this post was submitted on 24 Apr 2026
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What I'll defend, however, is fractional measurements when precision matters.
With decimal measurements, precision can't be nearly as granular. If your measurement is precise to one 1/8 of a unit, how do you represent that in decimal? 0.625 implies your measurement is precise to the nearest thousandth, but rounding it to 1 also isn't precise. 5/8, however, tells you the measurement AND the precision.
With fractional measurements, you can specify precision by changing the denominator to any number, whereas decimal is essentially fractional measurements, but with fixed denominator at powers of 10. For instance, a measurements of a half-unit with levels of precision between 0.1 and 0.10, fractional can be 6/12, 7/14, 8/16, 9/18, 10/20, 24/48, etc. Decimal can't specify that precision without essentially writing a sentance.
What's simpler to record? "24/48" or "0.5 +- 0.208333...."
My metric measurents are precise to 1/10th of a unit. Like 22.7°C or 34.7cm.
What if you get a new ruler that's 4 times as precise than the one you have that measures to 0.1cm? You don't want to record it as 0.70cm, because that's more precise than your measurement. But you could record it in 40ths with fractions.
Another way to look at it is that decimal is already a fractional system (1/10, 1/100, 1/1000) that doesn't allow you to use 90% of possible fractions.
If there's a technical need you can have your scale divided into whatever you want. There's nothing preventing you into dividing your scale every 0.25mm to get 1/4th precision. It's very rarely done because there's no need, but it's absolutely possible.
Thermometers have sometimes division per 0.5°C instead of 1°C
It does. If it were precise to less than that, you'd say 0.62 or 0.6 to indicate hundredths or tenths. Why would you say 0.625 if you're not precise to thousandths? You'd say 0.62500 if you wanted to indicate precision to hundred-thousandths.
But what if your precision is greater than 1/100 but not 10 times as precise?
That does make sense when you need absolute precision like when doing abstract math. Otherwise you can just use whichever unit and number of significant digits you need and be precise to that amount. That's what you do with imperial/American customary units as well; a 5/32" screw isn't going to be manufactured to the precision of a Planck length; manufacturers specify their sizes to three significant digits of an inch.
Let's say you have a machining project and your tools are precise to 0.1 mm. So you plan things out at a precision of 0.1 mm. It doesn't matter that a distance is 17/38 cm exactly. It doesn't matter that it's 4.473684210526315789... mm. You can't set the tool to anything better than 4.5 mm anyway.
Also note that the metric system doesn't prevent you from using fractions. You're perfectly free to work with fractions where useful. That's just not how people talk about lengths because those fractions have no meaning outside your specific use case.
But that 5/32 screw has its precision built into the measurement. Sig figs and error ranges aren't required for fractional, because both are built into the denominator.
If your 5/32 measurement is super precise you can record it as 160/1024ths, because the denominator has "+/- 1/2048" built into the measurement.
When precision matters, that precision is considered in the measurements. You would never put 0.5 +- 0.208333, you express it as 0.50 +- 0.21. The error value is just the standard deviation of the measurements and it doesn't make sense to use more than 2 significant digits.
Another example would be measuring large distances using a ruler with centimeter precision. In that case, a measurement would be expressed as 250 +- 1 cm. Converting the measurement from cm to mm, it is 2500 +- 10 mm. This is much more cumbersome with inches or feet as changing units means updating the precision, possibly reducing it.
Did I defend using imperial units?
I'm defending recording precision without having to add a qualifying statement because you can otherwose only increase precision by orders of magnitude in decimal.
This hurts my brain. Why do we care about all the weird fractions? +/- 0.1 is just another way of saying 1/10. You can still do that if you want without having to do fraction math in random denominators.
The fraction allows you to communicate length and tolerance in a single number. A decimal implies precision to the last number, a measure with a fraction can show 1/8 as more granular than 1/16. 1/8 of a cm is less precise than a mm, but if you wrote 1.125 cm, you are now implying sub mm level precision.
This matters because the level needed in building generally doesn't line up to 1/10 measurements. For example if you had a brick wall and a row had 1 cm height differences between bricks in a row it would be extremely obvious and look terrible. A 1mm height difference would be impossible to notice, but is also overkill to get that level. Ideal is about 5/8 cm or 6.35 mm difference over 3 meters of wall. The fractional measure often ends up easier to work with in practice.
I don't see how that isn't true of decimals, too. 0.1 indicates a precision of 1 digit, 0.12 indicates a precision of 2, 0.120 indicates a precision of three.
How do you account for doubling precision? Decimal only records 10-fold steps.
If I want to build something and I want it to be 23/48" ± 1/24" how would I write that? Because the way I understand it x/48" would imply a tolerance of ± 1/48".
If your tolerance is 1/24 your precision isn't fine enough enough to record 23/48.
23/48 has a built in tolerance of +/- 1/96, because outside of that range the measurement would read as either 22/48 or 24/48.
If you are drawing maps, a precision of meters is enough. If you are building a house, cm it is. If you are making furniture, mm. If you are working with metal, um (micrometer)