How Do You Play Sudoku With Numbers?

How do you play Sudoku with numbers? You choose a digit from 1 to 9 and put it in an empty cell. You keep it when that digit is missing from the cell’s row, from its column, and from its 3×3 box. The digit is a label for a slot. Adding the numbers in a row, or looking for a sequence, does not tell you what belongs in the blank.

The formal rules sit in the complete guide to how Sudoku works. This page is the smaller job: reading a digit, deciding whether it may be typed, and telling a finished number apart from a note. The first few placements, in order, are in how to solve Sudoku step by step.

What the numbers stand for

A classic grid has 81 cells and nine digits. Each row must contain 1 through 9 once. Each column must contain 1 through 9 once. Each 3×3 box must contain 1 through 9 once. A 4 in row 2 is the same kind of fact as a 4 in row 8: that label is used, so the rest of those three units lose it.

Nothing about the digit’s size matters. 9 is not “larger” in the logic than 1, and 8 does not have to sit next to 7. You could swap the labels for nine letters and the checks would be identical. Players use 1–9 because they are easy to write and easy to scan.

Printed digits, and the digits already filled when you open a puzzle on this site, are givens. Select one and the number pad will not replace it. Your entries are the cells that started empty.

The three checks before you enter a digit

Pick an empty cell and a candidate digit. Ask three questions, in any order:

1. Does this row already contain the digit?
2. Does this column already contain it?
3. Does this box already contain it?

One yes, and the digit is illegal there. Three nos, and the digit is still possible. Possible is not the same as forced. A cell can allow 6 and 9 at the same time. You enter a digit when the other eight are blocked, or when that digit has only one cell left inside the unit you are scanning.

Here is a cell with two survivors. Its row already holds 1, 4, and 7. Its column holds 2 and 8. Its box holds 3 and 5. The blocked digits are 1, 2, 3, 4, 5, 7, and 8. The cell may be 6 or 9. Leave it blank, or write both as notes. Typing 6 because it is the smaller option invents a fact the grid has not given you.

A forced case is quieter. Suppose column 2 already shows 1, 2, 3, 4, 5, 6, 7, and 8, and r9c2 is the only empty cell in that column. The column is missing 9. Before you type 9, look along row 9 and through the box that contains r9c2. If 9 is absent from both, r9c2 is 9. If 9 is already in row 9, stop and reread column 2. A clash means an earlier digit is wrong, or a given was skipped.

How to enter a number on the board

On Sudoku Unlimited, select an empty cell, then press a digit from 1 to 9. The digit fills the cell. Erase clears a cell you filled. Undo steps back through your own moves.

Notes mode, the pencil control, changes what the same digit button does. With notes on, the digit is drawn small inside the cell and can be toggled off with a second press. Those small digits are candidates. They are not answers, and a later scan should not treat them as placed. Turn notes off before you commit a digit you have proved.

The hint control lists digits still legal in the selected cell. The site gives 3 free hints each local day, and finishing a puzzle can add another. A hint is a list. If you use one, still name the row, column, or box that left only that digit.

An illegal digit is accepted into the cell and marked wrong. The header counts mistakes. Three mistakes end the puzzle unless zen mode is on. Clear the mark and restore the candidate you can defend.

Easy is the right board for this habit. Medium is the same entry method with fewer givens, so the notes stay useful longer. What those level names change is spelled out in easy vs medium vs hard.

The same digits on paper

A paper grid uses the same three checks. Write a proved digit large enough to match the givens. Write candidates small, in the same order every time: 1 2 3, then 4 5 6, then 7 8 9. When you place 4, strike 4 from the notes in the rest of its row, column, and box.

Sheets for Easy through Expert, with a solution page, are on printable Sudoku. Use the solution page to check a digit you have already justified.

Smaller grids use fewer numbers

Kids Sudoku keeps the same idea and shortens the alphabet. A 4×4 grid uses 1 through 4, once in each row, column, and box. A 6×6 grid uses 1 through 6. If nine digits are too many to scan, start on 4×4 and move to 6×6 when the four-digit check is automatic. The classic 1–9 board is the same action with a longer list.

When several numbers still fit

Stop typing. A cell with two legal digits is unfinished. Write both, then look for a unit where some digit has only one home. That home can sit inside a cell that still shows other notes. Placing it, and clearing the extras, is a hidden single. A cell whose own list has shrunk to one digit is a naked single.

The daily challenge is one classic grid per UTC day, shared by every player, with the same digits and the same three checks. The Sudoku solver can confirm a digit after you can say which unit forced it.

Cage sums are a different puzzle. Killer Sudoku still places 1–9 under the classic rules, and each cage must add up to the number in its corner. Classic Sudoku, the subject of this page, has no sums to compute.

Frequently asked questions

Do the numbers have to go in order from 1 to 9?

Scan in any order you can repeat. Many players pick one digit and hunt it through every unit, then move to the next digit. Others finish the fullest row first. The grid only cares that each digit lands once in each unit.

Can I change a number that was already on the grid?

Givens stay. You can erase and undo digits you entered. If a given seems to block every candidate, the mistake is in a digit you added earlier.

What if I want arithmetic?

Open Killer Sudoku. Classic play never adds a row to decide a cell. The only number you write is a digit that the three units still allow.