From an old value to a new one — increase or decrease.
Increase 80 by 25% → 100. Works live as you type.
Percentage change follows the standard formula: (new value − old value) ÷ old value × 100. Note that the base matters — a rise from 80 to 100 is +25%, but a fall from 100 back down to 80 is −20%, not +25% in reverse, because the two calculations use a different starting (base) value.
Applying a percentage to a number works differently and covers both directions: to increase a value by a percentage, use new = from × (1 + percent/100); to decrease it, use new = from × (1 − percent/100).
These are two genuinely different questions people often mix up — 'what percentage did this change by' versus 'what is this number after a percentage change' — and mixing up which formula to use is one of the most common everyday percentage mistakes, which is exactly why this calculator handles both cases separately rather than assuming one.
Because percentage change always uses a different base value depending on direction. Going from 80 to 100, the base is 80, giving +25%. Going from 100 to 80, the base is 100, giving -20%. The absolute change (20) is the same, but the percentage isn't, since the starting point differs.
Multiply the original number by (1 + percent/100). For example, increasing 200 by 15% is 200 × 1.15 = 230.
Multiply the original number by (1 - percent/100). For example, decreasing 200 by 15% is 200 × 0.85 = 170.
Confusing 'percentage change between two numbers' with 'applying a percentage to a number' — they use different formulas and answer different questions, which is exactly why this calculator keeps them as separate, clearly labeled tools.
