The Project Gutenberg FAQ - V-86

V.86. One space or two at the end of a sentence?

Whichever you prefer, but if using two spaces, please use them only at the end of a sentence, not after abbreviations like "Dr." and "per cent.", and not after non-sentence-ending punctuation like the question-mark in the sentence: "Must you go? when the night is yet so black!"

Many people have strong views on either side of the "one space or two?" question, and we're not about to try and argue with them. Use whichever is most natural for you.

However, if using two, you take responsibility for deciding where the sentence ends. You can't just place two spaces after every period, question-mark and exclamation mark, since periods are also used for abbreviations end ellipses, and question-marks and exclamation-marks don't always end sentences.