der Umfang
scope / extent
The extent, size, or reach of something.
Der genaue Umfang des Projekts muss noch definiert werden.
The exact scope of the project still has to be defined.

scope / circumference
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scope / extent
The extent, size, or reach of something.
Der genaue Umfang des Projekts muss noch definiert werden.
The exact scope of the project still has to be defined.

The word is made up of the preposition 'um' (around) and 'Fang' (from the verb 'fangen', to catch). So it literally describes what 'catches' or encloses something, whether a surface or an abstract topic.
Der Zaun markiert den gesamten Umfang des Grundstücks.
The fence marks the entire perimeter of the property.
You can easily distinguish the meanings from context. If it is about measurable lengths, as with a circle or a waist, the geometric meaning is intended. If it is about abstract things like projects, knowledge, or tasks, it means extent or reach.
Der Umfang des Schadens war größer als erwartet.
The extent of the damage was greater than expected.
In the abstract meaning 'Ausmaß' (extent), 'Umfang' (scope/extent) is often used in fixed expressions. Common ones are 'in vollem Umfang' (to the full extent), 'in großem Umfang' (on a large scale), or 'einen erheblichen Umfang haben' (to be considerable in scope).
Die neuen Regeln gelten in vollem Umfang für alle Mitarbeiter.
The new rules apply in full to all employees.
When referring to a measurable size, 'Umfang' (circumference) is often combined with the verbs 'messen' (to measure), 'berechnen' (to calculate), or 'haben' (to have).
Der Gärtner berechnet den Umfang des runden Blumenbeets.
The gardener calculates the circumference of the round flower bed.
area / field
'der Bereich' and 'der Umfang' can both describe limits or reach. 'Bereich' more often names the field itself: Which tasks, topics, or responsibilities belong to it? 'Umfang' more often describes how large something is or how much is included. The words can be confused when talking about work, responsibility, or a project. 'Mein Bereich' means: I am responsible for that. 'Der Umfang' means: This is how large the task is, or this is how much it includes.

Vor dem Wochenende besprechen wir den Umfang des neuen Projekts.
Before the weekend, we discuss the scope of the new project.
Kundenberatung gehört in unserer Firma zu Lenas Bereich.
In our company, customer consulting is part of Lena's area of responsibility.