Powerful GUI Calculator for Linux?

Most famous ones are:

• Gnome Calculator
• galculator
• xcalc
• kcalc

They all have basic and scientific modes.

By default Ubuntu comes with "Gnome calculator" unless you are using a specific flavor of Ubuntu.

Gnome Calculator:

For example in "Kubuntu" you should have "kcalc".

Kcalc:

galculator

There are other options available too like "xcalc", I use galculator myself, install it using:

sudo apt install galculator

It's lightweight and really fast, doesn't have much dependencies, easy to use and has nice features.

Paper Mode:

Scientific view:

Extcalc

If you are looking for something with more features then I guess you are looking for "Extcalc".

I am personally a fan of speedcrunch, because it has a decent list of common physical and chemical constants and remembers your history across sessions:

As noted in the comments by @Michael:

[It is also] easy to mix/convert hex, bin and decimal representations of numbers. With mask() and unmask() it’s also possible to reduce/extend numbers to certain bit widths.