According to the Oxford Dictionaries, one millimetre is the same as 0.039 inches in terms of length. Therefore, 25 millimetres is equal to 25 times 0.039, or 0.9775 inches.
The more frequent conversion is from inches to centimetres, where 1 inch equals 2.54 cm. In other words, one centimetre is equivalent to 1/2.54, or 0.39. However, 1 centimetre is equivalent to 10 millimetres, therefore 0.39 must be divided by 10 to determine the number of millimetres in 1 inch. The answer given by Oxford Dictionaries is 0.039.