I typed a percent change that started at zero. The page printed Infinity.
I wanted 15 percent of 80. I wanted the formula under the answer, with 15 and 80 in it, not P and V1. I wanted a page that would refuse the zero instead of dressing it up as a number.
The percentage calculator does that. Six forms. A URL per form. The base named under the result. Then it stops.
I ranked this sixteenth out of 1,754. Score 9.3. About six hours on the card.
I Wanted 15 Percent of 80
You are looking at a sale tag, or a grade, or a paycheck line. Someone said 15 percent. You want the number, then you want to see how it was made.
Search for percentage calculator and you get a page that asks you to pick a formula name first. Or you get a Calculate button. Or you get an essay, and the inputs are under it.
I wanted the question on the first screen. What is X percent of Y. Type 15. Type 80. Read 12. Read 15 / 100 × 80 = 12 under it.
I wanted the formula with my numbers in it. I didn't want Infinity.
That is the whole visit for most people. Two numbers. One answer. Close the tab.
Rank 16 of 1,754
I ranked this sixteenth out of 1,754, score 9.3, about six hours. The card said six modes on one page: of, increase, decrease, change, difference, reverse. Tabbed. A URL per mode so increase and reverse can rank on their own query.
The everyday math pile on that list also has the unit converter, which already shipped. A math solver sits later and costs more hours. This one sat at sixteen because the search is frequent and the grievance is specific. People are not asking for a new kind of percent. They are asking for the form they typed, without a scroll and without a fake number when the base is zero.
Bing filled 15 completions on percentage calculator. I didn't invent a volume behind them. I have the completion count from the ranking pass.
Six Searches, Not One
percentage increase calculator, percentage decrease calculator, percentage change calculator and percentage difference calculator each pulled 11 to 15 more completions. People added formula, uk, omni or excel on the end.
reverse percentage calculator pulled 6, including reverse tax percentage calculator.
That is 74 completions across the six forms. They are not the same search. A person who types reverse tax is not looking for 15 percent of 80. A person who types percentage points is not looking for the average of two numbers.
One page can still answer all six. It has to give each form its own address. Sharing one URL for six questions is how five of them get buried.
The Page With Six Tiny Boxes
I opened calculator.net/percent-calculator.html this sitting.
The first line asks you to provide any two values and click Calculate. There is a box for "what is of". There is a box for "is what % of". There is a box for "is of what". Further down there is a Percentage Difference Calculator. Further down there is a Percentage Change Calculator, with Increase and Decrease in the same block, and another Calculate button.
The article under those boxes starts with a classroom. 25 of 50 students are male. The ratio is 0.5. Times 100 is 50 percent. Then it writes the algebra as P × V1 = V2. If you came here with 15 and 80, that line is a different language.
Their difference example is 10 and 6. Average 8. Difference 50 percent. Same formula I use. Their change example is 500 up 10 percent to 550, or down to 450. Same multiply. Finding the box is the work. Then finding the form you actually typed.
Reverse, as its own form with its own address, is not there.
I am not going to invent an ad count from that fetch. The ranking file called the page ad-heavy. Tonight I can say what the markup gave me: stacked boxes, Calculate clicks, and a long "What is a percentage?" article after the inputs.
| What you open | What I saw this sitting | What this page does |
|---|---|---|
| calculator.net/percent-calculator.html | Several boxes, Calculate, one URL | Six tabs, live answer, ?mode= per form |
| omnicalculator, from the ranking file | Essay above the inputs, paid extras | Inputs first, no paywall on the six forms |
| A page that starts at zero | Infinity, or a blank | A named refusal |
I didn't open omnicalculator tonight. The ranking write-up said it wraps the calculator in a long article and puts extras behind a paywall. I have that file. I don't have a fresh screenshot.
The Essay Above the Inputs
That layout is a search trick. The article is for the crawler. The person who typed 15 and 80 has to scroll past "What is a percentage?" to reach the box.
I put the copy under the calculator. Four to six headings. The questions people actually ask. The inputs stay at the top.
If you landed here from a blog post, you can scroll. If you landed here from percentage calculator, you can type.
What is 15 Percent of 80
The first tab is the head term. What is X percent of Y.
15 percent of 80 is 12. The formula is 15 / 100 × 80 = 12. The page says 80 is the base the percent is taken of.
That sentence is the whole point of the page. A percent is always of something. If the page does not name the something, you cannot check the work.
Try the sample on that tab. It fills 15 and 80. You can edit either field. There is no Calculate click. The work is a multiply.
Raise It, Cut It
Increase and decrease are the next two tabs. Same two numbers, different question.
Raise 80 by 15 percent and you get 92. The amount added is 12. Cut 80 by 15 percent and you get 68. The amount taken off is 12.
A 100 percent decrease lands on 0. That is a real answer. The leftover is zero. A 100 percent discount the other way, when you are trying to find the original, is not a real answer. That is a later tab.
The URL writes ?mode=increase or ?mode=decrease. Paste that into a chat and the other phone opens the same form.
Change From A to B
Change measures from the starting number. 80 to 92 is a 15 percent change because 12 sits on 80. The formula is (92 − 80) / 80 × 100 = 15%.
