HOW TO USE
HOW TO USE PERCENTLAB — PERCENTAGE CALCULATOR
PercentLab answers the four percentage questions people actually run into. Pick a mode from the tabs at the top, type your numbers, and the result updates instantly as you type — along with the formula, so you can see how it was calculated.
MODE 1 — X% OF Y (find the portion)
The discount problem. Enter a percentage and a number to find the portion.
Example: “What is 25% of 80?” → 20
Formula: (25 ÷ 100) × 80 = 20
A visual bar shows the split at a glance.
MODE 2 — X IS WHAT % OF Y (find the ratio)
The comparison problem. Enter the part and the whole to find what percentage one is of the other.
Example: “30 is what % of 200?” → 15%
Formula: (30 ÷ 200) × 100 = 15%
MODE 3 — % CHANGE (increase or decrease)
Enter an original and a new value. The result is color-coded — green for an increase, red for a decrease.
Example: 50 → 65 is a +30% increase
Formula: ((65 − 50) ÷ 50) × 100 = +30%
MODE 4 — TIP CALCULATOR
- Enter the bill amount.
- Tap a quick preset (10% / 15% / 18% / 20%) or type a custom percentage.
- Use the − / + buttons to split between people.
Example: $75 bill, 20% tip, split 2 ways:
• Tip: $15.00
• Total: $90.00
• Per person: $45.00
MENTAL MATH ANCHORS
10% = divide by 10
25% = divide by 4
50% = divide by 2
100% = the whole
20% is just 10% twice. 15% is 10% plus half of 10%. Once you’ve got these anchors, a surprising amount of percentage math can be done in your head.
PercentLab
What is X% of Y?
Example: “What is 25% of 80?” → the answer is 20.
Result
20
25% of 80 is 20
How it’s calculated
(25 ÷ 100) × 80 = 20
Visual
Breakdown
Tip amount
$0
Total bill
$0
Per person
$0
10%
= ÷ 10
25%
= ÷ 4
50%
= ÷ 2
100%
= the whole
The 15% Tip Problem, and Other Reasons I Built a Percentage Calculator
Last week I stood in a restaurant, phone out, staring at a bill for $63.80, trying to figure out 15% in my head. I did the thing we all do — moved the decimal, got $6.38, added half of it, landed somewhere around $9.57, then second-guessed myself, gave up, and just rounded to ten bucks because the waiter was waiting.
I have a degree. I’ve done this math a thousand times. And I still, every single time, stand there doing arithmetic in public like it’s a pop quiz I didn’t study for.
The honest truth is that nobody’s bad at percentages. We’re bad at percentages under pressure — in a restaurant line, at a store trying to figure out if 30% off is actually a good deal, staring at a price increase and wondering if it’s “a lot.” The math is easy. The context is what scrambles your brain.
So I built PercentLab. It’s a percentage calculator. And I built it for the version of me who keeps doing long division next to a hungry waiter.
The three questions we actually ask (and get wrong)
When I sat down to think about it, almost every percentage problem in real life is one of three questions. Not fifty. Three. And they’re the same three my brain refuses to answer cleanly on the spot.
“What is X% of Y?” This is the discount problem. “What’s 30% off $80?” The store is counting on you being bad at this, by the way. A “30% off!” sign is designed to feel bigger than it is. The calculator answers it instantly, and you’d be surprised how often the answer is smaller than the sign wanted you to believe.
“X is what % of Y?” This is the comparison problem. You scored 42 out of 60 on something. You spent $120 of a $400 budget. Your team used 8 of its 20 vacation days. These are all the same question wearing different clothes, and the answer — the percentage — is what makes the number actually mean something.
“What’s the % change?” This is the one I get asked about most, and it’s the one people quietly get wrong all the time. Your salary went from $50k to $65k. That’s not a 15% raise — wait, no, it is… see? This is exactly the problem. The calculator just tells you. +30%, by the way. It was $50k to $65k, which is a 30% raise, and if you got that, congratulations and also can I work with you.
I also threw in a tip calculator, because that’s genuinely the most common percentage calculation in human existence, and the one with the highest social stakes. Nobody wants to be the person who tips 12% on a group dinner and spends the next three days thinking about it.
The part where it shows its work
Here’s the thing I care about, and the reason I didn’t just build a box that spits out a number.
Every result in PercentLab comes with the formula written out underneath it. Not because the formula is complicated — it isn’t — but because I got tired of calculators that hand you an answer and leave you trusting it blindly. When you see “(25 ÷ 100) × 80 = 20” written out, you don’t just get the answer. You get why. And once you’ve seen the why enough times, you start being able to do it in your head. Which is, I think, the actual point of any good calculator — it should be quietly teaching you to not need it anymore.
There’s also a little visual bar for the “X% of Y” mode, so you can see what 25% of something looks like instead of just reading it. Turns out our brains are dramatically better at judging a filled-in bar than a decimal point. Who knew.
Why percentages feel hard (and why they aren’t)
If there’s one thing I’d want you to take away from all this, it’s that percentages are secretly just division in a costume.
10% is dividing by 10. 25% is dividing by 4. 50% is dividing by 2. That’s it. That’s the whole trick. Once you’ve got those three anchors, you can do a shocking amount of percentage math in your head — 20% is just 10% twice, 15% is 10% plus half of 10%, and so on. I put those anchors right on the page, because that little bit of mental scaffolding is worth more than any formula.
The reason percentages feel hard isn’t the math. It’s that we only ever meet them in moments of mild panic — a tip, a discount, a raise, a sale ending in an hour. So we fumble, and then we decide we’re “bad at math,” and we carry that around forever.
You’re not bad at math. You’re bad at doing math while someone watches. That’s different, and it’s fixable.
What the tool does (and what it doesn’t)
PercentLab does four things — the four percentage calculations people actually run into — and it does them instantly, in your browser, with nothing sent anywhere and nothing to sign up for. It shows its work. It shows a visual when it helps. It has a tip calculator because of course it does.
It does not, and I want to be clear about this, do your taxes. It doesn’t do compound growth or mortgage amortization or anything fancy. It’s not trying to be a spreadsheet. It’s trying to be the thing you reach for when you’re standing in a restaurant line and you just want to know the tip without doing long division in public.
And honestly? That’s enough. That’s the entire job. Most tools are trying to be everything to everyone and end up being nothing to anyone. This one just wants to answer the three questions you actually ask, and let you get back to your dinner.