Percentage calculator
Five common percentage questions, each with its own short form. Type two numbers and the answer appears live.
What is X% of Y?
What is %
of ?
→ 30
X is what percent of Y?
is what % of
?
→ 20%
What is the percentage change from X to Y?
From
to :
→ +25%
Increase X by Y%
+ %
→ 115
Decrease X by Y%
− %
→ 85
What this tool does
The five most-asked percentage questions — each as its own input row. Change a number and the answer updates immediately.
The five questions
- What is X% of Y? Multiplies Y by X/100. "20% of 150" = 30.
- X is what percent of Y? Divides X by Y, multiplies by 100. "30 is what % of 150" = 20%.
- Percentage change from X to Y. (Y − X) / X × 100. From 120 to 150 is a +25% change; from 150 to 120 is −20%.
- Increase X by Y%. X × (1 + Y/100). 100 increased by 15% is 115.
- Decrease X by Y%. X × (1 − Y/100). 100 decreased by 15% is 85.
The two most-confused cases
Asymmetry of percentage change. If you go from 100 to 150, that's a +50% change. Going back from 150 to 100 is not −50% — it's −33.3%, because the denominator changes. The percentage change is always relative to the starting value.
Add then subtract. If a number is increased by 10% and then decreased by 10%, it does not return to its starting value. 100 × 1.10 × 0.90 = 99. This catches people in retail pricing, sales reporting, and weight-tracking apps.
Real-world examples
- Sales tax. If a tax rate is 8%, the "Increase X by Y%" calculator gives you the gross price from a net price.
- Discount. A 25%-off sale on a $80 item: use "Decrease X by Y%" → $60.
- Tip. An 18% tip on $42.50: use "What is X% of Y?" → $7.65.
- Year-over-year growth. Revenue went from $1.2M to $1.5M: use "percentage change" → +25%.
Privacy
The math runs in your browser.
Frequently asked questions
Why doesn't a 10% increase plus a 10% decrease return to the start?
Each percentage is relative to a different base. 100 × 1.10 = 110. 110 × 0.90 = 99. The decrease is taken from 110, not from 100. This is a common source of confusion in pricing and finance.
How do I work out tax-inclusive vs tax-exclusive prices?
To add tax to a net price, use "Increase X by Y%". To strip tax from a gross price, divide the gross by (1 + tax-rate/100). For example, $108 ÷ 1.08 = $100 net at 8% tax.
Why is my percentage change showing as negative when the value went up?
Double-check which value is "from" and which is "to". The formula is (to − from) / from × 100. If from is the larger number, a positive trip looks negative.
Can I enter decimals?
Yes. All five calculators accept decimals like 15.5 or 0.085. The output is shown to four decimal places.
Are the calculations rounded?
They display rounded to four decimals but compute at full floating-point precision. For accounting-grade math, treat any browser-based calculation as approximate.