Ƙa'idar rashin tabbas, wanda kuma aka sani da ƙa'idar rashin tabbas, ra'ayi ne a cikin injiniyoyi na ƙididdiga waɗanda ke bayyana cewa wasu nau'i-nau'i na kaddarorin jiki, kamar matsayi da ƙarfi ko makamashi da lokaci, ba za a iya auna su daidai ko kuma a san su a lokaci guda ba. Wannan ka'ida wani muhimmin al'amari ne na yanayin injiniyoyin ƙididdigewa kuma yana da tasiri mai mahimmanci ga fahimtarmu game da halayen barbashi a matakin atomic da subatomic. Ƙa'idar rashin ƙayyadaddun ƙa'idar sau da yawa tana haɗuwa da aikin masanin kimiyyar lissafi na Jamus Werner Heisenberg, wanda ya fara ba da shawara a cikin 1920s.