％ Percentage Calculator

The Percentage Calculator handles the three percentage questions people actually ask: what is X% of Y, X is what percentage of Y, and what is the percentage change from one number to another. Each one has its own tab so you never have to remember which formula to use. Every calculation runs instantly in your browser.

How to Use This Tool

Pick the tab for your question. For "X% of Y" enter the percentage and the value (20% of 150). For "X is what % of Y" enter the part and the whole (30 is what % of 150). For "% change" enter the original and new numbers (150 → 180). The answer updates as you type.

What This Tool Checks

Pure arithmetic: (a/100)×b for "percent of"; (a/b)×100 for "what percent"; ((b−a)/a)×100 for percentage change. There is no rounding beyond display, and nothing is estimated.

Understanding Your Results

A positive percentage change is an increase, a negative one is a decrease. Note that a 50% drop followed by a 50% rise does not return you to the start — percentage changes compound off a moving base, which is exactly why the dedicated "% change" tab is useful.

How to Fix Common Problems

If you get an unexpected result, check which tab you are on — "20% of 150" and "20 is what % of 150" are different questions with different answers. Percentage-change needs the original value in the first box; swapping the two numbers flips the sign.

SEO Best Practices

For price changes, discounts, tips, tax and commission, decide first whether you want a part of a whole (percent of) or a comparison between two numbers (percent change). For multi-step changes (a discount then tax), apply them one at a time rather than adding the percentages together.

Example

20% of 150 = 30. 30 is 20% of 150. Going from 150 to 180 is a +20% change; going from 180 back to 150 is a −16.67% change (not −20%), because the base is different.

Common Mistakes to Avoid

The most common mistake is adding percentages that have different bases — a 10% discount and a 10% tax do not cancel out. Another is confusing percentage points with percent: going from 5% to 7% is a 2-percentage-point rise but a 40% relative increase.