There was no immediate comment from ISIS on the estimates from the coalition, which is made up of countries including Britain, France and Jordan that have been bombing the extremists' positions, NBC News reported with reference to Reuters.
"We believe in Iraq it's about 40% ... And Syria, harder to get a good number, we think it's around 20," coalition spokesman U.S. Army Col. Steve Warren told a press briefing in Baghdad.
"Taking together Iraq and Syria .. they lost 30% of the territory they once held," he said.
We have retaken 40% of territory that ISIL once held. Enemy is weaker and on defensive. They have not gained one inch in Iraq since May.— COL Steve Warren (@OIRSpox) January 5, 2016
ISIS swept through a third of Iraq in 2014, seizing Mosul, the largest city in the north, and reaching the vicinity of Baghdad.
Counter-offensives by Iraqi and Kurdish armed forces, supported by the U.S.-led coalition, and by Iran-backed Shiite militias have forced ISIS out of several cities — including Tikrit, north of Baghdad, and Ramadi, to the west last month.
In Syria, ISIS is fighting the army of President Bashar Assad and other rebel groups opposed to his rule.
It is facing air strikes by the U.S.-led coalition and by Russia which has sent warplanes to support its ally, the Syrian government. Iraqi Prime Minister Haider al-Abadi last month said 2016 will be the year of "final victory" on the hard-line group.