The percentage change from initial value (Vinitial) to final value (Vfinal) is equal to the initial and final values difference divided by the initial value times 100%:
percentage change = (Vfinal - Vinitial) / Vinitial × 100%
Price percentage increase from initial value of $1000 to final value of $1200 is caluclated by:
percentage change = ($1200 - $1000) / $1000 × 100%
= 0.2 × 100% = 20%
Price percentage decrease from initial value of $1000 to final value of $800 is caluclated by:
percentage change = ($800 - $1000) / $1000 × 100%
= -0.2 × 100% = -20%