I designed and 3D-printed my own slide rule to help me play Balatro!

Balatro is a roguelike survival game where you need to multiply "chips" and "mult" together to meet a requirement each round. You get three chances to draft enough resources to survive. I designed my own slide rule to help with the mental multiplication - most of the fun of the game comes from the mechanics being slightly obscured from the player.

Since I designed this slide rule myself, I was able to make a couple unconventional design choices that fit my needs. For instance, mine has three octaves so it can represent numbers within the ones, thousands, or millions' range, for example; no need to track arbitrary powers of ten. Since it's a rotary rule, it wraps around. Eg. 353×24 shows on the device as 8.47, so you can think of it as 8.47 thousand, for example.

Holding a physical object in my hands while playing helps more than I thought it would. Should I take a card that increases chips by 600 or increases mult by 1.3×? Do I need to take a card to clear the blind in the short term, or do I have enough resources to draft a slower card that will scale better over time? Even just looking at how densely packed the marks are on the "Chips" side vs the "Mult" side of the device gives a visceral physical sense of what my build needs to focus on.

Pictures and .STL: https://www.printables.com/model/1026662-jimbos-rotary-slide...

Github repository: https://github.com/gcr/balatro-slide-rule

The actual plotting code used Marimo notebooks, which host a python in your browser via WASM. Take a look here: https://marimo.app/l/4i15d7

I entered it in Printables’ educational tools competition but the other entries were cooler. Maybe HN might like it. :-)

I do not use, and have never used, a slide rule. My grandfather was an aeronautical engineering / materials scientist for McDonnell Aircraft, and did a lot of foundational work on heat shields for early space flight (or so I am told). He was eventually named a McDonnell Douglas Fellow, back when there were fewer than 15 Fellows - the company, at the time, took out a full-page ad in Aerospace Magazine announcing it.

I have his slide rule, that he used for ages. It's a mystery in a box to me - I have not the foggiest clue how it is used - but I cherish it.

Back in 1969 I had a summer job in the engineering department of a factory. The Chief Engineer was an old guy who was a slide rule artist. He taught me how to solve second degree equations (ax^2 + bx + c = 0) with just a slide rule. The standard formula that I had learned in high school includes adding and subtracting, which a slide rule cannot do. His method involved pulling out the slider of the slide rule and inserting it backwards. Then, through some clever manipulation using the D and B scales (which now faced each other) and the A and C scales, he could actually solve the equation with just the slide rule. I wish I could remember exactly how it was done.

A big 6-foot K&E sliderule hung at the front of my high-school chemistry classroom, but was never used. At graduation ('91) I asked the teacher if I could have the slide rule and she said "sure".

I keep it now in my office, and once a year I bring to the data visualization class I teach at UChicago, to show how it works, and to show it as an example of a visual device in aid of computational thinking (nomographs being another great example).

I have one on my watch. It’s a citizen with a circular slide rule /E6B flight computer. I need my reading glasses to use it, but it’s fun.

It can reliably get me 2 sig figs, and a decent guess at a third. But… if I think about it for a minute, I can usually get that in my head anyway. Being able to setup a ratio is great though for unit conversions and things.

It’s also really good for answering that question when driving where you’re like, ok if I go 10mph faster how much sooner will I get there which is otherwise hard to do mentally.

Most of the benefit of using a slide rule in my experience comes not from using it, but from thinking LIKE you’re going to use a slide rule. You learn to freely use scientific notation with ease, and mental estimation to get the order of magnitude right.

And just my 2 cents, but circular slide rules are where it’s at.

I used them in high school in the late 1990s... just to get an extra challenge. I had my dad's and grandfather's and I picked up a couple at garage sales. I still have a collection, but haven't really used them in years. I'll probably break them out again when my kids get in middle school / high school math for fun. They are great for learning the rules of logs.

I did a little nostalgia threat on Mastodon recently. The sixth picture is of the slide rule I took to High School in 1973.

https://social.bau-ha.us/@raganwald/115979168665997624

Although slide rules are a "dead skill," Aviators typically learn to use something called an E6B Flight Computer, which works on the same principle as a round slide rule.

https://pilotinstitute.com/e6b-made-easy/

I have one in my flight bag and was required to demonstrate proficiency in its use. Of course we fly with connected digital devices these days, but having an analogue backup that operates even if the power fails is important.

I was issued one at King's Point Merchant Marine Academy in 1957 and used it constantly. A side benefit was the learned ability to estimate an answer and decimal point a skill which I still use. As I recall I mostly used multiplication, division, logs, and trig. I think I quit using it when I purchased a HP scientific calculator years later.

When I was in high school, (early 1990s,) there was a giant one mounted above the blackboard. It was clearly used for instruction in the past, but it looked so cool that no one wanted to remove it.

Every once in awhile a teacher would spend about 10-15 minutes showing how to use it. Everyone would "oooh" and "awww" and then we would all laugh about how we didn't need to use them now that we all had calculators in our pocket that were more powerful than the computers that put people on the moon.

