Use 'that is it' when you want to indicate something has come to a complete stop or ending point.

Signaling End

It implies no further action is required or something is satisfactory as it stands.

No Further Action

It can be used to assert limitation in a discussion, indicating no more can be contributed.

Express a moment of realization or discovery when the phrase is used with an exclamation mark.

To affirm that a solution or method is correct, 'that is it' can be used confidently.

Often used when someone provides the right answer or when realizing something correct.

Indicate the finality of a statement or that there will be no further discussion or change.

Shows assertiveness and is sometimes used to end a conversation decisively.

It's useful when you need to prevent further debate or indicate that you can't be persuaded.