A few years ago, a NASA climate satellite crashed into Mars because one team used metric and another used imperial. The cost: $125 million. The cause: somebody forgot to convert.
I think about this every time I try to bake an American recipe. The recipe calls for 2 cups of flour. I have a kitchen scale. How many grams is a cup of flour? It depends on how tightly you pack it, how humid the air is, and what kind of flour you're using. The answer is "roughly 120 grams" but the recipe writer assumed you'd just use a measuring cup and not ask questions.
The unit converter on Numberjoy started as a frustration project. I was tired of opening five different tabs every time I needed to convert something — one for length, one for temperature, one for volume — and each one had different UI, different defaults, and some of them had ads that made the page take forever to load.
What it covers
Four categories: length, mass/weight, volume, and temperature. Each category has the most common units for everyday use. Length goes from millimeters to miles. Mass covers milligrams through pounds. Volume does milliliters, liters, gallons, quarts, cups. Temperature handles Celsius, Fahrenheit, and Kelvin.
The temperature conversion is the trickiest part. Celsius to Fahrenheit is straightforward — multiply by 9/5, add 32 — but the other direction is less intuitive. Fahrenheit to Celsius: subtract 32, multiply by 5/9. Kelvin to Celsius: subtract 273.15. I had to write special-case logic for temperature because, unlike the other categories, it's not a simple multiplication. 0°C is not "0 units of cold" — it's the freezing point of water. The conversion factors don't work the same way.
I mention this because it's the kind of thing that's easy to get wrong, and a lot of unit converters do get it wrong. They treat temperature like any other linear conversion and the results are off by 32 degrees or more. The current implementation handles it correctly — I tested it against known values (0°C = 32°F, 100°C = 212°F, -40°C = -40°F, which is the one point where the scales meet).
The unit selection is clunky
Honest assessment: the dropdown menus for "From" and "To" rebuild themselves every time you change the category. This means if you switch from length to temperature, the selected units reset. It's functional but not smooth. I considered keeping the selection persistent across category changes, but that created edge cases — what happens when you switch from inches to Celsius? It didn't make sense. So the current behavior is intentional, but I wouldn't call it elegant.
There's also no way to favorite common conversions (like inches to centimeters, or Fahrenheit to Celsius). That would be a nice addition. You'd open the page and your most-used conversions would be one click away. Maybe in the next update.
Why the US still uses imperial
This isn't a political question, it's a practical one. The US tried metric in the 1970s. Road signs went up in kilometers. Gas stations posted prices per liter. And then... nothing happened. People ignored it. The signs came back down. The US remains one of three countries that haven't officially adopted the metric system (the others are Liberia and Myanmar).
The result is that anyone who works internationally — engineers, scientists, cooks, travelers — lives in two measurement systems at once. I can tell you my height in feet and my weight in kilograms. I measure distances in miles and cooking ingredients in grams. It's a mess, but it's the mess we have.
Until the US switches (don't hold your breath), a good unit converter is essential. This one is fast, it's free, and it handles temperature correctly. The rest will get better over time.