Gauging the difference between classical and instrumental music comes down to the definition of either form, and can be difficult to state precisely. Classical music has several definitions. It can refer to the music of a specific area or country, such as classical Romanian music. It also refers to a strict period of Western music production between 1750-1820 that was expressed with a range of different instruments, instrument arrangement and vocal productions. Composers of the classical music period include Mozart and Beethoven.

Sometimes people use classical music to refer to music written prior to the early 20th century that might be performed by an orchestra or a symphonic band. Classical music could be operas, solos, and small arrangements for chamber orchestras, quartets, quintets or trios. In this case the difference between classical and instrumental music is that instruments used in instrumental music can be any instruments. Electric guitars, synthesizers or any instruments developed after the early 20th century would be examples of those capable of producing instrumental but not classical music.

In defining the difference between classical and instrumental, one difference may be instrumental music that is produced in the style of classical music but written after the 20th century. In this case, we might refer to the difference between classical and instrumental music as a difference based on the era. Alternately, we might look at the difference between classical and instrumental music as the fact that classical includes vocal arrangements while instrumental implies no vocals.

From a time perspective, you could evaluate the difference between classical and instrumental music by saying, some modern film scores are instrumental music, evocative of the classical era. Here, you are conscious that the modern film's score may sound like classical music, but isn’t because it was written recently.

Instrumental music in strictest definition is music played on an instrument. Some people include the human voice as an instrument, but others do not. Therefore we can say one difference between classical and instrumental music is that only classical music without singing is instrumental. If voice is included in the definition of instrumental music, then many modern bands can’t even be classed as instrumental. Further they wouldn’t be considered classical, especially if they played in different genres like rock or ska. A musician like Bono from U2 in all definitions is not considered a classical musician.

When instrumental means without vocal accompaniment, any music composed without vocals would thus be instrumental. In this case, the difference between classical and instrumental music comes down to fine details and how strict the definition is of either form. To some, all classical music, vocal or not, is instrumental. To others, instrumental music means without vocals, so some classical music qualifies as instrumental. To others there is no difference between classical and instrumental music since both genres are produced on instruments.