Have you ever seen a dinosaur’s weight listed in a textbook or in a natural history museum and wondered about how they came up with those weights? After all, paleontologists and other scientists only have bones to work with, and those bones have been turned into rock through the process of fossilization. How can someone come up with a weight estimate for a living creature from fossilized bone?

The answer is that there are two methods to estimate the weight of a dinosaur. One method is by measuring the cross-sectional area of a weight-bearing bone, usually a limb bone such as a femur, and plugging that figure into a formula. The other method is by using scale models to estimate the volume of a dinosaur.

Cross-sectional Method:

The cross-sectional method is based on the idea that the more weight a limb bone bears, the larger and thicker it has to be. The equation is derived from modern, living animals such as the rhinoceros and elephant. However, this method produces questionable results because dinosaurs were so much larger than our living animals, and had quite different postures and shapes that require certain adjustments in the formula.

Scale Model Method:

The scale model method is the most ideal method because it works with the actual volume of the animal, not a derived equation. A model of known scale (such a one-fiftieth the length of a sauropod) is created or found. The volume of the model is calculated by seeing how much water it displaces. This number is then multiplied by the cube of the model’s scale in order to represent the volume of a full-sized dinosaur that is identical in shape to the scale model. The volume of the “real” dinosaur is then multiplied by 0.9 kg/liter, the mass of a liter of living crocodile, to get a mass in kilograms.

This method is the basis of most published dinosaur weights and is the most popular method used by paleontologists due to its more reliable nature. However, the estimated dinosaur weights are only as good as the models. They are still imprecise estimates because we still do not know how lean or fleshy dinosaurs actually were.

Weight estimates are only one way to state the size of a dinosaur. Many books prefer to use other size measurements such as skeletal length to avoid the uncertainty and give the most accurate description of a dinosaur’s size.