What is the geothermal gradient?

The geothermal gradient is the rate at which temperature increases with depth in the Earth. The global average is about 25–30 °C per kilometer (roughly 14–17 °F per 1,000 ft). It varies significantly by tectonic setting — volcanic areas can exceed 100 °C/km, while stable cratons average 15–20 °C/km.

Is the linear model accurate deep in the Earth?

The linear model (T = T_surface + gradient × depth) is a good approximation for the upper crust (roughly the top 10–20 km). At greater depths, heat production from radioactive decay and changes in thermal conductivity make the actual gradient non-linear. For research below about 15 km, more complex models are needed.

What surface temperature should I use?

Use the mean annual surface temperature (MAST) for the location — not the air temperature on a given day. In practice, the temperature at about 10–20 m depth is stable and equals the long-term mean. Global averages range from about 0 °C in arctic regions to 30 °C in tropical areas.

What gradient is typical for a geothermal energy site?

Commercial geothermal power plants typically require gradients above 60–80 °C/km to reach usable temperatures (150–300 °C) at drillable depths (2–5 km). High-enthalpy volcanic systems can exceed 200 °C/km. Enhanced geothermal systems (EGS) can work with gradients as low as 30 °C/km using deeper boreholes.

How do I convert °C/km to °F per 1,000 ft?

Multiply °C/km by 0.5486 to get °F per 1,000 ft. For example, 30 °C/km × 0.5486 ≈ 16.5 °F/1,000 ft. This tool handles the conversion automatically when you select imperial units.

Geothermal Gradient Calculator

Enter depth, geothermal gradient, and surface temperature to calculate subsurface rock temperature — or solve for any unknown.

Calculator Inputs

Solve for

Surface Temperature

°C

Mean annual surface temperature at the location

Geothermal Gradient

°C/km

Global average ≈ 25–30 °C/km

Depth

Common Gradients

Result

Enter values and click Calculate to see the result.

Summary

The Geothermal Gradient Calculator applies the standard linear geotherm formula — T = T_surface + (gradient × depth) — to estimate rock temperature at any depth in the Earth's crust. You can solve for subsurface temperature, the depth at which a target temperature occurs, the gradient implied by two known temperatures, or the surface temperature from downhole measurements. Presets for common geologic settings (global average, continental crust, volcanic arc, oceanic crust, stable craton) let you get a quick estimate without looking up reference values.

How it works

Select what you want to calculate: subsurface temperature, depth, geothermal gradient, or surface temperature.
Enter the three known values for your chosen mode.
Click Calculate to get the result instantly.
The formula used is T = T_surface + (gradient × depth), where depth is in km and gradient is in °C/km.
Use the preset gradients for common geologic settings (continental crust, volcanic areas, oceanic crust) for quick estimates.
Switch between metric (°C, km) and imperial (°F, miles) units as needed.

Use cases

Estimate borehole temperature at a planned drilling depth before spudding a well.
Back-calculate the geothermal gradient from measured surface and bottom-hole temperatures.
Assess whether a location is suitable for geothermal energy extraction.
Estimate depth to a specific temperature horizon for geothermal reservoir modeling.
Teaching the linear geothermal gradient model in earth science courses.
Cross-check downhole temperature logs against expected gradient values.
Plan well casing and cement specifications based on expected formation temperature.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu