Should you use one or two spaces after a sentence? This is one issue about which many people disagree. Many people assume that whatever they are used to doing must be the correct method.
The origin of the two-space rule was for typewriters. The problem typewriters had was that some letters are wider than others, but when typed they take up the same amount of space on the page. For example, the letter w is very wide and i is very thin. So a typewriter leaves small spaces between some letters. This method of printing is monospaced. To make sentences slightly easier to read a bigger space was needed between monospaced sentences. Hence when using a typewriter it is good advice to leave two spaces after each sentence. Before typewriters, traditional printers usually used thinner typeface for the letter i than for the letter w. This method of printing is proportional.
To show the difference, below are twenty letters w and i. The monospaced letters are the same length. However the w letters touch each other and the i letters have a wide space between each other. The proportional letters do not take up the same length. The w letters still touch each other, but the i letters are much closer to each other.
Today most typing is on computers and they usually use proportional fonts with a different amount of space for each letter. Having only one space after a full stop (period in US English) makes most text written on computers slightly easier to read. However some computer fonts, such as Courier are monospaced. They should be treated as if they were written on a typewriter.
The difference is quite small; generally it is not that important whether you use one or two spaces. However if you read a lot, small differences might make a text more pleasing. Small variations might be the difference between an A- or a B+ for your paper or selling your book or not.
It is better to leave two spaces after a sentence when using a typewriter or a monospaced font. With most other fonts it is better to leave only one space after the full stop. Use either one or two spaces after a full stop, do not switch between them. Most of the time keep doing what you are used to doing, trying to make this small change can be an extra burden. Whatever you are used to doing is the correct method for you.