It's always nice to learn about the past so we can appreciate what we have now.

Seen yesterday on someone's blog

[You Need A Kitchen Slide Rule](https://entropicthoughts.com/kitchen-slide-rule)

I have two or three that I inherited from my dad. I've never learned to use them because I haven't thought of something I'd use them for. The one thing I could think of is quickly doing fractional math while woodworking (what width will I have if I rip this 7.5" board into 4 pieces?) but in reality I just don't actually do that much math while woodworking.

I have four now, and even go out of my way to use them a bit.

My favorites from my collection are a little Dietzgen No 1773, which is surprisingly featureful for something that can fit in a pocket, and a Faber-Castell Novo-Biplex 2/83N, which is kind of one of the big guns for slide rules.

(Raises aged hand sheepishly)

I have one on my desk that I often use for quick estimations. It boots up in zero seconds.

The physics prof who gave me my start would always get this kind of fond, nostalgic look in his eye whenever anyone mentioned "the days of punch card programming". So, naturally, I took an old slide rule and mounted it in a shadowbox with lettering that said "IN CASE OF EMERGENCY BREAK GLASS" when I got ready to graduate. He got a tremendous kick out of it.

I used a circular slide rule when I first started flight training. It was used in flight planing to calculate ground speed and a few other things. You can still buy one.

https://en.wikipedia.org/wiki/E6B

I had a Keuffel & Esser slide rule, but it has long since disappeared. Check out some of the other stuff they made:

https://en.wikipedia.org/wiki/Keuffel_and_Esser

We learned to use them in high school (in Canada) in the mid-late '70s. Electronic calculators were just becoming widespread, and not everyone had them.

I think I can do basic calculations with them, although I really haven't touched one in many years.

I still use one of mine on occasion. I would say 2-6 times a month.

I have some from my father, but even though he showed me how to use them a long time ago, I never actually used them. I do however encourage anyone with an interest to go to the Arithmeum in Bonn (Germany) if you have the chance. It houses an large collection of regular and specialized sliderules as well as other (mechanical) computational devices.

https://www.arithmeum.uni-bonn.de/en/arithmeum.html

Yeah. Pickett N600-ES. Sometimes I will solve a problem with it just because I can. Keeps the mental model fresh. Mostly just use an HP 42s though.

I will note I didn't get it or use it until about 1998.

Yep. I have one and I keep showing off how to do math using it, to kids.I have also replaced the broken slider with a plastic piece after etching the log scale onto it.

I have a little collection of slide rules. I love those things.

I'm not old enough to have used them to do calculations, but I find them extremely useful to explain logarithms and how multiplication can be represented by the sum of logarithms. I actually work with grad students who should know these things, but watching it in a slide rule on their hands really helps to build intuition.

I own my grandfather's slide rule, he was a master toolmaker for Rolls Royce (aerospace engines) in North London during WW2.

I've picked it back up as an intentional way of slowing myself down during calculations.

Typically to do a calc I fire up Excel or the calc on my phone, bang in the numbers and accept the result without thinking. It's that "without thinking" part that is dangerous. The slide rule is slow and physical and forces my brain to think about the inputs. Another nice feature is that it can give you quick answers when you aren't sure of the accuracy of the inputs. eg if 2 * 4 was really 1.8*4.1, what would the answer be? It's quicker to see that on a slide rule (one tap on the ruler) than punch in 7 characters.

Following a post on HN a few weeks ago, I bought a used one to use in the kitchen for scaling recipes. It has to be a linear one for that, not circular, so you can set it and read it without touching it again. I also have one in my "apocalypse kit" in case of, I dunno, an EMP?

My dad was an engineer in the slide rule era and taught me how to use one when I was a kid. He said when he was in college all the engineering students had them hanging from their belts in leather sheaths like gladiator swords and they would slap when they walked.

in spite of my age, I'm one of first digital native. I never used it and nobody explained me how to use. At same time I avoid most of analog instruments: multimeters, scope meters, calipers.

Funny you ask ... I lieterally just now pulled out one of my slide rules to keep track of the required run rate for a cricket T20 game.

Fastest and best feedback for whether the batting team is ahead of the rate.

When I was in high school, calculators had just hit the scene. Some teachers allowed them, others didn't.

One of our teachers allowed us to bring a single page of notes to the exam. I wrote my notes on a photocopy of a slide rule. At exam time, I tore the sheet in half.

Of course the teacher thought I was being a smart-ass, and given that the tests were written when calculators were not allowed, they were never really all that useful.

In college chemistry, at each exam, they handed out a sheet that had the periodic table on one side, and a table of logarithms on the other.

Now that's a properly dead skill, surely. I have my dad's one somewhere, and know roughly how it works, but I've not touched it this century.

I also have one of these: https://archive.org/details/spencersdecimalr0000unse ; I believe they were popular around the time of the UK converting to decimal currency, to save people having to do the transitional arithmetic. Had a bunch of other tables in. A physical LUT.

