The contribution of Greek to the English vocabulary can be quantified in two ways, type and token frequencies: type frequency is the proportion of distinct words; token frequency is the proportion of words in actual texts.
Since most words of Greek origin are specialized technical and scientific coinages, the type frequency is considerably higher than the token frequency. And the type frequency in a large word list will be larger than that in a small word list. In a typical English dictionary of 80,000 words, which corresponds very roughly to the vocabulary of an educated English speaker, about 5% of the words are borrowed from Greek directly, and about 25% indirectly.
Since the living Greek and English languages were not in direct contact until modern times, borrowings were necessarily indirect, coming either through Latin (through texts or various vernaculars), or from Ancient Greek texts, not the living language. More recently, a huge number of scientific, medical, and technical neologisms have been coined from Greek roots—and often re-borrowed back into Modern Greek.
•(references:) Konstantinidis, Aristidis (2006): Η Οικουμενική Διάσταση της Ελληνικής Γλώσσας 'The Universal Reach of the Greek Language'