CodeKitHubWhat Is This Tool?
A percentage expresses a number as a fraction of 100 — 25% means 25 out of every 100. The three questions people actually need to answer are: "what is 20% of 350?" (percentage of a number), "48 is what percent of 60?" (ratio as a percentage), and "the price went from 80 to 95 — what's the change in percent?" (percentage change).
Each has its own small formula, and mixing them up is the most common percentage mistake. This tool keeps them as three separate, clearly-labeled calculators so you always use the right one. As a rule of thumb: if you know the rate and need the amount, use the first calculator; if you know both amounts and need the rate, use the second; and if a value changed over time, use the third.
Why Use It?
- All three everyday percentage questions on one page, clearly separated.
- Live results — numbers update as you type, no button pressing.
- Percentage change shows a signed result (+/-), so increases and decreases are unambiguous.
- Handles decimals and negative numbers correctly.
- Free, instant, mobile-friendly — perfect for shopping and restaurant math.
How to Use
- Pick the block that matches your question.
- Type your two numbers — the result appears immediately.
- First block: 15% of 240 → fill 15 and 240.
- Second block: 45 is what % of 180 → fill 45 and 180.
- Third block: from 80 to 95 → fill 80 and 95 for the percentage change.
Example
Input
What is 15% of 240?Output
36Formula: 240 × (15 ÷ 100) = 36. A 15% tip on a $240 bill is $36.
Common use cases
- Splitting a restaurant bill and calculating a tip amount before the total is divided among the group.
- Working out grade percentages from a raw score out of a total, or the reverse — how many points are needed on a final exam to reach a target grade.
- Comparing sale prices across stores that advertise discounts differently (a flat amount off vs. a percentage off).
- Reading investment or savings account statements that report gains and losses as percentage change over a period.
Why mixing up the three formulas is such a common mistake
All three percentage questions look superficially similar — they all involve two numbers and end in a percent sign — which is exactly why people plug numbers into the wrong formula so often. "20% of 350" and "what percent is 20 of 350" use completely different operations (multiplication vs. division) despite sharing the same two numbers, and swapping them gives a wildly wrong answer (70 vs. roughly 5.7%) rather than a slightly-off one, which is often the first sign something was set up incorrectly. Keeping the three calculators visually and functionally separate, rather than one clever universal input field, is a deliberate design choice to reduce exactly this kind of mistake.
Frequently Asked Questions
How do I calculate a percentage of a number by hand?
Multiply the number by the percentage divided by 100: 20% of 350 = 350 × 0.20 = 70. Mental shortcut: 10% is one decimal shift (35), then scale — 20% is double that.
How is percentage change calculated?
(new − old) ÷ |old| × 100. From 80 to 95: (95−80)/80 × 100 = +18.75%. A negative result means a decrease. Note the base is always the OLD value — that's the most common error.
Why is a 50% increase not undone by a 50% decrease?
Because the base changes. 100 + 50% = 150, but 150 − 50% = 75. Each percentage applies to its own starting value — this asymmetry trips up even professionals.
What's the difference between percentage points and percent?
Going from 10% to 15% is a rise of 5 percentage points, but a 50% relative increase. News often confuses these; this calculator's third block gives you the relative (percent) change.
How do I calculate a discount price?
Use the first block for the discount amount: 25% of 80 = 20, so the sale price is 80 − 20 = 60. Shortcut: multiply by (100 − discount)%: 80 × 0.75 = 60.
How do I use this as a discount percentage calculator?
The first block ("X% of Y") is the discount percentage calculator: enter the discount rate as X and the original price as Y to get the discount amount, then subtract it from the price yourself — or just use the (100 − discount)% shortcut to get the sale price in one multiplication. If you instead know the original and sale price and want to find what percentage was taken off, use the second block ("X is what % of Y") with the discount amount as X and the original price as Y.