The Charles University refectories (called "menza" in Czech) in Prague are open every working day. Students of the Faculty of Science can use all Charles University refectories.
The closest ones are Menza Budeč and Menza Albertov.
The system of ordering meals and payments is described here.
Registration
If you have your Charles University Student Card/ISIC Student Card, go to the cash desk in the refectory, put some money (CZK – cash and credit card payments possible) in your refectory account and ask for your login and password (a different account than for SIS, printing, etc.). Afterwards, you can log in the Webkredit (menu for the week, money on your account, ordering meals in advance). The same services offer applications MobilKredit 2 (Android) and MobilKredit 2 (iOs).
There are also several bistros/canteens on campus e.g. Bistro Živina (Albertov 6, Prague 2), Bistro Golda (basement of Hlavova 8, Prague 2), or student’s pub "Mrtvá ryba" (Dead fish, Benátská 4, Prague 2).