How it works
- You set a number and a percentage with two sliders. The headline label rewrites itself as you drag, so it always reads as the question you are asking rather than as a generic caption.
- The part is value × percent ÷ 100. The remainder, the value plus the percentage and the value minus the percentage are all derived from that same product, which is why the rows always agree.
- Output is rounded to two decimal places for display. The underlying arithmetic uses full double precision, so the proportion bar and the rows are computed from unrounded values and only the presentation is rounded.
- The proportion bar splits the number into the part and the rest. It is the same two figures as the rows, drawn to scale.
- Everything runs on each input event, in this page. Nothing is submitted, stored or logged.
Limitations
Percentage change is undefined when the starting value is zero. Growth from zero to any number is not a percentage — dividing by zero has no answer, and tools that report 'infinite' or '100%' there are making something up. This one refuses instead. If your baseline is genuinely zero, report the absolute change and say so.
A percentage increase followed by an equal percentage decrease does not return you to where you started, and this trips people up constantly. Add twenty percent to a hundred and you get a hundred and twenty; take twenty percent off that and you get ninety-six. The second percentage applies to a larger base. The tool computes each operation correctly; the intuition is what is wrong.
Rounding to two decimals means a chain of calculations can drift. If you are computing a percentage of a percentage of a percentage, carry the full figures through rather than retyping the rounded display value at each step.
Questions
Are my figures saved anywhere?
No. The numbers exist only in this page while it is open. There is no account, no history, and no analytics event carrying your amounts.
How accurate is the result?
The arithmetic is exact in double precision; only the display is rounded to two decimals. For money that is more precision than you need, and for scientific work you should carry the raw figures rather than the displayed ones.
Why can't I calculate the change from zero?
Because percentage change divides by the starting value, and dividing by zero has no answer. Any tool that gives you a number there has invented it. Report the absolute change instead — 'up by 40' rather than 'up by some percentage'.
Why doesn't +20% then −20% get me back to the start?
Because the second percentage applies to the increased number, not the original. A twenty percent rise followed by a twenty percent fall leaves you four percent down. To reverse a twenty percent rise you need a decrease of one sixth, not one fifth.
Do I need an account?
No. There is no sign-up, no email step and no usage counter. The tool is paid for by the ads on this page.
When people use it
Most people open this with one figure to check before they act on it: a tip, a commission line on a statement, a mark on a test, a supplier's quoted increase. The sliders sit directly under the heading, so the answer appears before you have finished reading the page.
The second use is sanity-checking someone else's arithmetic. An invoice says a fifteen percent discount was applied and the total does not look right; a report claims a rise that does not match the two numbers printed above it. Setting the two figures and reading the rows takes a few seconds and settles it either way.
Percentages usually end up inside something written to a limit — a report summary, a listing description, a status update with a character cap. Word count gives the live figure while you trim the sentence the number sits in.
How it compares
A phone calculator does this too, and does it faster if you already know which operation you want. What this page adds is that it shows all four related quantities at once — the part, the remainder, the value increased and the value decreased — so you do not have to decide in advance which of them was the question. Where a spreadsheet is better is any case with more than one row: percentages across a table belong in a column formula, not in a slider.