How do you check how much free space a tag has?
Read the tag with a tool that reports its available content space. That figure accounts for everything the tag reserves for itself and is the only number worth planning against. The capacity printed on the packaging is always larger and always misleading.
How to read it
Open a reading tool, hold the phone against the tag, and look for the line reporting available or free space. It appears alongside the chip, the tag type and the current content.
No writing is involved and nothing is changed. Reading a tag costs it nothing, so this can be done as often as you like, on as many tags as you like.
Which number a tool shows
Tools generally report the space available for content, which is what the tag itself declares in its structure. That figure already has the reserved areas and the container framing deducted.
What it does not deduct is the header each record will add, so a tag reporting a given free space will hold slightly less actual content than that number suggests, and slightly less again if you write several records.
Some tools also show total memory, which is the packaging figure and is not the one to plan with. If two numbers appear, the smaller one is the useful one.
When to check
Before the first write of any batch, always. It confirms the tag family, the state and the capacity in one operation.
Before committing to a content design, since discovering that a vCard does not fit after choosing to use vCards is an expensive way to learn it.
And when a write fails without a clear reason, because capacity is one of the two usual causes, the other being the tag being locked.
Leaving margin
Do not fill a tag. Content grows, and a tag chosen to fit exactly today is a tag that has to be replaced when the address gains a parameter or the name gains a middle initial.
A comfortable rule is to plan for roughly half the reported free space. That absorbs record headers, future growth and the difference between chip families in the same order.
If your content will not fit in half, the honest options are a shorter URL, fewer records, or a larger tag, in that order of cost.
Frequently asked questions
Why is free space smaller than the advertised capacity?
Because the identifier, the configuration, the lock bits and the container framing are all deducted first. The gap is normal and differs between chip families.
Does the free space change after I write?
The available space stays the same. What changes is how much of it is used, and a tool will typically show both the capacity and the current content size.
Can I trust the figure from any tool?
It comes from the tag rather than from the tool, so tools agree. What varies is whether a tool shows available space, total memory, or both, so check which number you are reading.
How much space does a URL need?
Roughly one byte per character, minus the abbreviated prefix, plus a small record header. A short address on a common consumer tag leaves plenty of room.