Overview
Percentages run the background math of money — discounts, tips, raises, interest, the growth figures in every meeting. Most people nail the easy ones and quietly approximate the rest, and approximation is where the expensive misunderstandings hide.
The design decision here is separation. “20% off 150”, “30 is what percent of 150” and “80 to 100 changed by what” look alike and get confused constantly. Giving each its own inputs removes the most common mistake before any typing happens.
What is a percent, precisely?
“Per hundred” — a fraction with denominator 100. So 20% is 20/100, and “20% of 150” is simply (20/100) × 150 = 30. The reverse question divides: 30/150 = 0.2, expressed per hundred as 20%. Two operations, mirror images.
Percent change is where intuition breaks. From 80 to 100 is (100−80)/80 = 25% — but back down from 100 to 80 is −20%, because the reference point moved. The calculator labels the direction explicitly so the asymmetry can't sneak past you.
Features
- Three dedicated modes with their own inputs — no mode confusion.
- Live answers updating with every keystroke.
- Direction labels on change mode: increase, decrease or flat.
- Clean decimals: floating-point dust like 22.49999999% is trimmed.
- Self-explaining defaults so each mode's purpose is obvious.
- Your numbers persist when switching between modes.
- Zero clutter: two boxes and an answer per mode.
- Private: all arithmetic stays in your browser.
Benefits
- End mental-math dread. Tips, discounts and raises in seconds.
- Ask the right question. Separate modes stop the classic mix-ups.
- Exact results. Clean decimal handling you can quote.
- Free forever, working offline once loaded.
Who can use it
Shoppers sanity-checking sale prices, diners splitting bills fairly, freelancers adding tax or commission to quotes, students doing homework, and managers who want growth figures stated correctly in the meeting. Anyone who has nodded along to a percentage they didn't follow will find the change mode especially clarifying.
How to use
- Choose the mode matching your question: of, share-of, or change.
- Enter the two numbers in the labelled boxes.
- Read the live answer as it updates beneath.
- On change mode, note the increase/decrease label for describing it properly.
- Switch modes freely with the tabs — your numbers stay where you left them.
Frequently asked questions
How do I work out 20% off $150?
First mode: 20% of 150 is 30, so the discount is $30 and you pay $120. Equivalently you're paying 80% of 150 — same $120. Either framing lands in the same box.
Why isn't a 25% rise reversed by a 25% fall?
Because percentages anchor to their starting point. Up from 80 to 100 is +25% (20/80); down from 100 to 80 is −20% (20/100). “Up 50%, down 50%” leaves you at 75% of where you started — the single most useful percentage fact there is.
What's the difference between percent and percentage points?
A move from 20% to 25% is 5 percentage points but a 25% relative increase (5/20). Interest rates and polls use percentage points precisely to dodge this ambiguity — borrow the habit in your own writing.
Can a percentage be over 100%?
Of course. 150% of 80 is 120; a 200% increase triples the original. “Percent” only means “per hundred” — nothing caps it at one hundred.
Are my numbers stored?
No. The arithmetic runs entirely in the page's memory and evaporates when you leave. Nothing is uploaded or remembered.
Conclusion
Percentages have exactly three everyday shapes — of, share-of, change. Learn to recognise which question you're asking, put it in the right box, and the money math stops being guesswork.