百分比计算器

The four calculations people actually need

1. What is X% of Y?

result = Y x (X / 100)

A 15% tip on an $80 meal is $12.

2. What percentage is X of Y?

result = (X / Y) x 100

$12 is 15% of $80.

3. Percentage change

result = ((new - old) / old) x 100

Always divide by the original value, not the new one. Dividing by the new value gives a different, wrong number — a common mistake in reporting.

4. Increase or decrease by X%

result = Y x (1 + X/100)

A price of $80 increased by 15% is $92, not $95. The 15% applies to the original.

A note on the classic "20% increase then 20% decrease" trap

These do not cancel out. A $100 price increased by 20% becomes $120. Decreasing that by 20% gives $96, not $100. You need a 16.67% decrease to return to the original. This matters when reading compounding claims in marketing copy.

Percentage points vs percent

These are different and often confused:

• Percentage points — the raw difference between two percentages. Interest going from 4% to 6% is a rise of 2 percentage points.
• Percent change — the relative change. Interest going from 4% to 6% is a rise of 50%.

Related

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