this post was submitted on 12 Mar 2024
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This article says that NASA uses 15 digits after the decimal point, which I'm counting as 16 in total, since that's how we count significant digits in scientific notation. If you round pi to 3, that's one significant digit, and if you round it to 1, that's zero digits.

I know that 22/7 is an extremely good approximation for pi, since it's written with 3 digits, but is accurate to almost 4 digits. Another good one is √10, which is accurate to a little over 2 digits.

I've heard that 'field engineers' used to use these approximations to save time when doing math by hand. But what field, exactly? Can anyone give examples of fields that use fewer than 16 digits? In the spirit of something like xkcd: Purity, could you rank different sciences by how many digits of pi they require?

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[–] [email protected] 116 points 7 months ago (2 children)

I'm a waitress, and pie is $12.50.

[–] dingus 10 points 7 months ago (1 children)
[–] [email protected] 13 points 7 months ago (1 children)

Not if it's a whole pie.

...

That is a whole pie, right?

[–] [email protected] 16 points 7 months ago (1 children)

Yes, but a small, 3-4 person one. Key lime though!

[–] Feathercrown 15 points 7 months ago
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[–] [email protected] 77 points 7 months ago (3 children)

I'm Australian. I normally manage a pie with 5 digits, unless it's particularly crumbly or runny, in which case I will sometimes use 10!

[–] [email protected] 16 points 7 months ago (1 children)

I find this comment absolutely hilarious.

I recognize your profile pic from a comment months back that was also a short, deadpan reinterpretation of the question that I found hilarious. I can't for the life of me remember what it was of course.

Thanks for making me laugh!

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[–] [email protected] 11 points 7 months ago

3628800?? thats a lot!

[–] [email protected] 7 points 7 months ago (11 children)

I bet all the Americans reading this are now imagining you eating some gooey dessert like key lime pie or pumpkin pie with your hands.

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[–] taiyang 51 points 7 months ago (2 children)

I'm a liar and I use all the digits of pi.

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[–] maniacal_gaff 42 points 7 months ago

I haven't typed the digits of pi for probably 20 years because it's defined as a double precision float in all the programming libraries I use.

[–] [email protected] 37 points 7 months ago (2 children)

Ya know, this thread has inspired me. I'm a sound engineer, and find myself yelling "check one two three four" in the michrophone to test it all the time. I'm gonna start reciting the digits of Pi instead, and then as I learn them, I'll progressively advance how many numbers of Pi that I use in my everyday job :D

I work at a library, though. I should probably just go with poetry or Douglas Adams or something, but this makes me sound much more impressive

[–] FlightyPenguin 16 points 7 months ago (1 children)

Or some Douglas Adams poetry: Vogon poetry.

[–] [email protected] 15 points 7 months ago* (last edited 7 months ago)

Oh freddled gruntbuggly,

Thy micturations are to me

As plurdled gabbleblotchits on a lurgid bee.

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[–] [email protected] 36 points 7 months ago (1 children)

As a mathematician, I don’t use any digits, but the symbol Ο€.

[–] [email protected] 13 points 7 months ago (4 children)

As a computer scientist, same, but it's called PI.

It's the computer that does the thing with the digits, not me. πŸ™ƒ

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[–] [email protected] 35 points 7 months ago

baker. I measure pie based on how much I can fit in my mouth

[–] dingus 26 points 7 months ago (1 children)

I work in healthcare and I've yet to use even a single digit of pi

[–] [email protected] 23 points 7 months ago

What are you talking about? I constantly explain the calculus of the flow rate in the push IV drug I'm giving by going through the (pi)r^2 * h of the syringe, with emphasis on the dh/dy. All my patients love hearing it. They constantly thank me as I finish giving them the dilaudid.

[–] [email protected] 21 points 7 months ago (2 children)

Software engineer.. we also use all 16 digits of pi

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[–] Asidonhopo 21 points 7 months ago

Retail, and to my knowledge among all my coworkers we have used zero digits of pi.

When I code in C++ I use 15 digits of pi after the decimal point (double float) but I have only rarely coded for money and have never used pi for those work products, so again, zero digits on the clock.

Ditto for restaurant work, although 2 decimal points would be more than enough if I needed the volume of a cake or other round food.

[–] [email protected] 21 points 7 months ago (1 children)

I am a farmer who has to graft pipe cuts at various angles. i use 3.14159. which is plenty since i am measuring my cuts to the nearest eighth of an inch and i am not sending this ish to the moon.

[–] [email protected] 10 points 7 months ago (1 children)

I would like to hear more about rice farming and if the rice is tall or you are.

