The metric system was created during the French Revolution to replace a chaotic patchwork of local measurement standards across Europe. Based entirely on tens and originally tied to the size of the Earth, it eventually became the dominant system of measurement worldwide. The United States legalized metric use in 1866, made a major push toward voluntary conversion in 1975, and in 1988 declared metric the preferred system for trade and commerce, but everyday American life never fully converted. What most people do not know is that American customary units are themselves defined in metric terms: an inch is exactly 25.4 millimeters.
Daniel’s friend in England measures distance in miles but temperature in Celsius, and that inconsistency opens a conversation about why measurement systems differ at all. The episode moves through the French Revolution origins of metric, the long American history with both systems, the hidden metric foundations of customary units, and the 1999 loss of the Mars Climate Orbiter, a $125 million spacecraft lost because one piece of ground software sent thruster data in customary units while NASA’s navigation system expected metric.
Daniel is nine. He asks the questions, refuses to accept easy answers, and picks every episode topic himself.
MomCo-host & Researcher
Mom does the digging - finding the sources, checking the claims, and translating them into something a nine-year-old will actually sit still for.
Things to remember
01
The metric system was designed during the French Revolution. Customary units accumulated over centuries with no unified logic.
02
The US legalized metric use in 1866 but everyday life never fully converted despite later pushes.
03
An inch is exactly 25.4 millimeters. American customary units are formally defined in metric terms.
04
The Mars Climate Orbiter was lost in 1999 because of a unit mismatch between ground software and navigation systems.
“Inventing a better standard can be easier than getting millions of people to change the habits and systems they already use.”
Mom
Full transcript12 min · 1,240 words
DanielMom, my friend in England told me they measure distance in miles like us, but temperature in Celsius. And weight in kilograms. How does that even work? Why doesn't everyone just use the same system?
MomDaniel, that question has a long and surprisingly dramatic answer. It involves a revolution. A metric transition the United States started, but never fully completed. And a spacecraft that was lost at Mars. Because different parts of the same mission were using different units.
DanielWhat? A spacecraft was lost because of measurement units?
MomYes. After traveling through space for nine months, we will get to the story shortly.
DanielLet's start at the beginning. Where did the metric system come from?
MomThe metric system was created during the French Revolution in the 1790s. Before that, measurement was a mess. Europe had thousands of local units and standards, many with ancient names connected to things like feet, hands, and other familiar objects. A unit with the same name could have different values, in different places.
MomAnd, revolutionary France decided to replace that patchwork with a universal decimal system, based on nature.
DanielSo, the metric system was designed. The old system just accumulated.
MomExactly. The meter was originally defined as one ten millionth of the distance from the equator to the North Pole along a meridian. Everything else followed from that, all in multiples of ten. One kilometer is a thousand meters. One centimeter is a hundredth of a meter.
MomNo memorizing arbitrary numbers, just tens all the way through. Today, the meter is actually defined using the speed of light, but the original idea was literally tied to the size of the Earth.
DanielHmm, so they started with the Earth and ended up at the speed of light. And U.S. Customary units are the opposite. One mile is 5,280 feet. One U.S. Gallon is 128 U.S. Fluid ounces. There is no pattern, you just have to know them.
MomThe older systems that U.S. Customary units grew from were not designed all at once. They accumulated over centuries. Different units for different trades, and different places, gradually standardized, but never rationalized. And once France adopted the metric system, metric eventually became the dominant measurement system around the world.
DanielSo, why doesn't the U.S. Follow it?
MomThe surprising part is that the United States has been connected to the metric system for a very long time. Congress legalized its use all the way back in 1866. But in 1975, Congress made a much bigger push with the Metric Conversion Act, encouraging a national transition to metric.
MomPresident Gerald Ford signed it, but the transition was largely voluntary.
DanielSo the law encouraged metric, but nobody had to actually use it.
