Scientists have realised dinosaurs were much smaller than we thought, after they discovered a mathematical mix-up in how they are measured.
Experts look at dinosaur skeletons then use a formula worked out 25 years ago to figure out how heavy they were.
But they'd been making the dinosaurs far too heavy, because the original sum had been worked out incorrectly.
So the whopping apatosaurus louisae, which was thought to weigh 38,000kg, was much smaller at just 18,000kg.
Dr Gary Packard made the claims after looking at the original sums.
"Giant dinosaurs probably were only about half as heavy as is generally believed," he said.
His new figures mean the diplodocus, loses a lot of weight - falling from 5,500kg to 4,000kg.
Dinos would still have been pretty impressive though, as Dr Packard's claims only affect their weight rather than how tall or long they were.