How to Calculate Percentage: 5 Methods With Examples

I Couldn't Split a Restaurant Bill — And That's How This Started

I'm going to be honest with you. I was sitting at a restaurant with four friends, the bill came to 187, and someone said "let's just tip 20 percent" and I sat there staring at my phone calculator like it was written in a foreign language. Not because I'm bad at math — I literally run a calculator website — but because in that moment, my brain just blanked on how to turn 20 percent of 187 into an actual number.

That's the thing about percentages. You use them constantly, every single day, and then one random Tuesday evening your brain decides it doesn't remember how they work.

So I figured I'd write down every method I actually use, with real examples, because there's more than one way to do this and some ways click better for different people. If you've ever frozen up trying to figure out a tip, a discount, a tax amount, or what percentage one number is of another — this is for you.

The 5 Methods (With Actual Numbers, Not Abstract Stuff)

Method 1: The Classic Formula

This is the one they taught you in school and you probably half-remember. It works for the most common question: "What is X% of Y?"

So for that restaurant bill: (20 ÷ 100) × 187 = 0.20 × 187 = 37.4. The tip would be 37.40. I mean, that's straightforward once you see it written out, but in the moment with everyone staring at you, it feels harder than it is.

This method handles about 80% of the percentage questions you'll ever run into (see what I did there?).

Method 2: Finding What Percentage One Number Is of Another

Different question entirely. Say you scored 42 out of 55 on a test. Or you sold 340 units out of a goal of 500. You want to know: what percentage is that?

For the test score: (42 ÷ 55) × 100 = 76.36%. Not bad, not great. For the sales goal: (340 ÷ 500) × 100 = 68%. You're two-thirds of the way there, basically.

I use this one constantly when I'm looking at project completion rates or figuring out how much of a materials order has actually shown up on site.

Method 3: Percentage Change (Increase or Decrease)

This is the one people mess up the most.

You need this when something went from one value to another and you want to express that change as a percentage. Like, your rent went from 1,400 to 1,550 — how much of an increase is that? Or a product dropped from 89 to 67 — what's the discount percentage? The formula is almost the same as Method 2, but you're using the difference between the two numbers as the "part," and the original number as the "whole."

Rent example: ((1550 − 1400) ÷ 1400) × 100 = (150 ÷ 1400) × 100 = 10.71% increase. That hurts.

Product discount: ((67 − 89) ÷ 89) × 100 = (−22 ÷ 89) × 100 = −24.72%. So roughly a 25% discount. The negative sign just tells you it went down.

The mistake I see people make? They divide by the new number instead of the old one. Always divide by where you started. Always. I got this wrong for an embarrassingly long time before someone corrected me, and the numbers are close enough that you don't always notice the error, which makes it worse.

Method 4: The "Move the Decimal" Shortcut

This is honestly my favorite for mental math.

10% of anything is just that number with the decimal moved one place to the left. So 10% of 250 is 25. 10% of 83 is 8.3. 10% of 1,400 is 140. From there you can build almost any percentage you need:

You Want	Start With	Then Do This	Example (of 250)
10%	Move decimal left once	That's it	25
5%	Find 10%	Cut it in half	12.50
20%	Find 10%	Double it	50
15%	Find 10% + 5%	Add them together	25 + 12.50 = 37.50
25%	Find the number	Divide by 4	62.50
1%	Move decimal left twice	That's it	2.50

So if you need 18% of something, you find 10% (move the decimal), then 8% (which is 1% times 8), and add them. It sounds like more steps but it's actually faster in your head than pulling out a calculator, once you get the hang of it.

Method 5: The Reverse Percentage (Working Backwards)

This one's sneaky useful. You know the final price after a discount or tax, and you need the original. Like, you paid 76.50 after a 15% discount — what was the original price?

The logic: if something is 15% off, you paid 85% of the original. So 76.50 is 85% of the original.

So: 76.50 ÷ 0.85 = 90. The original price was 90. And if you want to check — 15% of 90 is 13.50, and 90 minus 13.50 is 76.50. It works!

I had no idea this method existed until I was trying to figure out a pre-tax price from a receipt for an expense report. Took me way too long to realize I couldn't just subtract the tax percentage from the total (that gives you the wrong answer, and it's a really common mistake).

When to Use Which Method

Situation	Method	Example
"What's 20% of 350?"	Method 1 (Classic) or Method 4 (Decimal shortcut)	Tip, discount amount, tax
"42 out of 55 is what percent?"	Method 2 (Part ÷ Whole)	Test scores, completion rates
"It went from 200 to 260 — what % change?"	Method 3 (Percentage Change)	Price increases, growth rates
Quick mental math at a store	Method 4 (Move the Decimal)	Estimating discounts on the fly
"I paid 76.50 after 15% off — what was the original?"	Method 5 (Reverse)	Expense reports, pre-tax pricing

If you're doing a bunch of these at once — like calculating

, or figuring out

— just use a calculator. Seriously. That's what they're for. I built ours specifically because I was tired of second-guessing my own arithmetic.

And if you're dealing with adjacent math problems, you might also want to check out our

(because fractions and percentages are basically the same thing wearing different outfits), our

for shopping math, or the

so you never freeze at a restaurant again like I did.

For more involved number crunching — ratios, proportions, that kind of thing — the

and

have you covered.

FAQ

Why can't I just subtract the percentage from the total to find the original price?

Because the percentage was calculated based on the original price, not the final price. If something was 100 and you got 20% off, you paid 80. But 20% of 80 is only 16 — so if you try to "add 20% back" to 80, you get 96, not 100. The percentages are relative to different base numbers. You have to divide by (1 − discount rate) instead. It's annoying, I know.

What's the difference between percentage points and percent?

If interest rates go from 3% to 5%, that's a 2 percentage point increase — but it's actually a 66.7% increase. The distinction matters in finance and statistics. In everyday life, most people use them interchangeably (and honestly, context usually makes it clear what someone means).

Is there a quick way to calculate percentages without a calculator?

Method 4 above — the decimal trick. Find 10% by moving the decimal, then build from there. Also, a trick I love: X% of Y is always the same as Y% of X. So 8% of 50 is the same as 50% of 8, which is 4. Way easier to do in your head.