Percentage Formula Guide
Every percentage formula you need — percent-of, increase, decrease, change, and reverse calculations with examples.
4 min read · Last updated: July 13, 2026
Percent of a number
Result = (percent ÷ 100) × number. Example: 15% of 80 = 0.15 × 80 = 12. Use this for tips, tax, and discounts.
Percent increase and decrease
New value = original × (1 ± percent/100). A 20% increase on 50 = 50 × 1.20 = 60. A 20% decrease = 50 × 0.80 = 40. Note: increasing then decreasing by the same percent doesn't return to the original.
Percent change between two values
Percent change = ((new − old) ÷ old) × 100. If a stock goes from $40 to $50, change = (10/40) × 100 = 25%. The original (old) value is always the denominator.
Reverse percentage
Find the original when you know the result and percent applied. If $120 is after a 20% markup, original = 120 ÷ 1.20 = $100. Our Percentage Calculator handles all five modes instantly.