The raw difference, 12, sits under that. Some people want the 12. Some people want the 15. Both are on the card.
If the starting number is 0, there is no percent I can print. 10 is not a percent of 0. The page says the base is zero. It does not print Infinity.
I have seen that Infinity on other pages. It looks like an answer. It is a division by zero with a costume on.
Points Are Not a Change
If a rate moves from 8 percent to 11 percent, that is 3 percentage points. It is also a 37.5 percent relative change, because 3 sits on 8.
Those two numbers answer different questions. A headline that says "up 3 percent" when the rate moved three points is a different story than a 37.5 percent jump.
Tick "These numbers are already percents" on the change tab. The card shows 37.5% and a row labeled Percentage points with 3 in it.
I left that as a tick, not a second page. The numbers are the same either way. The label is what changes.
Difference Uses the Average
Difference is the form people mix up with change.
10 and 15 differ by 40 percent because the average is 12.5, and 5 sits on 12.5. The formula is |10 − 15| / 12.5 × 100 = 40%. The page says the average 12.5 is the base.
Change from 10 to 15 would be 50 percent, because that one uses 10 as the start. Same two numbers. Different base. Different answer.
If the two numbers average to zero, like 10 and -10, the page refuses. Same reason as a change from 0. I will not print Infinity.
Reverse Walks Backward
Reverse is the form the stacked pages skip, or bury.
85 after a 15 percent discount came from 100. The formula is 85 / (1 − 15 / 100) = 100. Taken off is the default.
115 after a 15 percent markup came from 100 too. Switch the toggle to Added on. The formula flips the sign.
A 100 percent discount has no original left to find. The leftover is already zero. Dividing by zero is how you get Infinity again. The page names that and stops.
Reverse tax is the completion people actually type. Sales tax is a markup. Added on, type the final and the rate, read the number before tax.
Zero is Not Infinity
I am repeating this because it is the only place these pages go badly wrong in a way you can see.
A percent of zero is not a number. JavaScript will give you Infinity. A lot of pages pass that through and call it the answer. A student copies it. A shopper stares at it.
This engine returns a named error, zero-base, and the page prints "The base is zero, so a percent of it is not a number I can print." The word Infinity does not appear.
The tests watch that path. Change from 0 to 10. Difference of 10 and -10. Reverse at 100 percent off. Those three used to be the places I would have shipped a costume.
The Tests That Had to Fail First
I wrote the tests before the page. That is the house rule here. A test that has never failed proves nothing.
Nineteen of them sit next to the math. 15 percent of 80 is 12. 80 up 15 percent is 92. 80 down 15 percent is 68. 80 to 92 is 15 percent. 8 to 11, with the percent tick on, is 37.5 percent and 3 points. 10 and 15 differ by 40 percent. 85 after 15 percent off is 100. 115 after 15 percent on is 100. A leftover binary float from 0.1 plus 0.2 prints as 0.3, not as 0.30000000000000004.
Eight more sit on the page. Empty state. Sample. Tab writes the URL. Zero start refuses. Points row. Reverse sample. Query string restore. Copy link.
I watched the first of those fail because the file was not there yet. Then I wrote the smallest code that passed. That is the whole method. I will not ship a percent page whose first test was written after I liked the layout.
The Link Holds the Mode
Each tab writes ?mode= into the URL. Of, increase, decrease, change, difference, reverse.
If you typed numbers, those go in as a and b. Reverse can add reverse=on. Change can add as=percent when the tick is on.
Copy link. Paste it. The other phone opens the same form. Nothing is stored on my side. If you don't want the person next to you to see 15 and 80, don't paste the link.
The sample on each tab is editable. Of uses 15 and 80. Change uses 80 and 92. Difference uses 10 and 15. Reverse uses 15 and 85, taken off.
What Leaves the Tab
The math runs in this tab. After the layout has loaded you can unplug the cable and still get 12.
Mixpanel and Google Analytics load with the rest of Softery.io. They see that someone opened the tool. They see which mode you picked. They get a money band for the size of the result, the same bands the invoice page uses. They do not get 15, or 80, or 12.
I will not write that nothing third party loads. That would be a different site.
After You Have the Number
If the 15 percent was a tip, that is a different page. Tip and Bill Split splits a check, even or by item, and still works when the dining room has no signal.
If the number you wanted was inches from centimeters, use the Unit Converter. One page per pair. No ad when the unit changes.
The catalog is on free tools. This is tool twenty-three.
Open the Calculator
- Open the percentage calculator.
- Stay on Of, or pick Increase, Decrease, Change, Difference or Reverse.
- Type two numbers, or press Try the sample and edit it.
- Read the answer, the formula, and the line that names the base.
- Copy the answer if you want the text. Copy the link if you want the same form on another phone.
If the starting number is zero, the page will say so. Try a different base. Don't paste Infinity into a homework box.
Last updated: September 8, 2026 | Reading time: 10 minutes
Written by Evgeniy Poznyak, who still does 15 percent of 80 on paper when the phone is in the other room.