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

Name: Geothermal Gradient Calculator
Availability: InStock
Author: Nham Vu

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

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

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