What percentage of the world's Christian population are also Roman Catholic?

Roman Catholic Answer

That depends on how you define "Christian". The obvious answer is 100% as all Christians are Catholic, even if they are not a visible member of the Church, if they have been validly baptised and profess the Nicene Creed in its traditional interpretration. Those who call themselves "Christian" but do not belong to the Church have classically been known as heretics, the explanation of which is below. The current usage of the word "Christian" to mean practially anything is to be deplored, it is even used of pagans who display some virtue which could be considered Christian. Those who have been raised in separated faiths, such as protestants, are only material heretics as explained below.

from Modern Catholic Dictionary by John A. Hardon, S.J. Doubleday & Co., Inc. Garden City, NY 1980

Heresy. Commonly refers to a doctrinal belief held in opposition to the recognized standards of an establish system of thought. Theologically it means an opinion at variance with the authorized teachings of any church, notably the Christian, and especially when this promotes separation from the main body of faithful believers.
In the Roman Catholic Church, heresy has a very specific meaning. Anyone who, after receiving baptism, while remaining nominally a Christian, pertinaciously denies or doubts any of the truths that must be believed with divine and Catholic faith is considered a heretic. Accordingly four elements must be verified to constitute formal heresy; previous valid baptism, which need not have been in the Catholic Church; external profession of still being a Christian, otherwise a person becomes an apostate; outright denial or positive doubt regarding a truth that the Catholic Church has actually proposed as revealed by God; and the disbelief must be morally culpable, where a nominal Christian refuses to accept what he knows is a doctrinal imperative.
Objectively, therefore, to become a heretic in the strict canonical sense and be excommunicated from the faithful, one must deny or question a truth that is taught not merely on the authority of the Church but on the word of God revealed in the Scriptures or sacred tradition. Subjectively a person must recognize his obligation to believe. If he acts in good faith, as with most persons brought up in non-Catholic surroundings, the heresy is only material and implies neither guilt nor sin against faith. (Etym. Latin haeresis, from the Greek hairesis, a taking, choice, sect, heresy.)

Of the some 2.2 billion people who are Christians approximately 1.2 billion are Roman Catholic. So slightly more than 50% of Christians are Roman Catholic.