I typed 350 into a converter because I needed Celsius for an oven. The page let me change the unit. Then it played a video.
I sat there with the number I already knew and an ad I did not ask for. The multiply itself is (350 minus 32) times 5/9. That is 176.667. I did not need a skip button to get there.
The unit converter on Softery does that multiply in the tab. One page per pair people actually type. Change the number. Follow the swap link. Nothing sits between those two clicks. The number you type never goes out as a number.
The Video Between the Number
A converter I opened this week did the same trick the phone apps do. The first number looked free. Changing Fahrenheit to Celsius was the bill. I closed the tab and did the arithmetic on paper.
I ranked this idea ninth out of 1,754, score 9.50, about ten hours of work. The card said one engine and one page per pair. It also said zero interstitial ads when a unit changes. That last line is the product. The rest is a multiply.
A post on r/SideProject said the quiet part. Score 2, which is not a crowd. It is still the sentence I kept coming back to:
I just want to convert 350F to C without fighting a website full of ads... The #1 complaint about converter apps is dark patterns (an ad triggered by changing units).
I can match the first half. No ad fires when you change a unit here. I cannot write "no tracking" over a site that already loads Mixpanel and Google Analytics. I built the smaller claim. The typed number never becomes a Mixpanel property.
Rank 9 of 1,754
The everyday math pile in that list is long. A percentage calculator. A grade tool that once returned infinity. The converter sat near the top because people type the pair, not the category.
Ninth was the rank because the query is a pair and the work is a table plus a URL. Ten hours got me the constants and 34 addresses. "No ad on a unit change" is what I shipped. The dropdown can wait.
The giant dropdown sites work if you already know which of a thousand units you want. A lot of people open one of those, change the unit, and get a video instead of 176.667.
Seventeen Completions Each
Bing's autocomplete returned 17 suggestions on "convert cm to inches" and another 17 on "convert kg to lbs". Those were the two highest single-pair terms in that file. "convert mm to inches" returned 14, including the fraction form. Across the pair cluster the count was 95.
People add those words because the first result wasted their time. They did not type "unit converter". They typed the two units they already had.
A from-scratch unit converter Show HN sat at 147 points and 130 comments. An open converter on GitHub had 710 stars. I did not go hunting for the item id. Those two numbers were enough. People will upvote a converter that stays out of the way.
If you land here, you get two boxes and a chart. The pair is already picked. Pressing nothing still shows a number, because an empty converter looks broken.
One Engine, Thirty Four Addresses
The engine is one function. Linear units go through SI. Temperature goes through kelvin. Height is the one split, because 173 cm is 5.67651 feet as a decimal and 5 ft 8.11 in in a classroom.
Seventeen seeds, each with a reverse. That is 34 URLs. cm to inches, kg to lbs, ml to oz, and 350 F to C. The hub at /tools/unit-converter is cm to inches so the first screen already has a number.
I did not build a category picker with a thousand units. That page already exists, a lot of times. I built the page you get when you already know the pair.
Swap is a real link. It opens the reverse pair with the current answer filled in. Someone can send you inches to cm after they started on cm to inches. Add ?v=10 if you want that number waiting when the page opens. Fahrenheit pages default to 350, because that is the number that started this.
The Inch Is 2.54
Length uses the international yard. One inch is 2.54 centimeters, exactly. One foot is 0.3048 meters. One mile is 1,609.344 meters. Those are not rounded guesses. They are the constants the page multiplies.
Weight uses the avoirdupois pound of 0.45359237 kilograms. Volume uses the US gallon of 3.785411784 liters, which is 231 cubic inches. A US cup on this page is 8 US fluid ounces. A US gallon is 128 fluid ounces.
I printed the formula under the boxes so you can see which gallon you got. If you needed an imperial gallon, this page will not pretend. Do the last step yourself, or use a page that names that gallon.
The formatter stops at six significant figures. 10 cm is 3.93701 in. 2.54 cm is 1 in. I do not want a float leftover that looks like a different inch.
350 F Is 176.667 C
Temperature is the odd one. It is not a multiply. Celsius to kelvin adds 273.15. Fahrenheit goes to kelvin the long way, then back. 350 F is 176.667 C on this page. 32 F is 0 C and 212 F is 100 C. Minus 40 is the one point both scales share.
I default the Fahrenheit pages to 350 because that is the oven number from the post. You can type over it. You can put ?v=212 in the URL and send boiling.
The kelvin pairs are there because people type them. C to K is a shift. F to K is the same shift after the Fahrenheit map. I did not hide them behind a dropdown of "more units".
If you type junk, the other box clears. Infinity and NaN fail the parser. A trailing unit in the field is stripped so "10 cm" still works. Commas are fine. I wanted the box to accept the way people actually paste a number.
Height Is Not a Decimal
173 cm is 5.67651 feet if you only do the multiply. Nobody in a doorway says that. They say 5 foot 8.
