Search the sides of your cassette tape for any numbers indicating length of playback. Look for a figure such as "C60" or "C90" which indicates the total playback length of both sides in minutes. For example, a C60 tape has 30 minutes of playback time on each side of the cassette.
Find a stopwatch or any other device that can measure time in minutes such as a mobile phone. Use minutes to measure the playback time of your cassette since all cassette tapes are measured in minutes, rather than seconds or hours.
Place the cassette tape in your cassette player and rewind it to the beginning of one side.
Press play on the cassette tape player and the stopwatch simultaneously.
Stop the stopwatch when the cassette player stops playing the cassette tape. The time it takes for the cassette's spools to pass and wind the length of its magnetic tape is the length of one side of the cassette tape.
Multiply the time it took for the cassette tape to play through one side by two. The figure you calculate is the length of the entire cassette tape.