The European Commission number, or EC number, also known as EC-No and EC#, is the seven-digit code that is assigned to chemical substances that are commercially available within the European Union. This number is assigned by the Commission of the European Union; the EC number is the official number of a substance in the European Union.

The EC number is made up of seven digits according to the pattern xxx-xxx-x.

?EINECS numbers start with number 200-001-8 (formaldehyde). There are currently 100,204 entries.
?ELINCS numbers start with 400-010-9 (trade name: "indosol yellow SF-2RL"). There are currently 4,381 entries.
?NLP numbers start with 500-001-0 ("2-methylpropene, trimers"). There are currently 703 entries.

The EC Number may be written in a general form as: NNN-NNN-R, where R is a check digit and N represents integers. The check digit is calculated using the ISBN method. According to this method, the check digit is the remainder of the following sum after division by 11:

If the remainder R is equal to 10, that combination of digits is not used for an EC number. To illustrate, the EC number of dexamethasone is 200-003-9. N₁ is 2, N₂ through N₅ are 0, and N₆ is 3.

The remainder is 9, which is the check digit.
The substance to which an EC number refers (and vice versa) can be looked up on the official "European Chemical Substances Information System" web site, ESIS.