For openSUSE Factory run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:Pantheon/openSUSE_Factory/X11:Pantheon.repo
zypper refresh
zypper install pantheon-calculator
For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:Pantheon/16.0/X11:Pantheon.repo
zypper refresh
zypper install pantheon-calculator
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:Pantheon/15.6/X11:Pantheon.repo
zypper refresh
zypper install pantheon-calculator