There's a scene in the TV show Fargo where a character explains why he can't trust a certain deal: "You know what a percent is? It's per hundred. Every hundred. People think they understand it, but they don't." He wasn't wrong.

Percentages are one of those things that feel simple until you actually need to use one under pressure. A restaurant bill arrives and you need to split it six ways with tip. A store has a "30% off, then an extra 15% off" sale (which is not 45% off, by the way — it's 40.5% off, because the second discount applies to the already-discounted price). A news article says inflation rose 2% — but is that 2 percentage points or 2 percent of the previous rate?

I built the percentage calculator for exactly this kind of everyday confusion.

The three percentage traps

There are really three distinct percentage problems, and most calculator sites only handle one of them well:

1. "What is X% of Y?" — This is the one everybody knows. 15% of 200 = 30. Straightforward. The percentage calculator handles this with two input fields.

2. Percentage increase/decrease. — If a product costs $80 and is now $100, what was the percentage increase? It's 25%. Most people guess something between 15% and 30%. I haven't added this mode yet, honestly. It's on the list.

3. Reverse percentage. — "I paid $85 after a 15% discount. What was the original price?" This is the one that trips up almost everyone. The answer is $100, and if you're surprised by that, you're not alone. Most people would say something like $97.75 (wrong) or $100.85 (also wrong).

The current calculator does the first one well. The other two modes need to be added. I'm not pretending it's perfect — it covers the most common use case, but if you're doing a lot of percentage work, you'll eventually want the other directions too.

Why mental math fails

There's a reason people struggle with percentages: they're not additive. A 10% increase followed by a 10% decrease doesn't get you back to where you started — it gets you to 99% of the original. This trips up even people who work with numbers every day.

I once watched an accountant (an actual accountant) confidently claim that a 25% markup and a 25% margin were the same thing. They're not. A 25% markup on cost means you sell for 125% of cost. A 25% margin means your profit is 25% of the selling price. Those are very different numbers. The accountant was wrong, and I was too polite to say anything.

What the calculator actually does

You type in the percent and the number. It tells you the result. It also shows the formula in the display area — "What is _% of _?" — so you can see what you're calculating. The Clear button wipes both fields. That's it.

No ads, no sign-up, no tracking. Your discount calculations stay between you and your wallet.

I should probably add a "percentage change" mode soon. And a tip calculator. And a "percent off vs final price" view. The current tool covers the basics, but "the basics" isn't the same as "everything people actually need." Building a calculator that handles every percentage scenario would be a bigger project than it sounds like.

Then again, most things are.