According to the CIA World Factbook it is estimated that the overall world literacy rate was 82%. So the illiteracy rate would be around 18%. Illiteracy is defined as those over 15 who can read and write.
Also from the CIA World Factbook: over two-thirds of the world's 785 million illiterate adults are found in only eight countries (India, China, Bangladesh, Pakistan, Nigeria, Ethiopia, Indonesia, and Egypt); of all the illiterate adults in the world, two-thirds are women; extremely low literacy rates are concentrated in three regions, South and West Asia, Sub-Saharan Africa, and the Arab states, where around one-third of the men and half of all women are illiterate (2005 est.)
Here is a link to more information on Wikipedia - en.wikipedia.org/wiki/Literacy