How to Lose a Spacecraft

In September 1999, after nine months in transit, NASA’s Mars Climate Orbiter arrived at Mars and was never heard from again.

The spacecraft was fine. The rockets were fine. The maths was fine, twice over, performed correctly by two teams of people who were extremely good at maths.

One team’s software produced its results in pound-force seconds. The other team’s software read those numbers as newton-seconds. Nobody converted. The orbiter, dutifully following instructions that were numerically correct and dimensionally nonsense, flew a trajectory nothing could survive, and roughly $125 million of spacecraft stopped existing.

Unit conversion is the most boring arithmetic on earth. It is also where careful people fail, because it is the step everyone assumes someone else handled.

Conversion Is Not Just Multiplication

The naive mental model is that converting units means multiplying by a number. Inches to centimetres: multiply by 2.54. Pounds to kilograms: multiply by 0.4536. Simple.

That model is correct for most of measurement, and it is completely wrong for one very common case.

Temperature is not a ratio scale. Zero degrees Celsius does not mean “no temperature”; it means the freezing point of water, which is an arbitrary place to put a zero. Fahrenheit puts its zero somewhere else entirely. Kelvin puts it at actual absolute zero, which is the only one of the three that means what zero normally means.

So converting temperature needs an offset as well as a scale. Fahrenheit to Celsius is subtract 32, then multiply by 5/9. Kelvin to Celsius is subtract 273.15, with no multiplication at all.

Our unit converter handles this by routing everything through Celsius as a pivot: convert whatever you gave it into Celsius, then out to whatever you asked for. It is not the flashiest design decision in the world, but it means no pair of temperature units can be wrong independently.

Any converter that treats temperature as a single multiplier is broken, and it will be broken in a way that looks plausible. That is the worst kind of broken.

Six Significant Digits and the Lie of Precision

Results display to six significant digits, with trailing zeros trimmed.

That second part matters more than it looks. A tool that shows you 2.54000 is making a promise about precision it has no business making. Trimming to 2.54 says what is actually known.

Six digits is a deliberate middle. Too few and you lose real precision on small measurements; too many and you produce the false confidence of a number like 0.393700787401575 inches, which invites you to believe your tape measure has opinions at the femtometre scale. It does not.

This is the quiet failure mode of conversion tools: not being wrong, but being more precise than the situation deserves, and letting you carry that fake precision into a calculation where it starts to matter.

A Cup Is Not a Cup

Here is one that ruins recipes across the English-speaking world.

There is no single thing called “a cup.”

• The US customary cup is about 236.588 mL
• The US legal cup, used in nutrition labelling, is 240 mL
• The metric cup, standard in Australia and New Zealand, is 250 mL
• The imperial cup, historically British, is about 284 mL

Between the smallest and the largest, that is a difference of roughly 20%. In a cake, 20% more liquid is not a rounding error. It is a different cake, and not a better one.

Our converter uses the US customary cup at 236.588 mL, which is the right default for the majority of recipes written in cups. We are telling you the exact number rather than just saying “cups” because you deserve to know which cup you are getting, especially if the recipe you are following came from Sydney or a 1970s British cookbook.

(The same trap sits inside “a pint” and “a gallon,” where the imperial and US versions differ by about 20% in the other direction. The tablespoon is worse. Do not get anyone started on the tablespoon.)

A Kilobyte Is Not a Thousand Bytes

In the data storage category, our converter treats a kilobyte as 1024 bytes, not 1000.

This is the older convention, and it is the one most operating systems still show you. Because computers address memory in powers of two, 2 to the tenth power, 1024, was the closest round number and the industry adopted it.

Standards bodies later pointed out, quite reasonably, that “kilo” means a thousand everywhere else in measurement, and introduced kibibyte for the 1024 version. The naming never caught on outside documentation.

Which is why your 1 TB drive shows up as about 931 GB. Nothing was stolen. The manufacturer counted in powers of ten and your operating system is counting in powers of two, and they are both describing the identical number of bytes.

We picked 1024 because it matches what your computer will tell you, which is usually the number you are trying to reconcile.

What Is Actually In It

Seven categories: length, weight, temperature, volume, area, speed, and data storage. Results appear instantly as you type, with no Convert button to press, because a button between you and a number you already asked for is just a button.

There are shortcuts for the conversions people actually run: centimetres to inches, kilograms to pounds, Celsius to Fahrenheit. It is built mobile first with large touch targets, on the reasonable assumption that a decent share of unit conversion happens standing in a kitchen holding a phone with one flour-covered hand.

And like everything else here, it runs entirely in your browser. No account, no upload, no record of what you converted. Your measurements are nobody’s business, including ours.

Mind the Gap

The Mars Climate Orbiter failed a review board’s post-mortem not because anyone was careless with arithmetic, but because two correct systems disagreed about what their numbers meant and no step in between caught it.

Most conversion mistakes are not that dramatic. They are a cake that did not rise, a part machined 2.54 times too large, a drive that seems smaller than advertised. But they all come from the same place: a number that travelled from one context to another without anyone asking what it was measured in.

Convert something. If it is money rather than matter, the percentage calculator and the tip calculator are nearby, and neither of them will fly you into a planet.

(We briefly wanted to add furlongs per fortnight. It is a real unit, it converts perfectly well, and roughly one person per year would use it. That person knows who they are, and honestly, we may still cave.)