Code 25, also known as the Interleaved 2 of 5 barcode, contains numeric data only and is typically used for coding on cartons of products and industrial applications. Code 25 barcodes are encoded in pairs of digits, meaning that the two digits are interleaved with each other to create the barcode symbol. The name 2 of 5 originates from the fact that 2 out of every 5 black lines and spaces are wider than the rest. The wider bars are usually 2-3 times the width of the narrower bars.
Since only an even number of digits can be encoded in the Code 25 barcode, if you do have an odd number of digits that need to be encoded, simply add a 0 before the first digit. The Code 25 barcode needs an even number of digits because the first digit needs to be encoded on the bar or black lines, whereas the second digit would need to be encoded on white lines or spaces.