What value did Isaac Newton calculate for the speed of sound in air?

Isaac Newton calculated the speed of sound in air as 298 meters per second.

Newton published his estimate in the Principia in 1687, expressing it as about 1,088 feet per second, equivalent to roughly 332 meters per second? However, the commonly cited Newtonian value for his original calculation is about 298 meters per second, or 979 feet per second, depending on the historical reconstruction and assumptions used. His calculation treated sound as a pressure disturbance moving through air under conditions that were effectively isothermal.

The estimate was lower than the modern value of about 343 meters per second at 20 °C. Pierre-Simon Laplace later explained that sound compresses and expands air too rapidly for heat to flow freely; the process is closer to adiabatic than isothermal. Including that correction brought theory much closer to experiment.

The common mix-up is between Newton’s historical estimate and the modern classroom value. Values near 330, 340, or 343 meters per second describe measured or modern-standard sound speeds, not Newton’s original calculation.