Percentage Calculator: Of, Increase, Decrease, Difference

I Kept Getting Percentages Wrong, and It Was Costing Me

I'll be honest — for years I thought I had percentages figured out. Then I was looking at a supplier invoice that said "15% early payment discount" on a total of 8,400 and I confidently told my partner we'd save 1,400. We wouldn't. The actual discount was 1,260. I'd done the math in my head, rounded up, and basically invented money that didn't exist. That's a 140 difference, which on a tight project margin is not nothing.

The thing is, percentages feel simple until they aren't.

There are really four flavors of percentage problems that come up over and over — "what is X% of Y," percentage increase, percentage decrease, and percentage difference. They sound similar but they work differently, and mixing them up is exactly how you end up quoting the wrong number to a client or miscalculating a tip or botching a material estimate. So I built a

that handles all four, because honestly I got tired of second-guessing myself.

The Four Types (and When You'd Actually Use Each One)

"What is X% of Y?"

This is the most basic one. You see a 20% off sign at the store. The item costs 85. What's 20% of 85? That's it — that's the question.

So 20% of 85 is 17. The discounted price would be 85 minus 17, which is 68. I use this constantly — splitting costs on a project, figuring out tax on materials, calculating how much of a budget has been spent. It's the workhorse percentage calculation.

Percentage Increase

You bought lumber last year for 620 per thousand board feet. This year it's 780. What's the percentage increase? This one trips people up because they sometimes divide by the wrong number (the new number instead of the old one).

The formula: ((New - Old) ÷ Old) × 100

So that's ((780 - 620) ÷ 620) × 100 = (160 ÷ 620) × 100 = about 25.8%. Not 20.5%, which is what you'd get if you accidentally divided by the new value. That kind of mistake matters when you're explaining cost overruns to someone.

Percentage Decrease

Same idea, reversed. Say your electric bill dropped from 340 to 285 after you installed better insulation. The formula is identical in structure — you're still dividing by the original.

((Old - New) ÷ Old) × 100 = ((340 - 285) ÷ 340) × 100 = (55 ÷ 340) × 100 = about 16.2% decrease.

People sometimes just flip the increase formula and get confused by negative signs. I find it easier to just always subtract the smaller from the larger and then label it "increase" or "decrease" depending on which direction things went. Less room for error that way.

Percentage Difference

This is the sneaky one. Percentage difference is for when there's no clear "old" and "new" — you're just comparing two numbers. Like, you got two quotes for a job: one is 12,500 and the other is 14,800. What's the percentage difference between them?

Here you divide by the average of the two numbers, not by either one specifically.

For our quotes: |12,500 - 14,800| = 2,300. Average = (12,500 + 14,800) ÷ 2 = 13,650. So 2,300 ÷ 13,650 × 100 = about 16.8% difference. That's different from what you'd get with a straight percentage increase calculation (which would give you 18.4%), and the distinction matters when you're presenting numbers fairly — like comparing competing bids without making one look artificially worse.

Type	When to Use It	You Divide By	Example
X% of Y	Finding a portion of something	Just multiply	15% of 200 = 30
% Increase	Something went up over time	The original (old) value	200 → 250 = 25% increase
% Decrease	Something went down	The original (old) value	250 → 200 = 20% decrease
% Difference	Comparing two values with no "before/after"	The average of both values	200 vs 250 = 22.2% difference

Notice something weird in that table? Going from 200 to 250 is a 25% increase, but going from 250 to 200 is only a 20% decrease. They're not the same! That asymmetry messes with people all the time and it's completely normal — it's because the base number changes.

A Worked Example That Actually Came Up Last Week

A friend texted me asking about a car listing. The sticker price was 28,500 but the dealer was advertising "12% off for end-of-year clearance" and she wanted to know the sale price. Then she found the same car at another dealer for 25,900 and wanted to know the percentage difference between the two final prices.

Step one — find 12% of 28,500:

(12 ÷ 100) × 28,500 = 3,420 discount

Sale price: 28,500 - 3,420 = 25,080

Step two — percentage difference between 25,080 and 25,900:

|25,080 - 25,900| = 820

Average = (25,080 + 25,900) ÷ 2 = 25,490

820 ÷ 25,490 × 100 = about 3.2% difference

So the two prices were only about 3.2% apart. At that point you're really just choosing based on which dealer you trust more (or which one throws in floor mats, or whatever). I sent her the

so she could play with the numbers herself.

Common Mistakes I See Constantly

Dividing by the wrong base number. This is the big one. If something increased from 400 to 500, you divide by 400, not 500. I've seen invoices, proposals, even published articles get this backwards.

Confusing percentage difference with percentage change. They give different answers! Use change when there's a clear before-and-after. Use difference when you're just comparing two things side by side.

Stacking percentages incorrectly. A 10% increase followed by a 10% decrease does NOT get you back to where you started. 100 goes up 10% to 110, then down 10% to 99. You lost a dollar. This drives people absolutely nuts when they first encounter it, but it's just how the math works.

And then there's the classic — adding percentages that shouldn't be added. If one product is 30% off and another is 20% off, your total savings is NOT 50% off your whole cart. Each discount applies to its own item's price. Sounds obvious when I say it like that, but I've watched people make this mistake at checkout.

If you're working with

, that's a whole other thing — but the logic is the same once you get the decimal form. And if you're doing something like figuring out

, percentages are usually the clearest way to express what you find.

For more number-crunching situations, I've also got a

(handy when you need that midpoint for percentage difference), a

for when things get more complex, and a straightforward

for the basics. Sometimes you just need to multiply two numbers and not overthink it.

Is percentage increase the same as percentage difference?

No, and this is one of the most common mix-ups. Percentage increase (or decrease) compares a new value to an original value — you divide by the original. Percentage difference compares any two values without a "before" and "after" — you divide by their average. For 200 and 250: the percentage increase from 200 to 250 is 25%, but the percentage difference between them is about 22.2%.

Why doesn't a 50% increase followed by a 50% decrease return to the original number?

Because the base changes. Start with 100. A 50% increase makes it 150. Now 50% of 150 is 75, so a 50% decrease brings you to 75 — not back to 100. The increase was calculated on 100, but the decrease was calculated on the larger number, 150. It's not a trick, it's just that percentages are always relative to whatever number you're starting from at that moment.

How do I reverse-calculate the original price from a discounted price?

Divide the sale price by (1 - discount percentage as a decimal). So if something costs 680 after a 15% discount: 680 ÷ (1 - 0.15) = 680 ÷ 0.85 = 800. The original price was 800.