[mrb]

My wife's family, who almost all have university degrees, who were all born and raised in the US, and who are very familiar with the imperial system NEVER seem to be able to do math with it. They ask me, a European raised with the metric system, for help.

Find out how many fl oz of milk are in a measuring cup graduated in units of cups? Ask mrb.

Convert my daughter's height from feet/inches to inches? Ask mrb.

Convert a package's weight from oz to lb/oz? Ask mrb.

Need to know how cold it needs to be outside in farenheit for water to freeze? Ask mrb.

I lost count of the number of times they accidentally mix up for example 1.3 feet with 1 ft 3 in. Sometimes it's due to miscommunication, eg. I have seen "six pound five" interpreted as 6 lb 5 oz by one when the speaker meant 6.5 lb. Or vice versa.

It's just comical to see someone trying to argue that the imperial system is "sometimes easier."

[big_chungus]

This is also just an anecdote, but I'll second thomk's opinion on woodwork/manual measurement of most kinds. Twelve has one, two, three, four, six, and twelve for divisors, and that is incredibly handy when you're trying to do manual measurement. Even sixteen has one, two, four, eight, and sixteen, making for easier division of pounds to even numbers of ounces; compare this to ten, which has only one, two, five, and ten. People can't eyeball or measure tenths or fifths nearly so easily as thirds and fourths. Even eights are easy to get from measuring a quarter and taking half; good luck eyeballing a fifth. And before all the "we have computers to do it for us" people come out of the woodwork: 1. Not always and 2. High-precision floating-point arithmetic is still computationally expensive and often hard to get right. I think every one has, at one point or another, hit some weird numerical glitch due to floating-point error.

Every schoolboy learns basic unit conversion; I'm not convinced by your tales of hapless relations. It's not that hard to remember eight fluid ounces per cup. It's not that hard to multiply the feet times twelve, add the inches, and divide by twelve to get just feet. It's not that hard to divide the ounces by sixteen to get pounds and ounces. It's not hard to remember that water freezes at thirty-two degrees. I've also never heard someone describe weights as "six pound five" with either meaning; the closest I've heard are descriptions of height as, say, "six foot two". This means six feet and two inches in every case, and everyone understands this.

At this point,

In conclusion, it is sometimes easier. It's just comical to see someone trying to argue that there are literally zero cases where the customary system is sometimes easier.

[Symbiote]

150mm has 2, 3, 5, 6, 10, 15, 25, 30, 50, 75 so that works very nicely if you want something that can be combined with many combinations. European kitchen cupboards and appliances are designed with a width of N×150mm.

[thrower123]

> My wife's family, who almost all have university degrees

Honestly, this is probably part of the problem.

If you ask the AP Calculus student to calculate these things, they probably don't have that down solid.

If you ask the sweathog vocational tech kid, they will know what the units of measure are and have no problem.

Practice makes perfect.

[mark-r]

I once worked with a guy who had just bought a surplus cylindrical water tank, and wanted to get an idea of what it would weigh when filled. I asked him for the dimensions and multiplied each by 2.5 to convert to approximately centimeters. Then I multiplied the diameter by 3, approximating pi. I converted each to the nearest power of 2 so I could multiply by adding the exponents. Assuming 1 cubic cm of water is 1 gram, I had the log2 weight in grams. Then I subtracted 10 to divide by 1024, getting approximate kilograms. Then added 1 to get approximate pounds. Finally took 2 to the power of the result. By the time he finished telling me the dimensions, I had an answer for him.

[jessaustin]

How would one use the diameter to determine weight? Do we square it then divide by four?

[mark-r]

In metric, the weight of water in grams is equal to the volume in cubic centimeters, by definition. The volume of a cylinder is the area of the circle times the length. The area of a circle is pi * r^2. So my description above skipped a step or two.

[zokier]

> In metric, the weight of water in grams is equal to the volume in cubic centimeters, by definition.

