[frithguild]

Widely cited estimates for the mass of Apatosaurus louisae, one of the largest of the dinosaurs, may be double that of its actual mass (38 tonnes vs. 18 tonnes)."

What's a tonne?

[agere_contra]

One tonne = 1000 kg

[ETL]

They actually estimate a dino’s mass (or weight) by dunking a toy model of it into a volume-calibrated container of water. When they see how much of the water is displaced, they know the volume of the model. And since about 97-99% of body weight is water, volume is closely proportionate to weight. To complete the approximation for the actual, full-sized creature they step up the calculation through simple multiplication.