List of 10 shortest-reigning

List of 10 shortest-reigning Popes
Urban VII (September 15 - September 27, 1590): reigned on 13 calendar days*
Boniface VI (April, 896): reigned on 16 calendar days
Celestine IV (October 25 - November 10, 1241): reigned on 17 calendar days
Sisinnius (January 15 - February 4, 708): reigned on 21 calendar days
Theodore II (December, 897): reigned on 21 calendar days
Marcellus II (April 10 - May 1, 1555): reigned on 22 calendar days
Damasus II (July 17 - August 9, 1048): reigned on 24 calendar days
Pius III (September 22 - October 18, 1503): reigned on 27 calendar days
Leo XI (April 1 - April 27, 1605): reigned on 27 calendar days
Benedict V (May 22 - June 23, 964): reigned on 33 calendar days
John Paul I (August 26 - September 28, 1978) died on the 33rd day after his election, having reigned on 34 calendar days
(*) Stephen II (March 23 - March 26, 752), died of apoplexy three days after his election, and before his consecration. He was excluded from the list of Popes for centuries, but has now been reinstated, causing a mismatch in the numbering of the Popes who followed him and took that name.