VPN basics · Plain-English answer
Yes. A VPN uses your existing data connection and adds a small encryption overhead, typically a few percent on top of whatever you would use anyway. It does not provide free or unlimited data, and everything you do through the VPN still counts against your mobile or broadband allowance.
Every packet sent through a VPN carries extra bytes for encryption and encapsulation. With an efficient protocol like WireGuard the overhead is small, usually in the low single digits as a percentage. If you stream a film that would normally use 1 GB, expect it to use only slightly more through the tunnel.
Reconnection handshakes and keepalive messages add a trivial amount on top. In practice, your data usage with a VPN looks almost identical to your usage without one.
A VPN does not bypass your mobile data cap. Your carrier still counts every byte, because the encrypted traffic travels over its network like anything else. What the carrier loses is visibility, since it can no longer see which sites and services the data went to.
One quirk worth knowing is that some carriers zero-rate specific apps, such as unlimited music streaming. Through a VPN the carrier cannot identify that traffic, so zero-rating may stop applying and the data may count normally.
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