How do you calculate percentage increase?

Percentage change between two numbers A & B can be calculated as:

(B-A)/A * 100

For example, if a stock price increases in value from $123 to $145 the percentage increase is:

(145-123)/123 * 100 = 22/123 * 100 = 0.179*100 = 17.9%

Alternatively, if a stock price decreases in value from $145 to $123 the percentage decrease is:

(123-145)/145 * 100 = -22/145 * 100 = -0.152*100 = -15.2%