How to use this tool
- Pick the calculation you need using the tabs above the calculator (percentage of a number, percentage change, discount, tip split, or reverse percentage).
- Type your values into the input fields — the result updates instantly, with no "Calculate" button to click.
- Use the direction toggle on the Reverse % tab to specify whether the original percentage was an increase or a decrease.
Common mistakes to avoid
- For percentage change, don't divide by the new value — always divide by the original (starting) value, or the sign and size of the result will be wrong.
- Stacked percentages don't add up: "25% off, then 10% off" is not 35% off — each discount applies to the already-discounted price.
How to calculate a percentage
To find X% of Y, multiply Y by X and divide by 100: (X ÷ 100) × Y. To find what percentage one number is of another, divide the part by the whole and multiply by 100:(part ÷ whole) × 100.
Percentage increase and decrease formula
Percentage change compares an old value to a new one: ((new − old) ÷ old) × 100. A positive result means the value went up; a negative result means it went down. This is the same formula used for salary raises, price changes, and growth rates.
Discount and tip calculations
A discount removes a percentage of the original price: discount = price × (percent ÷ 100), and the final price is price − discount. A tip works the same way in reverse — it's added on top of the bill, then optionally split evenly across a number of people.
Frequently asked questions
- How do I calculate a percentage of a number?
- Multiply the number by the percentage, then divide by 100. For example, 20% of 150 is (20 × 150) ÷ 100 = 30.
- How do I find what percentage one number is of another?
- Divide the first number by the second, then multiply by 100. For example, 30 is what % of 150? (30 ÷ 150) × 100 = 20%.
- How do I calculate percentage increase or decrease?
- Subtract the original value from the new value, divide by the original value, then multiply by 100. A positive result is an increase; a negative result is a decrease.
- How do I reverse a percentage to find the original value?
- Divide the resulting value by (1 + the percentage as a decimal) if it was an increase, or by (1 − the percentage as a decimal) if it was a decrease.
Every calculation on this page runs locally in your browser using plain JavaScript — nothing you type is sent anywhere.
How GetByteRush compares
Most free tools for this online work the same way: you upload your file to their server, wait, then download the result — which means your file (and often your email, for a "free" account) sits on someone else's infrastructure. Here's the actual difference:
| Feature | GetByteRush | Typical online tools |
|---|---|---|
| Runs without uploading your numbers | Yes — pure client-side JS | Often submits the form to a server |
| Works with no internet after the page loads | Yes | No — needs a live connection per calculation |
| Ads-driven redirect / interstitial pages | Never | Common on ad-heavy calculator sites |
| Sign-up required | No | Sometimes, for "save your results" |
("Typical online tools" reflects publicly documented behavior of common free file-processing sites as of 2026 — always check a specific competitor's own privacy policy and pricing page, since these details change.)