A hard hat fiddling with his tape measure on the 50th floor of a high-rise condo under construction site accidentally dropped it Monday — and it landed on a man below, killing him, authorities said.

The 58-year-old victim, also a construction worker, wasn’t wearing his hard hat at the time, having left it in the back of his car, witnesses said.

The freak accident occurred as an AJD Construction worker was preparing a wall for windows on the 50th floor of one of two partially built residential towers at 70-90 Columbus Drive in Jersey City, according to witnesses and authorities.

The worker was unfastening the 1-pound measuring tape clipped to his work belt when it slipped out of his hands, a worker at the scene said.

The instrument fell nearly 500 feet before striking a piece of construction metal about 10 feet above ground, officials said.

The measuring device would have been falling at nearly 140 mph when it hit the piece of metal, City College professor Robert Alfano told The Post.

The tool ricocheted off the metal and smashed into victim Gary Anderson, who had been leaning into a car window to talk to someone and had just pulled his head out when he was struck, witnesses said.

Anderson, who works as an independent contractor for New York-based trucking company George Hildebrandt Inc., had been delivering a load of drywall to the project.

He immediately fell to the ground after he was hit, and was rushed to Jersey City Medical Center, where he went into cardiac arrest while doctors were treating him, according to the police report.

Anderson was pronounced dead from his head injuries at 9:52 a.m. His wife and daughters were with him at the hospital.

“He was a very good worker,” said an employee at George Hildebrandt. “He’s been with us for a long time.”

A person who picked up the phone at Anderson’s home in Somerdale, NJ, only said it was a “difficult time.”

Workers at the construction site were temporarily cleared out after the accident, but resumed their jobs at the site later in the afternoon.

The federal Occupational Safety and Health Administration is investigating the incident, cops said.

A former construction worker near the site insisted, “You don’t work in this environment without a hard hat.

“He paid the price,” the man said sadly. “You gotta have it.”

AJD Construction declined to comment.

The development project at which the accident occurred includes two residential towers and an attached hotel.

It is being developed by Panepinto Properties and Ironstate Development.