Quick answer: the tool applies the theoretical house edge to your entered stake and rounds. Think of the result as a long-run estimate, not a forecast for tonight.
What does the estimate mean?
The estimate takes total turnover (stake multiplied by rounds) and applies the theoretical house edge, calculated as 100% minus RTP. That gives a long-run average under the assumptions entered. A real session can be far above or below it.
Session cost estimator
Add the stake per round, number of rounds, and published RTP. The result is a simplified long-run estimate, not a session forecast.
RTP describes a theoretical average over a very large number of plays. It does not guarantee the return from a single session, which can vary widely.
What should I keep in mind?
If the operator publishes an RTP for the exact game version, use that figure. Settings can differ between versions. Even with the right number, the next result remains unknown.