Read the bar graph to find the numerical value of each column. The value of a column equals the amount on the opposite axis. For example, if you have a vertical bar graph and the sunny days column goes up to "30" on the y-axis, you would have 30 sunny days.
Add the values of each of the columns to find the total amount of results. For example, if you have 30 sunny days, 20 partly cloudy days, 10 overcast days and 40 rainy days, add 30 plus 20 plus 10 plus 40 to get 100 total days.
Divide the number of specific results by total results to find the portion of data represented. For example, if you want to find the portion of sunny days, divide 30 by 100 to get 0.3.
Multiply the portion by 100 to convert to a percentage. Completing the example, multiply 0.3 by 100 to find that 30 percent of the days are sunny.