Percentage Calculator | Four Ways, Worked Out

Four different questions get called "percentage", and picking the wrong one is how the arithmetic goes wrong. The menu names each in full so there is nothing to infer, and the sentence under the answer reads it back to you in words.

Percent change and percentage points are not the same thing, and the confusion is worth more than it costs. A rate going from 4% to 6% has risen by two percentage points and by fifty percent. Both are true, they differ by a factor of twenty-five, and headlines pick whichever is larger.

A rise and a fall of the same percentage do not cancel. Take 100, add 10% to get 110, then take 10% off and you have 99. The second percentage is of a bigger number, so it removes more than the first added. That is why a stock down 50% needs to double to get back, not rise 50%.

For a discount, use the change mode with the original as A and the sale price as B: the answer comes back negative, which is what a reduction is. For adding tax or a tip, the fourth mode does it in one step rather than making you add the two.

How to use it

Choose which of the four questions you are asking.
Put your two numbers in A and B. The answer updates as you type.
Read the sentence underneath to check it says what you meant.

Questions

What is the difference between percent and percentage points?

A percentage point is an absolute step on a scale that is already in percent; a percent is relative to where you started. A rate moving from 4% to 6% is a rise of two percentage points and of fifty percent. Both describe the same move, and which one is quoted usually depends on which sounds bigger.

Why does adding then subtracting the same percentage not return the original?

Because the second one is taken from a different number. 100 plus 10% is 110, and 10% of 110 is 11, so you land on 99 rather than 100. The same arithmetic is why an investment that halves has to double to recover, not gain 50%.

How do I calculate a discount?

Use "percent change from A to B" with the original price as A and the sale price as B. The result is negative, which is the discount. To go the other way, from a percentage off to a price, use "add A% to B" with a negative A.

How do I add tax or a tip?

The fourth mode, add A% to B, does it in one step: A is the rate and B is the bill. Working it out as two operations is where people drop a decimal, and it is also how a tip ends up calculated on a total that already included the tax.

Is my calculation sent anywhere?

No. The arithmetic runs in WebAssembly compiled from Rust inside this tab, and the page is served with a header that forbids it from opening a connection to anywhere at all. Nothing you type is recorded, and it works offline.

The questions it gets asked by name

Two of the four have pages of their own, opening on the question they are about:

Your data stays on your device

Everything above runs inside your browser as WebAssembly compiled from Rust. Nothing you type is uploaded, logged or stored on a server. You can load this page once, go offline, and it still works.

This page makes no requests at all, to anywhere. That is not a promise in the copy: it is a Content-Security-Policy header your browser enforces, and connect-src on it is none. Open the network tab and watch nothing happen.