The cm to feet and inches page prints the classroom form. 173 cm is 5 ft 8.11024 in. The decimal cm to feet page stays a straight multiply. I kept both, because a height and a length are different questions that share a unit.
You can type 5'8" or 5 ft 8 in on the height pair. The parser accepts both. The other direction takes feet and inches and gives centimeters with the same 2.54.
On a plain cm pair that is not the height page, the result still hints at the split. 173 cm on cm to inches will mention the 5 ft form so you do not have to open a second tab for a doorway.
Both Boxes Move
Type in either box. The other one updates. I did not want a Convert button on a multiply. The pointer on a wheel is decoration around a pick that already happened. The same idea applies here. The result is the other box changing while you type.
Empty the field and the other side clears. That is on purpose. A stale 176.667 next to a blank Fahrenheit box would look like an answer you still had.
Copy puts the result on the clipboard. It does not send the number to Mixpanel. The event name is outputCopied with result. That is the whole payload for that click.
I put a table of common values under the boxes. 1, 2, 5, 10, 20, 50, 100 on cm to inches. The Fahrenheit chart starts at 350 because that is the number people came here with. The table is text. It is not a canvas you have to read.
Swap Is a Real Link
A lot of converters flip the units in place and keep you on the same URL. That is fine until you want to send the other direction to someone. Then you are sending a page that still says cm to inches while the boxes have swapped, or you are sending a page that forgot the number.
Swap here is an <a href>. It goes to /tools/unit-converter/inches-to-cm?v=3.93701 if you started at 10 cm. The reverse page has its own title, its own chart, and its own FAQ. Google can tell those two pages apart. So can you, when the tab label is the pair you wanted.
I treated the 34 URLs as the product. The engine is shared. The address is not. If I had shipped one dropdown and a query string, I would have shipped the page people already bounce off.
What Mixpanel Sees
Softery.io loads Mixpanel and Google Analytics on every page. I will not write "zero third-party scripts" over that layout. The queue card said it. I crossed it out.
The tool id is unit-converter on the hub and on every pair page. Opening the page sends Tool Viewed with that id. Changing pair sends settingChanged with the slug, not the number. Typing sends inputProvided with a size bucket of the string you typed, not the value. Copying sends outputCopied.
350 never leaves as 350. 10 never leaves as 10. If you open the network panel you will see Mixpanel or Google traffic from the layout. You should not see 176.667 in those payloads.
After the layout has loaded you can unplug the cable and still convert. There is no account and no upload. Refreshing keeps you on the same pair. The number itself is not stored.
The Chart Is Text
I wanted a small table of the values people already have in their head. 1 inch. 10 cm. 350 F. A chart library would have been a canvas and a hover. A table is a sentence you can copy.
Each pair carries its own list. cm to inches uses 1, 2, 5, 10, 20, 50, 100. Fahrenheit to Celsius uses oven numbers. I did not reuse one generic range and hope it looked right on kelvin.
The formula line under the table names the constant. 2.54. 0.45359237. 3.785411784. (F minus 32) times 5/9. If the page is wrong, that line is where you catch it.
I left currency off. A live rate is not a constant. I left recipe density off. A cup of flour is not a cup of water, and that table is a later page. This one does units that do not change while you are looking at them.
What I Left Out
I left out the imperial gallon and the stone. The research file also asked for a gas mark row. I skipped that too. The first ship is the pairs people type, with the US gallon named as the US gallon.
I did not add a thousand-unit search box. That is the incumbent. It answers a different query.
I did not hide Mixpanel. The honest line is already on the page. They see that someone opened the tool. They do not get the number.
I did not put a login in front of a multiply. I did not put a video in front of 176.667. Those two were the whole point of ranking this ninth.
How to Check It
Open the network panel. Type 2.54 in the centimeter box. The inch box should say 1. Type 350 on the Fahrenheit page. The Celsius box should say 176.667. You should see Mixpanel or Google traffic from the layout, and you should not see 2.54 or 350 in those payloads.
| What you type | What this page uses | What you get |
|---|---|---|
| 1 inch | 2.54 cm exactly | 2.54 cm |
| 1 pound | 0.45359237 kg | the SI mass |
| 1 US gallon | 3.785411784 L | 128 US fl oz |
| 350 F | (F minus 32) times 5/9 | 176.667 C |
| 173 cm | classroom split | 5 ft 8.11024 in |
If you type nope in one box, the other box goes empty. That is the parser refusing junk, not a freeze.
Type the Number
If you have a number, type it. If you have the other unit, type in the other box.
- Open the unit converter, or the pair you already wanted, like cm to inches or Fahrenheit to Celsius.
- Type the number. Watch the other box.
- Copy the result if you need it somewhere else.
- Follow Swap if you meant the other direction.
- Open the network panel once if you do not believe the number stays put.
The other tools on Softery follow the same habit. The password generator never sees the password. The wheel spinner never sends the names. This one never sends the number you typed.
Last updated: September 5, 2026 | Reading time: 11 minutes
Written by Evgeniy Poznyak, who did the oven math on paper once and got tired of that.