Salamun alaykum I would like to know if the end time for isha is fajr, and if not how do we calculate it?

All praise be to Allah and may His blessings and peace be on His messenger, Muhammad,

According to Imam Ahmad, it is not permitted to delay the isha prayer after midnight without a reason.

According to the majority, it is only disliked.

The opinion of the majority is based on a hadith in which the Prophet (blessings and peace be upon him)  said:

"أمَا  إِنَّهُ لَيْسَ فِي النَّوْمِ تَفْرِيطٌ إنما التَّفْرِيطُ على من لم يُصَلِّ الصَّلَاةَ حتى يَجِيءَ وَقْتُ الصَّلَاةِ الْأُخْرَى".

"There is no negligence in sleeping, but in not praying a prayer until the time of the next one comes." (Muslim)

The Morning Prayer is an exception to this by consensus.

The position of Imam Ahmad is based on the following statement of the Prophet (blessings and peace be upon him):

"وَوَقْتُ صَلَاةِ الْعِشَاءِ إلى نِصْفِ اللَّيْلِ الْأَوْسَطِ."

"The time of isha is until midnight." (Agreed upon)

The holders of both positions accept both reports but have different ways of reconciling them.

The position of the majority is stronger, though that of Ahmad is safer.

The way you calculate the time of midnight is to determine the midpoint between sundown and sunrise, according to the stronger position.

Allah knows best.