[–] [email protected] 8 points 7 months ago

hmm rice can get pretty tall. i guess my username should be talltallricefarmer

[–] MissJinx 17 points 7 months ago* (last edited 7 months ago)

I'm an auditor. Zero digits is the norm if I have to use Pi there is something VERY wrong

[–] [email protected] 15 points 7 months ago

I'm a consultant and I use whatever Android calculator gives me

[–] [email protected] 14 points 7 months ago (1 children)

When I worked on a website with a map on it I used 15

50000kms is the kind of distances you get going around the earth so to get it down to a millimeter precision from 50k I think 8 or 10 digits required?

So I just put 15

[–] victorz 11 points 7 months ago

As an aside, you don't need to/shouldn't pluralize km. It's just 50,000 km. πŸ‘

[–] Witchfire 12 points 7 months ago (5 children)

Software Engineering. 16 sigfigs across 64 bits

[–] [email protected] 15 points 7 months ago (1 children)

Software Engineering too, I just use std::numbers::pi. Don't know how many digits it is offhand.

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[–] [email protected] 11 points 7 months ago (1 children)

I'm British, not American, so I use 7/22.

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[–] [email protected] 11 points 7 months ago

As a pilot I can't think of a time I've ever done numbers math with pi. Private pilots are taught to use an E6B flight computer, which is basically a device for accurately drawing and measuring the triangle you're looking to solve instead of doing algebra and arithmetic.

In the wood shop, if I do have to do algebra rather than just drawing a circle with a compass, I'll use 3.14, and I still have to round to the nearest 32nd of an inch.

[–] [email protected] 10 points 7 months ago (1 children)

Engineering student. I typically use whatever number of digits the calculator gives me in calculator computations, but that's unnecessary. IMO for a design, an engineer should use at least as many digits of pi as needed to not lose any significance due to truncating pi specifically. Practically, this means: keep as many significant digits as your best measurement. In my experience, measurements have usually been good for 3 significant digits.

For back-of-the-envelope or order-of-magnitude calculations where I only need to get in the ballpark of correctness, I'll use 3 (i.e., one significant digit). For example, if I order a pizza with a diameter of 12 inches, A β‰ˆ 36 * 3 in^2 = 108 in^2 is a fine ballpark approximation that I can do in my head to the real area A = 36Ο€ in^2 β‰ˆ 113.097... in^2 that my calculator gives me.

[–] [email protected] 8 points 7 months ago

I like your idea of using 3 as an approximation to get ballpark figures - if you wanted to add a smidge of extra accuracy to that you can just remember that in doing so, you’re taking away roughly 5% of pi.

0.14159265 / 3 β‰ˆ 0.04719755

Add in around 5% at the end and your approximation’s accuracy tends to gain an order of magnitude. For your pizza example:

108 in^2 x 1.05 = 113.4 in^2 which is accurate to three significant figures and fairly easy to calculate in your head if you can divide by twenty.

You could even fudge it a little and go β€œ108 is pretty close to 100. 5% of 100 is obviously 5, so the answer is probably around 108+5=113”

[–] [email protected] 9 points 7 months ago

I work in trails, like what you might walk on, and in the rare case I need pi, I use 3.14. This week, we have a meeting to talk about some things on 14 March. I will be using cornbread because I do not know how to make a pie but I will eat the pie others make. I look forward to this meeting.

[–] nycki 9 points 7 months ago

Answering my own question: I work in web development and my usual value for pi is the standard JavaScript Math.PI. JavaScript uses 64-bit floats, which are accurate to about 15 decimal places. But that's how many digits the computer uses. For practical math, I don't think I've ever needed more than 2 digits of accuracy in an equation involving pi.

[–] [email protected] 9 points 7 months ago (1 children)

I'm a programmer and I remember 33 digits, but in practice I never use pi because I never have to deal with geometry

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[–] [email protected] 8 points 7 months ago* (last edited 7 months ago) (3 children)

Structural engineer, and it depends. If I am doing structural or quantity calculations, I can get away with 3.14 (3 digits).

If I'm dealing with survey coordinates defined by horizontal curves, I'll have to use at least 10 digits.

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[–] ProfessorProteus 8 points 7 months ago

If anyone here wants a fun way to memorize more digits, enjoy this song!

Really glad I have a legit reason to share this old thing

[–] HarriPotero 8 points 7 months ago (1 children)

Mostly I'll just go with M_PI.

355/113 = 3.14159292035398 is close enough for my needs.

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[–] [email protected] 8 points 7 months ago* (last edited 7 months ago) (2 children)

Prototyping. I just checked and my slide rule has a notch for pi. So, all of them.

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[–] mangaskahn 8 points 7 months ago (1 children)

The State of Indiana tried to define it to 1 digit by law.

https://en.wikipedia.org/wiki/Indiana_pi_bill?wprov=sfla1

Thankfully, the bill was never passed.

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[–] [email protected] 7 points 7 months ago

I rarely use pi. But when I do I only use 3 digits. I wish I could use less, but we still need to use change when we make purchases.

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