kostenlos
free
Without having to pay for it; free of charge.
Der Versand ist bei einer Bestellung über 50 Euro kostenlos.
Shipping is free for orders over 50 euros.

free
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free
Without having to pay for it; free of charge.
Der Versand ist bei einer Bestellung über 50 Euro kostenlos.
Shipping is free for orders over 50 euros.

The word is made up of 'Kosten' (costs) and the suffix '-los' (-less). The suffix '-los' means 'without,' so 'kostenlos' literally means 'without costs.'
Die Teilnahme am Webinar ist für alle kostenlos.
Participation in the webinar is free for everyone.
'Kostenlos' can be used like an adverb to describe a verb. In this case, it is not declined and often appears at the end of the sentence or after the verb.
Sie können die App kostenlos im App Store herunterladen.
You can download the app for free from the App Store.
The word 'kostenlos' (free) is often combined with services or offers. Typical examples are: 'kostenloser Eintritt' (free admission), 'kostenloses WLAN' (free Wi-Fi), or 'eine kostenlose Probe' (a free sample).
Viele Cafés bieten ihren Kunden kostenloses WLAN an.
Many cafés offer their customers free Wi-Fi.
free
'frei' and 'kostenlos' can both mean that you do not have to pay anything. In this meaning, they are very similar. However, 'kostenlos' is clearer and almost always fits when the topic is money. 'frei' can also mean many other things, for example available or independent. That is why 'frei' can be misleading if the context is not clear. If you want to say clearly that something does not cost anything, 'kostenlos' is often more precise.

Im Urlaub nutzt Julia das kostenlose WLAN im Hotel.
On vacation, Julia uses the free Wi-Fi at the hotel.
Der Eintritt ins Museum ist heute frei.
Admission to the museum is free today.