MomAnd customary units were deeply embedded in American life. Road signs, machinery, construction, products, recipes, everyday habits. Converting all of that involved cost, coordination, and public resistance. Because much of the transition was voluntary, there was no single moment when everyone had to change together.
DanielSo the U.S. Did not stay on customary units because it thought they were better. It stayed because changing everything at once was genuinely difficult.
MomThen, in 1988, Congress went further. It declared metric the preferred system for U.S. Trade and commerce, and directed federal agencies to use it in many of their activities where feasible. But everyday American life never fully converted. So, today, the two systems live side by side.
DanielLike, how is that exactly done?
MomLike the spacecraft. In 1999, NASA launched the Mars Climate Orbiter. It traveled for nine months. When it reached Mars and fired its engines to enter orbit, something went wrong. The spacecraft approached Mars far lower than planned, disappeared behind the planet, and was never heard from again.
DanielBecause of the units?
MomYes. One piece of ground software, supplied by Lockheed Martin, produced thruster data in pound-four seconds. NASA's navigation software expected that data in newton-seconds, a metric unit. The conversion never happened. That mismatch caused errors to build up in the spacecraft's predicted path, until it approached Mars far too low and was lost, at a cost of about $125 million.
DanielSame spacecraft, same mission, different units, and nobody caught the mismatch until it was too late.
MomWhich is an extreme example, but it illustrates something real. Measurement systems are not just a matter of preference. When different systems are in use at the same time, the gaps between them can produce errors. Sometimes small ones, sometimes not.
DanielIs the U.S. Ever going to switch?
MomNobody knows. What is interesting is that America has already metricated in pieces. Scientists work heavily in metric. Medicine uses metric for many measurements and doses. Many industries use metric, alongside customary units. But road distances, weather temperatures, body weight, and everyday conversation still rely heavily on customary units.
MomSo, instead of one giant switch, America has ended up with a mixture.
DanielWait, so American customary units and metric are completely separate systems?
MomYes, here is the part that surprises everyone. American customary units are themselves defined in terms of metric. An inch is exactly 25.4 millimeters, and a pound is exactly 0.45359237 kilograms. So, even when Americans measure something in inches, the definition underneath those inches is tied to metric.
DanielSo, the American system is secretly metric at its foundation.
MomThe U.S. Has been measuring, in customary units, that are formally defined in metric terms for well over a century. It just does not advertise that.
DanielSo, the U.S. Is not really using one system or the other. It is using both at the same time, depending on what you are doing.
MomWhich means anyone living here has to hold two sets of units in their head simultaneously, and anyone working across that boundary has to be extremely careful about which one they are using at any given moment.
DanielEvery time someone in the U.S. Converts miles to kilometers or pounds to kilograms, they are dealing with a measurement history that started centuries ago and never completely resolved itself.
MomWhich is a reminder that, inventing a better standard can be easier than getting millions of people to change the habits and systems they already use. So, Daniel, what is one thing you are taking with you today?
DanielDo you want it in inches or centimeters? Let's see. The metric system was created during the French Revolution to replace a patchwork of local measurement systems across Europe. It is based on tens and designed to be logical and universal. The U.S. Legalized metric use in 1866, made a push toward voluntary conversion in 1975, and in 1988 declared metric the preferred system for trade and commerce, and required federal agencies to use it where feasible.
DanielBut, everyday American life never fully converted. And, here is the part that surprises everyone. American customary units are themselves defined in metric terms. An inch is exactly 25.4 millimeters. Now, this is the most fascinating part. In 1999, the Mars Climate Orbiter was lost after one part of its ground software supplied thruster data in customary units, while NASA's navigation software expected metric units.
DanielScience and medicine in the U.S. Are largely metric. And, everyday life mostly is not.
MomOne spacecraft, two unit systems, 125 million dollars, and a lesson that is still being learned.
DanielI am going to think about the French Revolution every time someone asks me how many feet are in a mile.
MomWhich is a very reasonable response to a very unreasonable question.