I wonder if there's anyone with abacus skills here. I hear that held out against calculators a lot longer, for shopkeeper uses.

I used a slide rule for some of my high school physics tests in the 2010s, just for fun. I lost a few points from messing up order of magnitude calculations, but it was totally worth it. I've since collected a few more slide rules from estate sales in my neighborhood. I wish more people knew about them—they're such a neat and elegant part of engineering history, which is why I keep them around.

I have no use for them on a day-to-day basis, though. An abacus is more useful for things like counting board game points and adding up taxes.

Yes, but only as a fun retro thing. Not for most calculations, although I have to say for back of the napkin, almost all in your head style things, they can be handy.

You mean like this one:

https://www.flickr.com/photos/jonsphotos/218609214/

Yeah, to intimidate kids when they don't deliver academically :)

Related (60 comments, How slide rules work)

https://news.ycombinator.com/item?id=45985196

I bought a vintage (pre-WW1) circular slide rule. But it's too much of an "artifact" to use - I'm afraid I'd damage it.

I don't use one "seriously", or even really know how to use one to be honest. But I did buy one last year, with the intention to learn to use it. Why? Novelty mostly, and just general intellectual curiosity. I haven't really had time to dig into working with it yet though. :-(

Naw. I used to, but it's so long ago, I'm not sure I could do it now if you put one in front of me. What you don't use atrophies.

I sort of did for a lark a few years ago, then promptly forgot how to since I could do reasonable math approximations in my head. But my father used his pretty effectively.

I worked at a countertop shop and used a sliding rule a bit. But I also used a sticky paper with marks to get measurements off my screen. A lot of blueprints provide no dimensions for cabinets and desks.

Countertops is an industry with all the modern tools but 5000yo approach.

My grandfather taught me, I have long since forgotten. But I do recall that the killer use case was cube roots, which as opposed to multiplication or even square roots is difficult to do with pen and paper and much harder to mentally estimate.

I have half a dozen of them (including my father's from college) that I cherish, but do not use. I love the simplicity and elegance of the design. (Slide rules do a lot with operations that essentially boil down to addition, subtraction, and looking up function values in tables.)

I've acquired one for curiosity (a relative used to teach engineering, he was thrilled I wanted one, gave me his own)

I used it a few times, it works, of course, ... but it's not fast and not precise so I don't think anybody would use it to be productive in 2026

It sits in a box

Yes, but just as a ruler. It belonged to my Dad who was an avionics engineer starting back in the 1950s. He worked all over the world seeing out the end of the British Empire. It smells of old cigarettes and is very worn and chipped.

My grandfather taught me the basics of using it and I still have his, but haven’t used it in decades and need a refresher.

I built my own slide rule in school for fun! It looked pretty cool to me at the time. The template is still out there if you search something like "paper slide rule".

I don't use it, but if you are interested in math, it's a nice device to take a look and try to understand, at least for the basic operations.

I had to learn how to use a circular slide rule for my pilot school (CRP-5). Easily top 5 exciting pieces of technology I had the opportunity to use

I'm building a slide rule in my head: https://magworld.pw

Knew a mechanical engineer at a place where I interned. Asked him about it and he joked that he didn't trust those transistors before explaining that it's just muscle memory to him and while a calculator would be faster he'd still earn the same per hour. Apparently I was the first to ask him in over a decade as everyone had moved on to do stuff in software and no one was pushing him to use a calculator anymore. Interns didn't inquire because they thought it must be some esoteric/religious practice. Last I heard he was still working there, management asked him to stay on past retirement age for his invaluable skillset. While its probably some other skill I just like to imagine the suits in a meeting where they decided to keep him on for this particular "skill" that no one else in the company had anymore.

Yes. Still use my E6B, no matter how much hate I get from other pilots.

I bought one a few months ago off of ebay after I realized how important logarithms are in many different domains (including machine learning & information theory)

I was not allowed to use a calculator in my high school calculus class but I was permitted to (and did) use a slide rule.

As a pilot, I'm using it all the time. Even got a fancy watch with physical dials to spin

A slide rule is casually used by the General in "Fantastic Voyage"

In 1995 I used slide rules in Fort Sill, Ok. Haven't used them since.

Not in a few years, but now that you've reminded me, I should pick the habit back up.

Obligatory Asimov quote:

"The Analytical Rule might be considered a distant relation – as a skyscraper is to a shack – of that kindergarten toy, the logarithmic Slide Rule. Darell used it with the wristflip of long practice. He made freehand drawings of the result and, as Anthor stated, there were featureless plateaus in frontal lobe regions where strong swings should have been expected."

I'd really love an Analytical Rule, this hoverboard of the early atomic era.

Slide rules are awesome.

Knowing how they work can be useful when imagining machinations of an algorithm and if the code is really necessary to do all the steps that way. :)

My father is an electrical engineer and uses one

Besides Cliff Stoll? ;-)

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