Does anyone know a simple Qt/KDE calculator except the one which comes with KDE?
Because it has one 'feature' that i cannot stand - when you press an operation button twice, it does not replace the last operation, but calculates some result again.

If you enter, for example, '5' and then '+', and then again '+' you get 10.
On regular calculators if you press several times in a row an operation button
it doesn't (re)compute the result - if you press '+' and then '-' it uses '-'
as the operation directive when you enter next number.

https://bugs.kde.org/show_bug.cgi?id=276331

For me it's a bug. Today i wanted to calculate sum of numbers from several
web-pages. So i opened first page and entered in calculator first number and
pressed '+'. Then i opened another page and entered next number in calculator
and pressed '+'. Then i opened another page and found smth interesting on it
and got distracted. Then i wanted to add number from this page and went to
calculator, but couldn't recall if i already pressed '+', so i pressed again
'+', as people usually do on simple real calculators. Guess what happened?
Usually to repeat last operation '=' is pressed.

Do you know a calculator for KDE which behaves differently then the one from KDE SC?

there's a calc widget & speedcrunch (QT) is simple if you ignore the sci keys (can't seem to disable them)

Obviously there's also various non-KDE/QT calc's like gcalctool and gcalculator

Thanks! Looks like speedcrunch is a good alternative!

I usually just use the one in KRunner (Alt+F2) for quick calculations. It's powered by Qalculate! so it's quite powerful despite its simple interface. There's also a Qalculate! plasmoid and the Qalculate backend was just included in Cantor.