Geothermal Gradient

The geothermal gradient tells you how much Earth’s temperature increases as you go deeper below the surface. It exists because the planet’s interior is hot from leftover formation heat, radioactive decay, and heat moving outward from the mantle and core, while near-surface rocks are much cooler. The formula assumes this increase is roughly linear over a limited depth range, so multiplying the gradient by depth gives the temperature change.

Use this when estimating temperatures in shallow drilling, geothermal energy studies, mining, or geology questions about how hot rocks may be underground. It is most useful for approximate calculations in the crust, not for very deep Earth conditions where the gradient can change.

• Using metres with a gradient in °C per kilometre without converting the depth first
• Treating the result as the absolute temperature instead of the temperature increase
• Assuming the same gradient applies equally deep into the Earth without checking whether the linear approximation is valid