Center Dot Sudoku
A Center Dot board: the nine highlighted cells - the middle of each 3×3 box - must together hold 1-9, on top of the usual rules.
Center Dot Sudoku takes Classic and adds exactly one thing: the nine cells in the middle of each 3×3 box form a tenth region that must also hold the digits 1-9. Everything else is unchanged, which makes it one of the gentlest steps beyond Classic - and a good first variant if Killer or Samurai feel like a lot.
The rule
- All the Classic rules apply - 1-9 exactly once in every row, column and 3×3 box.
- The nine box centres (marked with a ring on the board) must also contain 1-9 exactly once between them.
In the usual notation those cells are r2c2, r2c5, r2c8, r5c2, r5c5, r5c8, r8c2, r8c5 and r8c8. Notice something useful about them: no two of them share a row, a column or a box. The extra region therefore never repeats a constraint you already had - every bit of it is new information.
How to play, step by step
- Start on the marked cells. Each one belongs to four regions instead of three, so they are the most constrained cells on the board. They usually give up a digit before anything else does.
- Treat the centres as a unit. When you place a 7 in one centre, no other centre can be a 7 - even in a distant box that shares neither row nor column with it. That is the deduction Classic solvers keep forgetting.
- Count what the region still needs. Once six of the nine centres are filled, the remaining three take the three missing digits between them. That often forces a placement no ordinary scan would find.
- Then solve normally. Away from the centres it is plain Classic Sudoku - naked singles, hidden singles, pairs, and so on.
Why it is worth playing
The extra region cuts both ways, and that is what makes the variant pleasant. It is one more thing to check, but it is also one more way to deduce - a stuck grid often opens up the moment you look at the nine centres as a group rather than as nine unrelated cells.
One honest note about difficulty: a Center Dot puzzle starts you with the same number of digits as a Classic one at the same level, but they are not the same job. The extra region is another constraint the setter had to satisfy, so the same count of givens pins the grid down differently - the label describes how hard it plays, not just how bare it looks.
Common mistakes
- Forgetting the region exists. The most frequent error is solving it as Classic and only checking the centres at the end. Use them from the first move - they are your best cells.
- Assuming the centres must be 1-9 in order. They hold each digit once, in any arrangement. There is no pattern to the order.
- Thinking every marked cell constrains its neighbours specially. A centre cell affects its row, column and box exactly as any other cell does. The extra rule links it only to the other eight centres.
Difficulties & scoring
This type can be played on all five difficulties. The score multiplier for Center Dot Sudoku is ×1.15 - so every point counts for more on this variant. The "normal time" below is tuned for this type.
| Difficulty | Base score | Max (×1.15) | Normal time | Hints |
|---|---|---|---|---|
| easy | 150 | 345 | 5 min | 5 |
| medium | 300 | 690 | 11 min | 4 |
| hard | 600 | 1380 | 16 min | 3 |
| expert | 1000 | 2300 | 21 min | 2 |
| evil | 1500 | 3450 | 32 min | 1 |
How the score is calculated
Your score builds up from 0 as you place correct numbers, up to the full base when the board is finished. Then two bonuses are added:
base × your progress × (1 + combo bonus + time bonus) × 1.15
- Progress: the live score climbs from 0 to the base as the grid fills with correct numbers.
- Combo bonus (up to +50%): keep a clean run for the full bonus. Each error or hint is a combo break that shrinks the bonus - but it never takes Sparks away below the base.
- Time bonus (up to +50%): beat the normal time and you earn extra, proportional to how much faster you were. Finishing slower than normal costs nothing - there is no time penalty.
- The total is multiplied by ×1.15 and never drops below 0.
Coins
The coins you earn reflect your score - the more Sparks, the more coins (about 1 coin for every 45 Sparks, minimum 1). Spend coins in the Shop on hints and Jokers.
Tip: a clean, hint-free solve under the normal time scores the highest. The Daily Challenge is always Classic and fully scored.
Where to play it
Center Dot Sudoku scores at 1.15× here, because the extra rule is real work on top of classic logic: Classic plus one extra region: the nine box centres must also hold 1-9.
Difficulty and type are two separate dials here: the type sets the extra rule, the level sets how much of the grid you start with. Hard Sudoku leaves 26 of the 81 cells filled, where scanning gets you started and then leaves you stranded; Expert Sudoku leaves 25, where the beginner tools dry up almost entirely. Take Center Dot at the gentler end while the pattern is still new, and move up once it stops being a fight.