Look at the vertical lines that are used to divide the the musical staff. These vertical lines represent “measures.” The measures are also referred to as “bars.”
Reference the “Time Signature” that is located at the beginning of the sheet of music. The top number of the time signature tells how many beats are in a full bar/measure. The bottom number reflects which note gets 1 beat. For example, in the time signature “4/4” there are 4 beats in a measure and the ¼ note gets 1 beat. In the time signature “6/8” there are 6 beats in a measure and the 1/8 note gets 1 beat.
Start counting each measure of music that has the amount of beats specified by the time signature. For example, if the time signature is 4/4, you will count each bar that has 4 full beats. If the time signature switches to 6/8, you will count each bar that has 6 full beats.
Don't count an incomplete measure as a full bar. Quite often a piece of sheet music will start with a short measure that does not have the amount of beats specified by the time signature. In most cases, the incomplete measure will be completed by the last measure on the sheet of music. For example, if the time signature is 4/4 and the first measure only has 1 beat, you will find the remaining 3 beats in the last measure of the song. Therefore, you will combine the first measure and the last measure to make one bar.
Continue counting all of the vertical lines that are on the musical staff to get an accurate count of the amount of bars that is in the musical piece.