Just to clarify this is not exactly true. It was the original idea, but pretty quickly they realized that it wasn't a practical definition and kilogram ended up being defined by a standard reference object which was carefully stored in a vault in Paris and thus being disconnected from the meter and the density of water. Finally in 2019 kilogram was redefined again in terms of natural constants in the great SI unit upheaval. Although sadly the new definition is not very intuitive to understand.

[mark-r]

I'm sure the updates to the Kilogram definition tried to keep it as close to the original as possible, right? The difference pales in comparison to the other shortcuts I was taking.

[abduhl]

I have no idea what the GP is doing but you would just do

Pi * D^2 / 4 * H * 62.4

Where D is tank diameter in ft H is tank height in ft and 62.4 is the unit weight of water in lbs per cubic foot. Divide by 144 of D is in inches and 12 if H is in inches.

Or if you want to be real lazy you can see Pi/4*62.4 is ~49 and round to 50.

[Symbiote]

Do most people know 62.4 lb/ft³?

The equivalent metric factor is 1 g/cm³ or 1 kg/L [1] so with D and H in centimeters the calculation is

  π D²/4 × H

kilograms of water.

[1] Assuming we're talking about a normal water tank, not a space rocket. Density varies with temperature, it's 0.9970474 g/cm³ at 25 °C.

[abduhl]

The unit weight of water being 62.4 pcf is probably not known by most people - most people don’t think in cubic feet. The more likely (and less useful) number most people know is ~8 lbs per gallon.

[nwallin]

> Find out how many fl oz of milk are in a measuring cup graduated in units of cups? Ask mrb.

8. Volume is base 2. Two tablespoons in an ounce, 8 ounces in a cup, (there used to be other units in between, but nobody used them, like deci in si) two cups in a pint, two pints in a quart, four quarts in a gallon.

If you were going to sell me on switching units, it would be based on hexadecimal instead of base 10.

> Convert my daughter's height from feet/inches to inches? Ask mrb.

5' is 60". Add and subtract from that. 5'6” is 60"+6" = 66" inches, 4'4" is 60"-8"=52", etc. "Normal" humans cluster around 5' so this takes you pretty far.

> Convert a package's weight from oz to lb/oz? Ask mrb.

Again, base 2. As a programmer this is easy because it leverages all the same neural pathways that I use for converting between base 10 and base 2/16. Hell, maybe learning to cook in base 2 has made me a better programmer.

> Need to know how cold it needs to be outside in farenheit for water to freeze? Ask mrb.

This is definitely the worst example. 0°F is really cold. 100°F is really hot. Fahrenheit is objectively better than Celsius as a common parlance unit.

> I lost count of the number of times they accidentally mix up for example 1.3 feet with 1 ft 3 in.

I've never seen anyone do this ever. I've seen dumb computer systems do shoddy conversions on inputting numbers into a program, but that's why you normalize your inputs and show it back to the user. I've never seen or heard of a living breathing human make this mistake.

> "six pound five"

That's like saying "100 centi 57 meters". Those words have meanings on their own, but they don't have a meaning in that order. The only meaningful response to that is to be confused, and then realize they're confused.

How long did they live in the US and how long have they lived where they are now? This sounds like someone who lived in the US until they were ten and moved somewhere where they not only didn't use customary units but didn't speak English. Then spent the next few years scrambling to master the language and hit adulthood understanding neither US customary units nor metric units.

Honestly, culture, customs, and normality run deep. Those handful of nations that use , as the decimal separator and . as the thousands (or otherwise) separator would have a much easier time and much more benefit swapping their separators than the average American would switching from customary units to SI. Nearly everybody uses a 24 hour day, 60 minute hour, 60 second minute. 3600 second hour, 86400 second day, etc.

Oh did I say 24 hour day? Well I lied but it's close enough.

Astronomers happily use astronomical unit, light-year, parsec, and z= redshift to measure distance all in the same context. For me, having "dumb, arbitrary" units is way less important than having familiar units. Despite the fact that humans are dumb and arbitrary, we're still pretty clever.

