By far the best GUI calculator I used is Qalculate: <a href="http:&#x2F;&#x2F;qalculate.sourceforge.net&#x2F;" rel="nofollow">http:&#x2F;&#x2F;qalculate.sourceforge.net&#x2F;</a><p>It&#x27;s ridicolously fast and lightweight for the amount of features it contains, and it&#x27;s also free and open source.