Should there be a space before a percent sign?

Solution 1:

According to Wikipedia there is no consensus on this in English:

There is no consensus as to whether or not to include a space between the number and percent sign in English. Many authorities prescribe that there should be a space, whilst others advise against it. The brochure of the International System of Units declares in chapter 5: "a space separates the number and the symbol %". The ISO 31-0 standard also specifies a space, and the TeX typesetting system encourages using one. This is in accordance with the general rule of adding a non-breaking space between a numerical value and its corresponding unit of measurement. However, style guides – such as the Chicago Manual of Style – commonly prescribe to write the number and percent sign without any space in between.

Solution 2:

There may be no consensus among the standards bodies, but outside of technical writing at least, it doesn't matter what the ISO says. Modern U.S. usage overwhelmingly uses no space. Note that Wikipedia uses no space, as in the article for Percentage.

Demonstration

As a demonstration, one can download the first billion bytes of an English Wikipedia database dump (commonly used as a test file for data compression benchmarks):

user@host:/run/shm$ wget http://mattmahoney.net/dc/enwik9.zip

and count the (approximate) number of occurrences of 50 % and 50%:

user@host:/run/shm$ unzip -p enwik9.zip | fold | grep '50 %' | wc --lines
71

vs.

user@host:/run/shm$ unzip -p enwik9.zip | fold | grep '50%' | wc --lines
9216

(The first 1 usages of 50 % are:

On efficacy measures, a successful antidepressant trial involves just 50 % or mo
 signifies a mere 50 % or greater reduction in depression symptoms as opposed to
* '''Beta brasses''', with 45-50 % zinc content, can only be worked hot, is hard
* '''White brass''' contains more than 50 % zinc and is too brittle for general 
was very profitable for the V.O.C., initially yielding profits of 50 % or even m
s 40 to 50 % of the capacity of the elevator. The grooves in the drive sheave ar
nt process, usually requiring at least 50 % more electricity than the energy sto
 50 % and this in turn affected both the trade-in value of used vehicles and the
er molecules. Water containing 50 % H and 50 % D actually contains about 50 % HD
he 1980s. About 50 % of these moves were within the same prefecture; the others 

and of 50%:

 50% higher than nearby forested areas because snow does not cover the trees as 
<table border=0 cellpadding=2 cellspacing=2 width=50%>
 Afghanistan as being [[Persian language|Persian]] (local name: [[Dari]]) 50% an
Note: Albania has a large gray economy that may be as large as 50% of official G
rriages occur very early in a pregnancy. Approximately 10-50% of pregnancies end
tal, and a 20% increase since 1981, 50% since 1971. Major towns are Peterhead (1
MND is typically fatal within 2-5 years. Around 50% die within 14 months of diag
nosis. The remaining 50% will not necessarily die within the next 14 months as t
n up to 50% of SOD1 cases. In people of [[Scandinavia]]n extraction there is a r
[[Cognitive]] change can and does occur in between 33&ndash;50% of patients.

)

Solution 3:

When the symbol % is used, there should be no space. When the "percent" word is used, there should be space.

Examples from the Chicago Manual of Style Online:

Fewer than 3 percent of the employees used public transportation.

With 90–95 percent of the work complete, we can relax.

A 75 percent likelihood of winning is worth the effort.

Her five-year certificate of deposit carries an interest rate of 5.9 percent.

Only 20% of the ants were observed to react to the stimulus.

The treatment resulted in a 20%–25% increase in reports of night blindness.

The manual explicitly advices the following:

Note also that no space appears between the numeral and the symbol %.