Percentage Calculator — online, without uploading your files
Each mode is one explicit formula: percent-of multiplies the value by the percentage over 100; what-percent divides the part by the whole and multiplies by 100; percentage change divides the difference by the absolute value of the starting number and multiplies by 100. Using the absolute value means a change that starts from a negative number still reports a sign that follows the direction of the move. The first mode returns a plain number and the other two append a per cent sign. Results are displayed to at most eight decimal places, grouped according to your browser locale.
Quick Summary / Direct Answer
Pick one of the three modes — "X% of Y", "X is what % of Y", or "% change" — fill in the two boxes and press Calculate. Twenty per cent of 150 is 30, 30 is 20% of 150, and a move from 80 to 100 is a 25% change.
Tool Specifications
Runs on
Your browser
Server upload
None
Account
Not required
Cost
Free, no paywall
How to use Percentage Calculator in 3 simple steps
Three steps, all of them inside this browser tab.
Open the Percentage Calculator workbench
Open the workspace in your browser. The code it needs loads with the page — there is nothing to install and no account to create.
Provide your input files or parameters
Select, drag, or configure your input. The file is read into the page and processed there, so there is no upload to wait for.
Save your output
Preview the result and click download. The file is written from the page straight to your disk; nothing is stored on a server.
Local Browser Compute vs. Traditional Cloud Converters
What changes when the tool runs in your own tab instead of on someone else's server.
| Feature / Aspect | OpenTools (Local) | Standard Cloud Converters |
|---|---|---|
| Where your file goes | Stays in the page; it never touches a server | Uploaded to and processed on the provider’s servers |
| Waiting to upload | None — there is no upload step | Upload time, then a place in the server queue |
| Price | Free, with no paywall and no sign-up | Often a free tier with daily limits or a subscription |
| Retention | Nothing to retain; the file was never sent | Stored on the provider’s disks under their policy |
| Account & tracking | No account, no third-party trackers, no client-side analytics | Often an email sign-up and advertising trackers |
How this page is locked down
This page is served with a Content Security Policy that blocks every network connection the page could make (connect-src 'none'). Your browser will not send the file, its name or its contents to any server, including this one. The exact header is below.
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Frequently Asked Questions
Which three questions can it answer?
"X% of Y" gives (X ÷ 100) × Y, so 20% of 150 is 30. "X is what % of Y" gives (X ÷ Y) × 100, so 30 out of 150 is 20%. "% change" gives ((end − start) ÷ |start|) × 100, so 80 rising to 100 is a 25% change.
Why can the starting value not be zero for a change?
Because the change is divided by the starting value, and there is no meaningful percentage increase from nothing. That case stops with "The starting value cannot be zero". In the same way, a whole of zero in "X is what % of Y" stops with "The reference value cannot be zero".
What happens with negative numbers?
Percentage change divides by the absolute value of the starting number, so the sign of the answer reflects whether the value rose or fell rather than flipping with the sign of the start. A move from -50 to -25 is reported as a rise, because the value went up.
How is the answer rounded?
Display is limited to eight decimal places and uses your browser locale for grouping and the decimal mark. The calculation itself is done in double-precision floating point before that formatting, so a result that repeats endlessly is rounded for display rather than reported exactly.
Can it work out VAT, GST or a discounted price?
Not directly — it has these three modes and nothing else, so there is no reverse-percentage mode and no tax mode. For tax at a rate you supply, the finance workbench has a GST calculator that reports the tax and the total, for example 1,000 at 18% giving a total of 1,180.
Does Percentage Calculator upload my files or data to any server?
No. The work happens in the page you have open. This page is served with a Content Security Policy of connect-src 'none' and webrtc 'block', so the browser will not let it open a connection to any server, including this one.
Is Percentage Calculator free?
Yes. There is no paywall, no usage limit and no account. If the tool is useful to you, you can choose to support the project.
Can I use this for confidential legal or financial records?
Your file is not sent anywhere, so it is not exposed to this site or to a third party. What remains is your own device and browser — treat it the way your organisation's policy treats any local file.
Does the site collect anything when I visit?
The server records one coarse metadata event per page visit (see the security policy for exactly what). There are no third-party trackers and no client-side analytics, and nothing from the tool itself — your files, inputs or results — is included.
Ready to use Percentage Calculator?
Open the workspace in your browser. No account, no paywall, and your file never leaves the page.