Percentage Calculator — Three Modes, Instant Answers

How to use the percentage calculator (3 steps)

1. Pick a mode. "% of a number" answers what is 15% of 240? "What % is X of Y" answers 18 is what percent of 90? "% change" answers from 120 to 156, what is the increase in percent?
2. Enter the two numbers. The answer updates live — no equals button, no rounding surprises (full precision is kept internally; the display rounds to 4 decimal places and trims trailing zeros).
3. Read the sign. In % change mode a positive result is an increase, a negative result is a decrease — the label says which, so a price drop is never mistaken for a gain.

The three formulas, shown plainly

Percent of a number: X% of Y = Y × X ÷ 100. What percent: (X ÷ Y) × 100. Percent change: ((new − old) ÷ old) × 100. Note the percent-change formula divides by the old value — the most common mistake is dividing by the new one, which gives the wrong answer in every real case (a $50 item rising to $60 is +20%, not +16.7%).

Who needs this

Students checking homework, shoppers verifying a discount ("20% off $89.99 — is that really $17.99 saved?"), freelancers computing raise and rate changes, and anyone doing margin or tax math at a desk without Excel open.

FAQ

• Can I use decimals? Yes — enter 3.5 or 0.75 in any field; the math is exact floating point.
• What about negative numbers? They work. A price falling from $100 to $80 correctly shows −20% in the change mode.
• Does this tool send my numbers anywhere? No — it is pure client-side JavaScript.