Handling Numbers

When writing prose it is common to write single digit numbers, i.e. one to nine out in full and to write numbers larger than 10 in numerals. The same is true with contracts, although it is not unusual to see numbers larger than 10 written out in full too.
In contracts, numbers are usually followed by their numerical equivalent in parentheses.
There is no legal requirement for this practice, however it ensures that confusion, misreading, and misunderstandings are avoided.

In English speaking countries it is common to use a comma in larger numbers (e.g. 10,000).

 However, you may wish to note this:

“In Continental Europe the opposite is true, periods are used to separate large numbers and the comma is used for decimals. Finally, the International Systems of Units (SI) recommends that a space should be used to separate groups of three digits, and both the comma and the period should be used only to denote decimals”

Taken from: http://www.dailywritingtips.com/10-rules-for-writing-numbers-and-numerals/

This means that in some countries “10 000 or 10.000” might be more widely used, so if looks like one of the parties involved might be located in a non-English speaking country, then writing numbers out in full with serve as a way of avoiding misunderstandings.

Be sure to use half-width characters for the yen sign.
If the file corrupts on your clients computer and the character becomes garbled (often as a “\”).
When in doubt use “yen” or “JPY”: 10,000 yen or 10,000 JPY.

Monetary amounts are sometimes written out in full too: “one million one hundred thousand yen (￥1,100,000)”.　However, with the Japanese monetary system, numbers get large very quickly and writing “three million six hundred thousand yen (3,600,000 JPY)” can slow the reader down.

Percentages are also sometimes written out in full too: eighty-five percent (85%)

Whether you decide to write numbers larger than 10 out in full is ultimately your decision, and once you have made your decision be sure to be consistent, especially throughout the same document!

(Updated from an article published by Orian in August 2012)