With the end of President Bashar al-Assad’s regime in Syria, the task of tallying the cost in human lives of a nearly 14-year-long civil war is continuing. Death toll estimates from the conflict are as high as 620,000, a staggering number in a country with a prewar population of 22 million.

Experts say establishing the true scale of death is complicated, as estimates are drawn from different sources and methods and calculated in varying ways. Here’s what we know:

What is the latest death toll?

As of March, the Syrian Observatory for Human Rights had documented the names of 507,567 people who had died in Syria since the outbreak of the conflict in March 2011. The independent Syrian-run organization, which is based in Britain and collates information from multiple sources, said that it had verified another 110,343 deaths of people who were not named, bringing the total of civilians and combatants killed throughout the war to 617,910.

The Syrian Network for Human Rights, another independent human rights organization that has been tracking the toll since the start of the conflict, had counted a total of 231,495 civilian deaths through June.

What does the United Nations say?

The Office of the U.N. High Commissioner for Human Rights last released an accounting in 2021, when it estimated that at least 580,000 people had been killed to that point, including 350,209 “identified individuals.” The U.N. human rights chief, Michelle Bachelet, cautioned at the time that the figure was “not a complete number of conflict-related killings” but that it indicated a “minimum verifiable number, and is certainly an undercount of the actual number.”

Before 2021, the United Nations had not released a report on deaths in Syria since 2014 because it said conditions on the ground made accurate documentation impossible. However, when the war reached its 10th year, the organization responded to the need for updated information with a new report. By then, the war had shifted from large-scale military hostilities to regional clashes between multiple armed groups, making access and accuracy even more complicated, so the U.N. stopped reporting numbers.

How is the death toll calculated?

The U.N. and the Syrian human rights organizations say they rely on on-the-ground interviews, news reports and publicly available information such as death certificates and hospital records. They then identify and exclude duplicates and records with only partial information from the total and check the results against other statistical estimates.

Cases where people had been taken into government custody with no documentation — the forcibly disappeared — are not part of the total, nor are cases in which victims are killed and their bodies not recovered.

The Syrian Network for Human Rights focuses on tallying civilian deaths. The Syrian Observatory for Human Rights also tracks deaths among the Syrian military, rebel factions, foreign militias and armed units sent by other governments that are party to the conflict, such as Israel, Russia and Iran.

Will we ever know the final toll?

The end of the conflict will bring some clarity. When Damascus fell on Dec. 8, rebel groups opened Syria’s prisons, freeing thousands of surviving detainees to the relief of friends and families. But thousands of other people remain missing.

Fadel Abdul Ghany, the founder of the Syrian Network for Human Rights, has counted more than 100,000 men, women and children who were “forcibly disappeared” by the Assad regime since 2011 and who were not included in any death toll. So far, he says, very few have emerged from the prisons, and he has been able to identify only about 3,000 death certificates.

Based on what he has seen from the jails and the documents his team has obtained, Mr. Ghany estimated that at least 85,000 of those forcibly disappeared have been killed in al-Assad’s detention centers.