How percentage change works
% change = (new − old) ÷ old × 100
Percentage change compares two values against the starting value. If the result is positive it is an increase; if negative, a decrease. It answers questions like "my rent went from 80 to 92 — how big a rise is that?" without you having to eyeball the difference.
Worked example
From 80 to 92: the absolute change is 92 − 80 = +12. Divide by the old value: 12 ÷ 80 = 0.15, so the percentage change is +15% — an increase.
The reverse change is not the same
To undo a change you measure against the new base. Getting from 92 back down to 80 needs (80 − 92) ÷ 92 × 100 ≈ −13.04%, not −15%. This asymmetry is why a stock that falls 50% must then rise 100% just to break even. The calculator shows this reverse figure for every calculation.
Applying a percentage change
result = number × (1 + change ÷ 100)
The second section works forwards: give it a starting number and a percentage (negative for a decrease) and it applies the change. For example, 250 with a −10% change becomes 250 × 0.90 = 225 — handy for discounts, raises and markups.
When it is undefined
Percentage change from an old value of 0 has no meaning: you would divide by zero. In that case report the absolute change instead ("up 12 units") or use a different baseline.