Jay wanted
Binary to BCD converter that will convert from 0 to 18

Richard suggested a PROM; Bordyn replied
enourmous hex file made.

Not really - only 19 entries (0-18) are required!
Since 18 requires 5 bits, it'd be sensible to decode all 5, and decide on an output for "out-of-range" inputs.
Even so, that's still only 32 entries - hardly "enormous!?"