagonal line are clues. The number in the top-right corner is the total for the run of white cells to its right (across). The number in the bottom-left corner is the total for the run of white cells below it (down). Cells with no clue on a side simply start no run that way.

  2. 2

    Fill runs with different digits

    Every white cell holds a digit from 1 to 9. Inside a single run, each digit may be used only once, and the digits must add up to the clue. A run of three cells with the clue 7 can only be 1, 2 and 4 in some order.

  3. 3

    Cross-check every intersection

    Each white cell belongs to two runs, one across and one down, and must satisfy both. A digit that fits its across run may break its down run. Solving is the process of narrowing the options where the two runs meet.

  4. 4

    Finish the grid

    Keep placing digits until every run is complete. A clue turns green once its run is full and correct. Repeated digits and over-sized sums are marked in red as you go. The puzzle is solved the moment the last run adds up.

Rules

The complete rules

The objective
Place a digit from 1 to 9 in every white cell so that each across run and each down run adds up to its clue.
No repeats in a run
A digit can appear only once within any single run. The same digit may appear again elsewhere in the grid, including in a crossing run, as long as it is not in the same run twice.
Runs and clues
A run is an unbroken line of white cells, two to nine long. Its clue sits in the dark cell immediately before it. Every white cell belongs to exactly one across run and one down run.
One solution
Every puzzle here has exactly one valid solution. When a grid would have had more than one, a few cells are pre-filled and locked to remove the ambiguity, so you never need to guess.
Hints, checks, and timing
The clock starts when you press Play. A hint fills one cell with its correct digit and locks it; Check highlights digits that differ from the solution. Both are optional, and your best time without hints is tracked separately.

Controls

Touch, mouse, and keyboard

  • Everywhere

    Tap or click, then a number

    Select a white cell, then press a digit on the number pad below the board or on your keyboard. The two runs through the selected cell are highlighted so you always see which sums you are working against.

  • Notes button or P

    Pencil notes

    Turn on Notes to mark candidate digits in small type instead of committing to one. Placing a digit automatically clears that digit from the notes of every cell sharing a run with it.

  • Desktop

    Keyboard

    Arrow keys move between white cells. 1–9 enter a digit, Backspace or Delete clears a cell, U or Ctrl+Z undoes, P toggles notes, H reveals a hint, C checks for wrong digits, N starts a new puzzle, and M mutes the sound.

Grid sizes

5×5, 7×7, or 9×9

The size is the playable area, not counting the row and column of clue cells. Your times and streaks are stored separately for each.

  • 5×5

    Small — a quick warm-up

    A 5×5 playable area with short runs. Most solves take a few minutes, which makes it the best size for learning how clues and intersections interact.

  • 7×7

    Medium — the everyday puzzle

    Longer runs and more intersections mean you will need pencil notes and the unique-combination shortcuts. A comfortable coffee-break puzzle.

  • 9×9

    Large — the full challenge

    Runs of up to nine cells and a dense web of crossings. Expect to chain deductions across several runs before the grid opens up.

Where it runs

Any modern browser

  • Desktop browsers

    Chrome, Edge, Firefox, and Safari all run the game with no plugin. Click a cell and type a digit, or use the arrow keys to move around the grid.

  • Phones and tablets

    The layout is touch-first. Tap a white cell and tap a number on the pad. The grid, number pad, and tool buttons fit on one screen in portrait, and starting a new puzzle is instant.

  • Progress on this device

    Best times, best no-hint times, solve counts, and streaks for each grid size live in local browser storage. They stay until you reset them or clear site data, and they do not sync between devices.

Want to solve faster? See the strategy and tips page, including a cheat sheet of unique sum combinations.

Play Cross Sums free