QKitBoxPercentage calculator
The three percentage questions people actually ask, answered live as you type — no formulas to remember.
What is X% of Y?
X is what percent of Y?
Percent change (old → new)
How it works
The three modes map to the three percentage questions: a share of an amount, a share as a fraction of a total, and the relative change between two values. Percent change is (new − old) ÷ |old| — the absolute value in the denominator keeps a drop from doubling as a gain.
Frequently asked questions
Why does percent change from 100 to 50 show −50%, but 50 to 100 shows +100%?
Because percent change is relative to the starting value. Halving is always −50%, and recovering from 50 back to 100 is +100% — percentages are not symmetric.
Can I use negative numbers?
Yes — useful for losses or temperature deltas. The change mode handles sign direction explicitly.
Written and maintained by Steven Huang · Last reviewed October 29, 2026.