Brand-to-brand size at a fixed US label — matched by foot length (cm), not guesswork.
Two charts · cm matchHoka US 9 maps to about 26.1 cm on that brand’s chart. The nearest On row at that length is US 9 / EU 42.5.
Same chart cm can still feel different on foot. Read the guides:
Hoka: Bondi 8 (true to size) · Clifton 9 (true to size) · Speedgoat 5 (true to size)
On: Cloudmonster (true to size) · Cloud 5 (runs small) · Cloudsurfer (true to size)