Brand-to-brand size at a fixed US label — matched by foot length (cm), not guesswork.
Two charts · cm matchUGG US 7 maps to about 24.4 cm on that brand’s chart. The nearest Crocs row at that length is US 6.5 / EU 39.
Same chart cm can still feel different on foot. Read the guides:
UGG: Classic Mini (true to size) · Classic Short II (true to size) · Tasman Slipper (true to size) · Classic Ultra Mini (true to size)
Crocs: Classic Clog (runs wide) · Echo Clog (true to size)