How to use this percentage calculator
Each row on the page is a complete sentence with the numbers left blank. Find the sentence that matches your question, overwrite the example values, and the answer at the end of the line updates as you type. To find a tip, type 18 and the bill total into the first row. To see what share of a 400-question exam you passed with 50 correct, use the second row (12.5%). The third row handles before-and-after comparisons such as a rent rising from 80 to 100 and tells you whether that is an increase or a decrease and by how much. The fourth row applies a percentage in both directions at once, and the fifth reverses the whole thing: 30 is 15% of what number?
The percentage change formula behind the third row is (new − old) ÷ old × 100. Because the starting value is the base, the same two numbers give different answers depending on direction: 80 to 100 is +25%, but 100 back to 80 is −20%. The copy button beside each result puts just the answer on your clipboard so you can paste it into a message or spreadsheet.
Why the points row and the reverse row matter
The final row deals with the most common mix-up in news reports and survey results. If a mortgage rate moves from 10% to 12%, it has risen by 2 percentage points, but relative to the old rate that is a 20% increase. Both statements are true and both are useful, yet they describe very different things. Swapping them is how a modest change in an interest rate or a poll number gets reported as a dramatic one. Enter the two rates and the tool shows both figures side by side, so you can quote the right one.
Reverse percentages catch people out when tax is involved. A receipt showing 230 with 15% tax included does not mean the tax was 15% of 230; the tax was charged on the pre-tax price, so you divide by 1.15 to get back to 200. The "is what percent of what number" row and the increase and decrease row together let you check a price both ways in a few seconds, without keeping any formulas in your head.