Latin word ratus comes from Proto-Indo-European *reh₁-, and later Proto-Italic *rēōr (Reckon, calculate.. Think, deem, judge.)

Detailed word origin of ratus

Dictionary entry	Language	Definition
*reh₁-	Proto-Indo-European (ine-pro)	to put in order
*rēōr	Proto-Italic (itc-pro)	Reckon, calculate.. Think, deem, judge.
reor	Latin (lat)	I reckon, calculate.. I think, deem, judge.
ratus	Latin (lat)	Established, authoritative. Fixed, certain.

Words with the same origin as ratus