How to start a Killer Sudoku: a first-move checklist

Start a Killer Sudoku by finding single-cell cages and short cages with very small or large totals. Establish their possible digit sets, use row-column-box restrictions to locate digits, then check regions nearly covered by complete cages for a forty-five deduction.

Read the board before filling notes

Your first task is to find a constraint that is unusually tight. A large cage with a middling sum often permits many combinations. A two-cell cage totaling 3 permits only 1+2. Begin with the latter even if it is farther from the top-left corner.

Trace each promising cage's whole outline. A cage that seems to contain two cells may continue across a box boundary. Getting its size wrong invalidates every sum calculation that follows, so a brief boundary check saves more time than premature candidate writing.

Make a short opening pass

1. Enter the target of any single-cell cage.
2. Mark the digit sets in restrictive two- and three-cell cages.
3. Check their individual cells against existing row, column and box digits.
4. Revisit any region affected by a new placement.
5. Look for a box or line almost covered by complete cages.

You can make progress without placing a digit. Learning that three cells must contain 1, 2 and 3 is useful information. Record that restriction and move to a connected cage rather than choosing an arrangement at random.

Opening clue	Immediate conclusion	Still unresolved
Two cells total 3	Digits are 1 and 2	Which cell gets each
Three cells total 6	Digits are 1, 2 and 3	Their arrangement
Three cells total 24	Digits are 7, 8 and 9	Their arrangement
One leftover cell inside a complete region	Difference from forty-five	Whether peers allow that digit

For the last row, count only complete interior cages. The leftover cell is inside the selected full row, column or box. Name that region explicitly before subtracting.

Turn an anchor into a placement

Suppose cells A and B form a two-cell total of 3. If A's column already contains 1, A cannot be 1, so A is 2 and B is 1. Now inspect the rows and boxes containing A and B: either placement may remove the last alternative from another cell.

If neither position is settled, the pair can still help. When A and B lie in one row, the row's 1 and 2 are confined to them. Other cells in that row cannot contain either digit. If A and B lie in different rows, do not apply that row exclusion to both rows.

This is a useful opening rhythm: establish a set, check where it can go, then check what its restriction changes elsewhere. Avoid scanning the whole board from scratch after every small observation.

Try a simple region subtraction

If eight cells of a box are covered by whole cages totaling forty, the remaining cell is 5. This works even when you have not solved the eight interior cells individually. The fixed box total supplies the missing information.

Before using the subtraction, confirm that none of the counted cages extends outside the box and no cell is counted twice. If two cells remain, keep their combined sum as a note. Do not pretend the difference identifies each one.

When the first pass finds nothing

Pick one short cage and enumerate its complete combinations. Then inspect each cell's peers and remove arrangements that cannot fit. A moderately flexible cage can become restrictive once the board's other constraints are applied.

Next, scan for a naked single: a cell with one candidate remaining. Also check whether a particular digit has only one possible cell in a row, column or box. Killer arithmetic supplements these familiar Sudoku checks throughout the solve.

If you use a hint, read its reason before entering the suggested move. Identify whether you missed a cage combination, an ordinary Sudoku restriction or a region sum. That tells you where to look on your next opening pass.

Build a repeatable starting habit

Do not judge the opening by how many digits you enter immediately. A sound pair restriction that creates a placement later is real progress. The goal is a sequence of justified deductions that keeps the candidate notes consistent.

On your next puzzle, choose an anchor in a different part of the grid and apply the same checklist. Being able to start away from a familiar corner shows that you are following constraints rather than a fixed visual route.

Where to play

Kudoku has an interactive introduction, five independent difficulty tracks and logical hints in regular play. The app is free to download, with manual notes and a campaign playable in order. Use an easier track to practise a complete opening scan before moving to harder puzzles.