diff --git a/cara/apps/calculator/model_generator.py b/cara/apps/calculator/model_generator.py
index c4593b49..196487bc 100644
--- a/cara/apps/calculator/model_generator.py
+++ b/cara/apps/calculator/model_generator.py
@@ -324,7 +324,7 @@ class FormData:
         # Initializes a ventilation instance as a window if 'natural_ventilation' is selected, or as a HEPA-filter otherwise
         if self.ventilation_type == 'natural_ventilation':
             if self.window_opening_regime == 'windows_open_periodically':
-                window_interval = models.PeriodicInterval(self.windows_frequency, self.windows_duration, min(self.infected_start, self.exposed_start))
+                window_interval = models.PeriodicInterval(self.windows_frequency, self.windows_duration, min(self.infected_start, self.exposed_start)/60)
             else:
                 window_interval = always_on