What does formatting an NFC tag actually do?
Formatting writes the small structure that lets any compliant reader locate content, and usually an empty message so the tag is valid rather than merely described. What it writes differs by tag type, and a format interrupted partway leaves a tag that reads as unformatted or broken.
What it writes
The capability container, which tells a reader which version of the format applies, how much space is available and whether the content may still be rewritten. Then, in most tools, an empty message so the tag is immediately valid.
Nothing else changes. The identifier stays, the configuration stays, and anything stored outside the content region stays exactly where it was.
It differs by tag type
The word covers different operations depending on what the tag is. Android exposes formatting as a capability separate from writing, listed alongside the technology classes in its reference documentation, because a tag can be formattable without yet being formatted.
On the common consumer families the operation is small and fast. On card style tags it involves creating a file structure rather than writing a few bytes, and on vicinity tags the layout is different again.
Practically this means a tool may offer formatting for one tag and not another, and that is a property of the tag rather than a defect in the tool.
Formatting a tag that already holds content
The content goes. Formatting rebuilds the structure and leaves an empty message, so anything previously written is gone with no way back.
That is sometimes exactly what you want, and it is a heavier operation than simply writing new content over the old. If the goal is to replace what is on a tag, write the new content directly: it is faster, it spends fewer write cycles, and it cannot leave the structure inconsistent.
When formatting fails
Three causes, and the first two are recoverable.
The phone moved and the operation stopped partway. The tag now has a structure that is neither the old one nor a complete new one, and it reads as unformatted or unknown. Formatting again, properly, fixes it.
The tag family is not supported by the tool or the device. Nothing you do to the tag changes this, and another handset may succeed where yours does not.
The tag is locked. Locking makes the memory read only, and that includes the structure, so a locked tag can never be formatted again. This is the case with no remedy.
The failure that looks like a dead tag
A tag with a half written structure powers up, answers, reports its identifier, and offers no content. Everything works except the map to the content. Before concluding a tag is faulty, try formatting it.
Frequently asked questions
Should I format a tag before every write?
No. Writing new content replaces the old content directly, which is faster and spends fewer write cycles. Format only when a tag is unformatted or its structure is damaged.
Why does my tool offer formatting for some tags and not others?
Because formattability depends on the tag family and on what the platform exposes. A tag that is already formatted also has nothing to format, which some tools present as the option being unavailable.
Does formatting remove a password?
No. Password protection is a chip configuration rather than part of the content structure, so it survives formatting. Removing it requires the password and the right operation.
Can formatting fix a tag that reads as unknown?
Often yes, when the cause is a damaged structure and the tag was never locked. It is the first thing to try and it costs nothing.