Most of the people who complain strongly about customary units, which are dumb and arbitrary, speak languages where inanimate objects have gender. (for the record, I also think grammatical gender is dumb and arbitrary in English) "Auto" (meaning car) is neuter and "Wagen" (meaning car) is masculine. If one considers a scale where 100°F is really hot and 0°F is really cold a dumb, arbitrary system, you should stop to consider whether to_lower(str) and to_lower(to_upper(str)) yield the same results.

Humans are dumb, arbitrary creatures. The fact that US customary units are tend towards base 2 units instead of base 10 is way less arbitrary than daylight savings time, or the fact that France is in the wrong time zone, or the fact that Denmark has enshrined in law its own national time basis and then completely ignores it and then refuses to change the law which is ignored by literally every single person in Denmark.

[int_19h]

> nobody used them, like deci in si

What makes you believe that nobody uses them? Where I'm from, "decimeter" is in use, for example. Centimeters are way more common, sure - but it does come up when eyeballing distances, or in well-established terms such as "decimeter band".

[mrb]

The conversion techniques you explained, I use them myself (that's why they always ask for my help, I can do it in my head.) But the fact many Americans are unable to do that is, in itself, evidence the imperial system doesn't work that well for them.

> This is definitely the worst example. 0°F is really cold. 100°F is really hot.

This might be a sufficient explanation to give to a 5-year-old, but as soon as you need to do actual work with temperatures, this doesn't work so well. Case in point: many Americans don't know the freezing and boiling point in ⁰F. So, no, it's not "objectively better."

> That's like saying "100 centi 57 meters"

No European ever says this. In contrast, many American routinely say "six pound five".

> How long did they live in the US and how long have they lived where they are now?

Their whole life. Born and raised in America.

[Gormo]

American here. I've never once in my 40 years on this earth heard someone say "six pound five". That parlance is often used for feet and inches (almost always in reference to human height), but I've never encountered anyone using it for pounds and ounces.

[nkurz]

I'm American, have lived in many parts of the US, and I agree: "six pound five" is not something that we say. As Gormo says, this construction is frequent for feet and inches ("six foot five"), but would not be standard for weight.

And personally, if I were to use this nonstandard construction, I'd probably pluralize "pounds". I don't know why "foot" is usually singular in the parallel construction.

The most charitable interpretation I can come up with is that your relatives are from the American South, and are actually saying "six point five", meaning 6 and half pounds, and you are mishearing them.

[sfRattan]

> But the fact many Americans are unable to do that is, in itself, evidence the imperial system doesn't work that well for them.

The set of anecdotes you have put forward about Americans you know could be evidence of any number of things. You hypothesis is that the US customary system doesn't work well for them. An equally valid hypothesis is that public education doesn't work for them.

You keep asserting that your anecdotes are broadly indicative, but multiple people are telling you the opposite (and presenting an equivalent level of anecdotal evidence with similar limits in broad applicability).

For my part, I've lived in several parts of America my entire life and never met another American:

* who says "six pound five" for weight.

* who doesn't know that water freezes at 32°F.

* who would express length/distance as tenths of feet (e.g. 1.3). [1]

But I wouldn't therefore conclude anything about Americans generally.

In fairness, you could probably take a camera and get an 'average man on the street is dumb' effect if you looked hard enough and cut a bunch of disparate interviews together. I suspect you are encountering selection bias. All the people who are bad at math (independent of measuring system or educational background) have found and glommed onto you for help. All the people who don't have trouble with unit conversion (who possibly outnumber the former group) don't trouble you with their unit conversions.

[1]: I have seen halves and quarters (X.25, X.5, X.75) of feet expressed in decimal.

[Symbiote]

NB your fluid calculations are for US customary measures. The Imperial system has 20 fl oz in a pint, and people never use cups or quarts. Take care if you come across an old British recipe, or drink beer in pints in Britain.

0°F is colder than the coldest temperature I've ever experienced, your definition seems very arbitrary.

[beerandt]

Except 3 teaspoons to a tablespoon.