1) Be rigorously consistent in the use of figures or words to express a number.
Realise that generally, a reader comprehends figures more readily than their reflection in words. If a provision or contract contains both figures and words, use either figures or words for all the numbers of the same category.
Zero and one. Clarification may nevertheless be desirable in the case of 0 (zero), or if a number in a contract clause is a remarkably low one (e.g. as in “the purchase price for the Shares is EUR 1 (one euro)”).
2) Use words for simple figures from one to ten. Use figures for numerals from 11 upwards and for all figures that include a decimal point or a fraction.
Best practice rules 1 and 2 compete for priority. For example:
3, 14, 975 and 6,650
4.25 and 4¼
During the initial three years of the Term, Purchaser shall order at least 4, 8 and 21 containers, respectively.
It may occasionally be desirable to write out round numbers up to twenty, if this can be defended in terms of consistency.
3) Always use figures in percentages, for cross-references and serial numbers, for ranges denoted by a dash, in tables, for statistics and for votes.
Percentages: 4 percent
Cross references and serial numbers: page 25, Article 9, Section 3, Part 2
Ranges denoted by a dash: sections 3.14 to 3.15
Statistics: 3 managing directors were appointed in 2008, 2 in 2009…
Votes: 6 members were in favour, 3 against, and 2 abstained
Avoid starting a sentence with a figure; otherwise, write the number in words instead.
4) Best practice rules 1 to 3 also apply to ordinal numbers (e.g. second, fourth, 12th, 51st).
5) In English contracts, use commas to separate grouped thousands and use points to separate round numbers from decimals.
Note that this may be different in other languages. For example:
EUR 2,750.75 plus EUR 1,249.25 equals EUR 4,000.
6) Write out hundred and thousand in words or figures as is required for consistency. For rounded millions or higher use figures, words or their combination.
500 or five hundred but not 5 hundred
EUR 3,000 or three thousand euro but not EUR 3 thousand
2.5 million, 3 million, 31 billion
7) Avoid abbreviating millions by M or mln.
In several languages, the abbreviation M or m is used for thousands (e.g. in French, Italian and Spanish thousand is ‘mille’ or ‘mil’). Note that in Roman numbering, M indicates one thousand. Do not abbreviate billions (or more).
8) Avoid combining single-digit figures and words by using hyphens, but write out instead.
For example: a three-year term; a five-door car. But note custom phrases such as 40-hour week, 24-hour services, 4-wheel drive.
9) Do not add two decimal zeros after round (cardinal) amounts. For figures smaller than 1 add zero before the point.
An exception may apply when consistency or precision so requires, for example in tables or when other amounts are not round amounts (and all are part of one calculation). For example:
EUR 2,750.75 plus EUR 6,000.00 plus EUR 1,249.25 equals EUR 10,000.
…the default interest shall be further increased by 0.85.
Do not write out decimals.
10) When two numbers are adjacent, spell out one of them.
Usually, it would be the first. For example:
140 fifty-kilogram packages
Seventy 44-eurocent stamps
11) In English, compound numbers that are written out, take a hyphen.
In English, numbers below one hundred are compounded. For example: thirty-first; nineteen hundred sixty-six.
12) Use figures in a combination with units of measurement that are denoted by a symbol or an abbreviation.
250 kW or two hundred and fifty kilowatts
205 μg or two hundred and five micrograms
5 °C or five degrees Celsius
The opposite does not hold. If the units of measurement are spelled out, the numbers may be written as figures: 250 kilowatts, 500 metres.
13) In contract clauses, use the official (ISO) currency abbreviation with the related amounts (that appear in figures).
Examples of the official ISO currency abbreviations are EUR, USD and GBP. Note that the official notation of U.S. Dollars is USD and not US$. Accordingly, do not use the currency symbol (e.g. €, $, £). For example:
EUR 50 or one hundred euro
If, despite the above best practice rule, you do use the currency symbol, never put it behind the amount unless the national rule so prescribes for that symbol (i.e. never write 50€, 134$ or 13£).
The official plural of euro is euro (i.e. not euros). The official rule is not to capitalise euro (i.e. not Euro).
14) Avoid using fractions. When used, fractions should always be spelled out in words, even when the figures are higher than ten, unless they relate to round numbers.
a two-thirds increase
increased by two thirds
two-thirds completed, but not: ⅔ completed
Never contrast or compare a fraction with a decimal (i.e. avoid the applicable interest rate shall be 6¼ percent instead of 5.1 percent).
16) When a range of figures is indicated by a dash, do not repeat the symbol or multiple if they do not change. Close up the dash between the figures.
For example: 5,000-6,000, 5-6 percent and 5 billion-6 billion (not 5-6 billion).
In running (contract) text, use to (5,000 to 6,000 employees).
 The standard official abbreviations are listed in ISO 4217.