1" (see big caveat below) is the outside diameter of the steerer. The internal diameter of the head tube may be 30.0mm or, more likely, 30.2mm (it sometimes matters; a 30.0mm cup is too loose in a 30.2mm head tube, although the reverse works ok).
The other key dimension is the crown race seat diameter, which may be 26.4mm or 27.0mm. These are in no way interchangeable.
27.0 goes with 30.0 (JIS headset)
26.4 goes with 30.2 (ISO headset)
Older Raleigh bikes are dimensioned like JIS but the fork is threaded at 26tpi rather than the usual 24tpi. They are an absolute pain to deal with.
Because it involves removing metal, an annoying JIS frameset, generally found with cheaper bikes, can be easily converted to ISO with the correct reamers.
Big Caveat:
Some older French bikes have their own, totally metric, system. The steerer is 25.0mm with a 1mm thread pitch. If you have one of these, you need an actual "French" headset, not ISO or JIS.
