Overview
Percentages are the background arithmetic of money: discounts, tips, raises, interest, growth figures in every meeting. Most people handle the easy cases and quietly approximate the rest — and approximation is where the expensive misunderstandings live. This page separates the three questions people actually ask into three dedicated calculators.
The separation is the feature. “20% off 150”, “30 is what percent of 150” and “80 to 100 is what change” look similar and are routinely confused; giving each its own inputs removes the most common error before any number is typed.
What is a percent, exactly?
Literally “per hundred”: a fraction with denominator 100. Twenty percent is 20/100, so “20% of 150” is (20/100) × 150 = 30. The reverse question divides instead: 30/150 = 0.2, multiplied by 100 to express it per hundred — 20%. Two operations, mirror images of each other.
Percent change is the troublemaker. From 80 to 100 the change is (100−80)/80 = 25%, measured against the starting value. Reversing the trip, 100 down to 80, is a 20% fall — not 25% — because the starting point moved. That asymmetry is why the calculator labels the direction outright.
Features
- Three purpose-built modes, each with its own inputs: of, is-what-percent-of, and change.
- Live computation on every keystroke — no calculate button in sight.
- Explicit direction labels on change: increase, decrease or no change.
- Clean decimal handling that trims floating-point artefacts like 22.49999999%.
- Self-explaining defaults with worked examples preloaded in each mode.
- Private arithmetic — everything runs in your browser.
Benefits
- End mental-math dread. Tips, discounts and commissions in seconds.
- Stop mixing up the modes. Separate boxes prevent the classic confusion.
- Exact answers. No rounding mysteries, no spreadsheet needed.
- Free and offline-ready once the page has loaded.
Who can use it
Shoppers checking sale prices, diners splitting bills with tip and tax, freelancers adding commission to quotes, students working through homework, and managers sanity-checking growth figures before presenting them. Anyone who has nodded along to a percentage in a meeting while unsure will find this quietly liberating.
How to use
- Pick the mode matching your question.
- Enter the two numbers.
- Read the live answer as you type.
- On change mode, note the increase/decrease label.
- Switch modes freely — your numbers are kept.
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. The same result comes from asking what 80% of 150 is — the calculator handles either framing, since paying 80% and saving 20% are the same transaction.
Why is 80→100 a 25% rise but 100→80 only a 20% fall?
The reference point changes. Upward: (100−80)/80 = 25%. Downward: (80−100)/100 = −20%. Percentages are always relative to where you started, which is why “up 50%, down 50%” leaves you below where you began.
What is the difference between percent and percentage points?
A move from 20% to 25% is 5 percentage points, but a 25% relative increase (5/20). Polls and interest-rate news use percentage points precisely to dodge this ambiguity — a habit worth borrowing.
Can a percentage be over 100?
Of course. 150% of 80 is 120, and a 200% increase triples the original. “Per hundred” describes the unit, not a ceiling.
Are my numbers stored?
No. The calculations run entirely in your browser and the values disappear when you leave the page.
Conclusion
Three shapes cover nearly all percentage questions: of, is-what-percent-of, and change. Learn to tell them apart — separate boxes help — and the everyday money math stops being guesswork.