Qalculate Hacks

A developer documents advanced hacks for Qalculate, an open-source calculator, including bandwidth estimates, password entropy calculations, and currency conversions. The tool supports command-line and graphical interfaces with features like unit conversions and exchange rate updates.

Qalculate hacks This is going to be a controversial statement because some people are absolute nerds about this, but, I need to say it. Qalculate https://qalculate.github.io/ is the best calculator that has ever been made. I am not going to try to convince you of this, I just wanted to put out my bias out there before writing down those notes. I am a total fan. This page will collect my notes of cool hacks I do with Qalculate. Most examples are copy-pasted from the command-line interface qalc 1 , but I typically use the graphical interface as it's slightly better at displaying complex formulas. Discoverability is obviously also better for the cornucopia of features this fantastic application ships. Qalc commandline primer On Debian, Qalculate's CLI interface can be installed with: apt install qalc Then you start it with the qalc command, and end up on a prompt: bash anarcat@angela:~$ qalc Then it's a normal calculator: bash anarcat@angela:~$ qalc 1+1 1 + 1 = 2 1/7 1 / 7 ≈ 0.1429 pi pi ≈ 3.142 There's a bunch of variables to control display, approximation, and so on: set precision 6 1/7 1 / 7 ≈ 0.142857 set precision 20 pi pi ≈ 3.1415926535897932385 When I need more, I typically browse around the menus. One big issue I have with Qalculate is there are a lot of menus and features. I had to fiddle quite a bit to figure out that set precision command above. I might add more examples here as I find them. Bandwidth estimates I often use the data units to estimate bandwidths. For example, here's what 1 megabit per second is over a month "about 300 GiB" : 1 megabit/s 30 day to gibibyte 1 megabit/second × 30 days ≈ 301.7 GiB Or, "how long will it take to download X", in this case, 1GiB over a 100 mbps link: 1GiB/ 100 megabit/s 1 gibibyte / 100 megabits/second ≈ 1 min + 25.90 s Password entropy To calculate how much entropy in bits a given password structure, you count the number of possibilities in each entry say, a-z is 26 possibilities, "one word in a 8k dictionary" is 8000 , extract the base-2 logarithm, multiplied by the number of entries. For example, an alphabetic 14-character password is: log2 26 2 14 log₂ 26 × 2 × 14 ≈ 79.81 ... 80 bits of entropy. To get the equivalent in a Diceware https://en.wikipedia.org/wiki/Diceware password with a 8000 word dictionary, you would need: log2 8k x = 80 log₂ 8 × 000 × x = 80 ≈ x ≈ 6.170 ... about 6 words, which gives you: log2 8k 6 log₂ 8 × 1000 × 6 ≈ 77.79 78 bits of entropy. Exchange rates You can convert between currencies 1 EUR to USD 1 EUR ≈ 1.038 USD Even fake ones 1 BTC to USD 1 BTC ≈ 96712 USD This relies on a database pulled form the internet typically the central european bank rates https://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml , see the source https://github.com/Qalculate/libqalculate/blob/4acd56ce0c2703af80d6a8857c963d4400f2091f/libqalculate/Calculator-definitions.cc L3950-L3966 . It will prompt you if it's too old: It has been 256 days since the exchange rates last were updated. Do you wish to update the exchange rates now? y As a reader pointed out https://en.osm.town/@mdione/113979012459360601 , you can set the refresh rate for currencies https://qalculate.github.io/manual/qalculate-units.html qalculate-currency , as some countries https://en.wikipedia.org/wiki/Economy of Argentina will require way more frequent exchange rates. The graphical version has a little graphical indicator that, when you mouse over, tells you where the rate comes from. Other conversions Here are other neat conversions extracted from my history teaspoon to ml teaspoon = 5 mL tablespoon to ml tablespoon = 15 mL 1 cup to ml 1 cup ≈ 236.6 mL 6 L/100km to mpg 6 liters / 100 kilometers ≈ 39.20 mpg 100 kph to mph 100 kph ≈ 62.14 mph 108km - 72km / 110km/h 108 kilometers − 72 kilometers / 110 kilometers/hour ≈ 19 min + 38.18 s Completion time estimates This is a more involved example I often do. Background Say you have started a long running