If it is what is its name? Or do I need install it manually? Then tell me please the name of the package. In Debian I tried to install using word “Calculator” and it found no package with this name. A calculator like in Tails would be enough for me. 
On Debian templates I find “gnome-calculator”, “deepin-calculator”, “kcalc”. To find these I wrote apt search calculator. On fedora qubes instead you search with dnf search calculator.
But searching though the package managers (dnf/apt) yields some unrelated / bad results sometimes. So my suggestion is to just use a search engine in your case I would look for best calculator for debian for example. I wrote more in detail in my software installation guide:
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This may not what OP wants, but I think this could still be a viable solution.
I run python3 in a qube terminal, this is a great calculator, supports variables and allows to write an entire expression on a single line. It also retains history, so when you come back, you could reuse older values 
It’s not the standard calculator for simple math, but it’s really effective.
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If you have a JDK installed you can use jshell instead; it’s similar to @solene’s Python suggestion, i.e. allows things like:
jshell> 5+4
$1 ==> 9
jshell> var a = 55.2
a ==> 55.2
jshell> a / 2.7
$3 ==> 20.444444444444443
jshell> int fact(int number) {
...> return number == 1 ? 1 : number*fact(number-1);
...> }
| created method fact(int)
jshell> fact($1)
$5 ==> 362880
And it also remembers your last commands and allows saving code snippets etc…plus Java > Python 
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No strong opinion, I hate both but python is light and already there.
I often use sbcl or ecl for math, they are common lisp interpreter. But I thought it’s only interesting for people who prefer doing (* 8 8 (+ 4 4)) instead of 8*8*(4+4)
This notation is actually interesting when you do an operation with a lot of numbers because you don’t have to repeat the operator each time.
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Debian Template has, Settings/Applications/LibreOffice Math
I have never played with it. but I bet someone on the internet has developed a page to go into LibreOffice Math to be like a Calculator.
Is there something safe to install in dom0?
It stinks having to start a qube up just to use a calculator.
I think Galculator is the best calc to install on dom0. It requires no extra dependencies.
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If I am not wrong, Sven mentioned in a thread that he puts the calculator to his vault VM which boots up automatically. I would say it is pretty safe to have i.e. the gnome calculator next to your KeepassXC.
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What I really appreciate is this command. Thank you, mate! I didn’t know it before and now I know how to search packages to install.
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That’s certainly an option, and it doesn’t have to be vault. (I basically only open vault when I need a password.) One could imagine a very minimal “utilities” qube for such things.
As for galculator, I’m getting an immediate error from qubes-dom0-update so I guess I get to do something else this evening instead of what I had planned. (Apparently /usr/lib/qubes/qubes-download-dom0-updates.sh has disappeared from my system…)
Yes this is a digression: my issue with /usr/lib/qubes/qubes-download-dom0-updates.sh certainly wasn’t what I thought it was. In the first place, I didn’t realize the file was missing from sys-net…the message was in red but I didn’t catch on, that it wasn’t complaining about something wrong on dom0. (It’s not supposed to be on dom0 so it’s a good thing it wasn’t there.) Basically I had never until 4.2 used sys-net as my update proxy rather than sys-firewall, so my minimal template based sys-net never needed the package to install it until now.
The missing package was qubes-core-agent-dom0-updates. (It existed in sys-firewall.)