From the position of the sun for your city, with the calculation method and Asr setting chosen for this site. You can change both for yourself in My IlmHub.

Fajr and Isha depend on how far the sun is below the horizon – calculation methods use anything from 15° to 19.5°. Some masjids also add a few minutes of caution. Choose the same method and minute adjustments as your masjid and the times will match.

North America commonly uses ISNA (15°/15°); Europe and much of Asia use the Muslim World League; the Gulf uses Umm al-Qura; Pakistan, India and Bangladesh use Karachi; Egypt uses the Egyptian General Authority. When in doubt, ask your local masjid which method its timetable follows.

Above roughly 48° latitude true dawn and dusk can merge in summer. The angle-based rule is used by default, with the middle-of-the-night and one-seventh options – the same rules used by major fatwa councils for northern Europe and Canada.