copy job and you don't have the luxury of having a pipe you can insert pv 1 into to get a nice progress bar. For example, rsync or cp -R can have that problem but not tar . Yes, you can use --info=progress2 in rsync , but that estimate is incremental and therefore inaccurate unless you disable the incremental mode with --no-inc-recursive , but then you pay a huge up-front wait cost while the entire directory gets crawled. Extracting a process start time First step is to gather data. Find the process start time. If you were unfortunate enough to forget to run date --iso-8601=seconds before starting, you can get a similar timestamp with stat 1 on the process tree in /proc with: bash $ stat /proc/11232 File: /proc/11232 Size: 0 Blocks: 0 IO Block: 1024 directory Device: 0,21 Inode: 57021 Links: 9 Access: 0555/dr-xr-xr-x Uid: 0/ root Gid: 0/ root Access: 2025-02-07 15:50:25.287220819 -0500 Modify: 2025-02-07 15:50:25.287220819 -0500 Change: 2025-02-07 15:50:25.287220819 -0500 Birth: - So our start time is 2025-02-07 15:50:25 , we shave off the nanoseconds there, they're below our precision noise floor. If you're not dealing with an actual UNIX process, you need to figure out a start time: this can be a SQL query, a network request, whatever, exercise for the reader. Saving a variable This is optional, but for the sake of demonstration, let's save this as a variable: start="2025-02-07 15:50:25" save "2025-02-07T15:50:25"; start; Temporary; ; 1 = "2025-02-07T15:50:25" Estimating data size Next, estimate your data size. That will vary wildly with the job you're running: this can be anything: number of files, documents being processed, rows to be destroyed in a database, whatever. In this case, rsync tells me how many bytes it has transferred so far: rsync -ASHaXx --info=progress2 /srv/ /srv-zfs/ 2.968.252.503.968 94% 7,63MB/s 6:04:58 xfr 464440, ir-chk=1000/982266 Strip off the weird dots in there, because that will confuse qalculate, which will count this as: 2.968252503968 bytes ≈ 2.968 B Or, essentially, three bytes. We actually transferred almost 3TB here: 2968252503968 bytes ≈ 2.968 TB So let's use that. If you had the misfortune of making rsync silent, but were lucky enough to transfer entire partitions, you can use df without -h we want to be more precise here , in my case: Filesystem 1K-blocks Used Available Use% Mounted on /dev/mapper/vg hdd-srv 7512681384 7258298036 179205040 98% /srv tank/srv 7667173248 2870444032 4796729216 38% /srv-zfs Otherwise, of course, you use du -sh $DIRECTORY . Digression over bytes Those are 1 K bytes which is actually and rather unfortunately Ki , or "kibibytes" 1024 bytes , not "kilobytes" 1000 bytes . Ugh. 2870444032 KiB 2870444032 kibibytes ≈ 2.939 TB 2870444032 kB 2870444032 kilobytes ≈ 2.870 TB At this scale, those details matter quite a bit, we're talking about a 69GB 64GiB difference here: 2870444032 KiB - 2870444032 kB 2870444032 kibibytes − 2870444032 kilobytes ≈ 68.89 GB Anyways. Let's take 2968252503968 bytes as our current progress. Our entire dataset is 7258298064 KiB , as seen above. Solving a cross-multiplication We have 3 out of four variables for our equation here, so we can already solve: now-start /x = 2996538438607 bytes / 7258298064 KiB to h actual − start / x = 2996538438607 bytes / 7258298064 kibibytes x ≈ 59.24 h The entire transfer will take about 60 hours to complete Note that's not the time left , that is the total time. To break this down step by step, we could calculate how long it has taken so far: now-start now − start ≈ 23 h + 53 min + 6.762 s now-start to s now − start ≈ 85987 s ... and do the cross-multiplication https://en.wikipedia.org/wiki/Cross-multiplication manually, it's basically: x/ now-start = total/current so: x = total/current now-start or, in Qalc: 7258298064 kibibytes / 2996538438607 bytes 85987 s 7258298064 kibibytes / 2996538438607 bytes × 85987 secondes ≈ 2 d + 11 h + 14 min + 38.81 s It's interesting it gives us different units here Not sure why. Now and built-in variables The now here is actually a built-in variable: now now ≈ "2025-02-08T22:25:25" There is a bewildering list of such variables https://qalculate.github.io/manual/qalculate-definitions-variables.html , for example: uptime uptime = 5 d + 6 h + 34 min + 12.11 s golden golden ≈ 1.618 exact golden = √ 5 + 1 / 2 Computing dates In any case, yay We know the transfer is going to take roughly 60 hours total, and we've already spent around 24h of that, so, we have 36h left. But I did that all in my head, we can ask more of Qalc yet Let's make another variable, for that total estimated time: total= now-start /x = 2996538438607 bytes / 7258298064 KiB save now − start / x = 2996538438607 bytes / 7258298064 kibibytes ; total; Temporary; ; 1 ≈ 2 d + 11 h + 14 min + 38.22 s And we can plug that into another formula with our start time to figure out when we'll be done start+total start + total ≈ "2025-02-10T03:28:52" start+total-now start + total − now ≈ 1 d + 11 h + 34 min + 48.52 s start+total-now to h start + total − now ≈ 35 h + 34 min + 32.01 s That transfer has ~1d left, or 35h24m32s, and should complete around 4 in the morning on February 10th. But that's icing on top. I typically only do the cross-multiplication and calculate the remaining time in my head. I mostly did the last bit to show Qalculate could compute dates and time differences, as long as you use ISO timestamps. Although it can also convert to and from UNIX timestamps, it cannot parse arbitrary date strings yet? . Other functionality Qalculate can: - Plot graphs; - Use RPN input; - Do all sorts of algebraic, calculus, matrix, statistics, trigonometry functions and more ; - ... and so much more I have a hard time finding things it cannot do. When I get there, I typically need to resort to programming code in Python, use a spreadsheet, and others will turn to more complete engines like Maple https://en.wikipedia.org/wiki/Maple software , Mathematica https://en.wikipedia.org/wiki/Wolfram Mathematica or R https://en.wikipedia.org/wiki/R programming language . But for daily use, Qalculate is just fantastic. And it's pink Use it Gotchas There are a couple of things that get me with Qalc, unfortunately. Decimals precision I mentioned set precision above: set precision 6 1/7 1 / 7 ≈ 0.142857 set precision 20 pi pi ≈ 3.1415926535897932385 Fractional displays But sometimes, I want fractional displays obviously not for π because it is irrational https://en.wikipedia.org/wiki/Irrational number . For example, sometimes I work in inches, and this would look better as a fraction: 8973/12 8973 / 12 = 747.75 The trick here is to change the fraction setting, from the qalc 1 manual: fractions, fr -1 = auto, 0 = off, 1 = exact, 2 = on, 3 = mixed, 4 = long, 5 = dual, 1/n Determines how rational numbers are displayed e.g. 5/4 = 1 + 1/4 = 1.25 . 'long' removes limits on the size of the numerator and denonimator. Normally, this should be set to auto , so if you've changed it, change it back: set fractions auto Then you get the nice mixed output: 8973/12 8973 / 12 = 747 + 3/4 = 747.75 The dual setting is also nice: set fractions dual 747.75 = 2991/4 = 747 + 3/4 = 747.75 Strangely, I couldn't figure out how to get the same output in the graphical interface. The closest menu item is Mode Rational Number Form. Further reading and installation This is just scratching the surface, the fine manual https://qalculate.github.io/manual/ has more information, including more examples https://qalculate.github.io/manual/qalculate-examples.html . There is also of course a qalc 1 manual page https://manpages.debian.org/bookworm/qalc/qalc.1.en.html which also ships an excellent EXAMPLES section https://manpages.debian.org/bookworm/qalc/qalc.1.en.html EXAMPLES . Qalculate is packaged for over 30 Linux distributions https://repology.org/project/libqalculate/badges , but also ships packages for Windows and MacOS. There are third-party derivatives as well including a web version https://qalculator.xyz/ and an Android app https://f-droid.org/en/packages/com.jherkenhoff.qalculate/ . Updates Colin Watson liked this blog post and was inspired to write his own hacks https://www.chiark.greenend.org.uk/~cjwatson/blog/qalculate-time-hacks.html , similar to what's here, but with extras, check it out You can use your Mastodon account to reply to this post https://kolektiva.social/@Anarcat/113971985775192655 . Edited . https://gitlab.com/anarcat/anarc.at/-/commits/main/blog/2025-02-08-qalculate-hacks.md