Two rolls can both be "4×6 direct thermal labels, 500 per roll" and still be completely incompatible with your printer, because they're wound around different core sizes. It's a spec that's easy to overlook until a roll arrives and simply won't sit on the spindle.
What core size actually means
The core is the cardboard (or sometimes plastic) tube in the centre of a label roll, and the core size is its internal diameter. Your printer's spindle has to match that diameter closely enough to hold the roll steady while it unwinds — too loose and the roll wobbles and feeds unevenly; too tight and it won't go on at all.
Why 25mm is the default
Most desktop thermal printers built for shipping labels — the models covered in our thermal printer buying guide — use a 25mm (1") core as standard. It's small enough to keep the printer's footprint compact for a packing bench, while still being sturdy enough to support several hundred labels of roll weight without sagging.
What happens if you order the wrong core
At best, a mismatched core sits loosely and causes uneven feeding, print skew, or the roll working itself off-centre mid-batch. At worst, it simply doesn't fit the spindle and the roll is unusable in that printer — meaning you either return it or find a workaround like an external unwinder, both of which cost time you didn't plan for.
How to check your printer's core size
The spec is usually listed in the printer's manual or on the manufacturer's product page under "media specifications." If you no longer have the manual, measuring the internal diameter of a roll you already know works in that printer is the fastest way to confirm it — a standard tape measure across the tube opening will tell you in seconds.
Core size vs. roll diameter
It's worth not confusing core size with overall roll diameter — the latter is how fat the wound roll is once labels are on it, which determines whether it physically fits inside your printer's label compartment, separate from whether it fits on the spindle. Both matter, but core size is the one that causes a hard incompatibility rather than a tight squeeze.
If you're setting up a second printer, or connecting one printer to several computers on a packing floor, it's worth checking core compatibility before you standardise your label ordering — see our guide on connecting a thermal printer to multiple computers.
25mm core, every time
Our rolls are wound on a standard 25mm core, sized to drop straight into Zebra, Citizen, TSC and Brother desktop printers.
Shop 4×6 Labels