Geothermal Gradient Calculator
Find the geothermal gradient from a surface and a measured borehole temperature, then extrapolate temperature to any target depth.
🌋 What is Geothermal Gradient?
The geothermal gradient is the rate at which temperature increases with depth below Earth's surface, usually expressed in degrees Celsius per kilometer. It exists because heat continuously flows outward from Earth's hot interior toward the cooler surface, driven mostly by radioactive decay in the crust and mantle plus residual heat from planetary formation.
Geologists, petroleum engineers, and geothermal energy developers rely on geothermal gradient measurements constantly, to estimate subsurface temperatures at drilling depths before a well is ever drilled, to identify promising locations for geothermal power development where useful temperatures occur at shallower, cheaper-to-reach depths, to model regional heat flow and lithospheric thickness, and to understand hydrocarbon maturation in petroleum source rocks, which depends heavily on the temperature history rocks have experienced at depth.
A common misconception is that the geothermal gradient is a single fixed number that applies everywhere on Earth. In reality it varies enormously by tectonic setting, active volcanic regions and mid-ocean ridges can show gradients many times the continental average, while old, thick, stable continental interiors (cratons) typically show gradients well below it. Even at a single location, the true geotherm curves rather than staying perfectly linear once depths become large enough for changing rock conductivity and heat sources to matter.
This calculator applies the standard local-linear-gradient relation to derive the geothermal gradient from a surface temperature and one measured borehole temperature at a known depth, then extrapolates temperature to any other target depth, plotting the resulting geotherm on a temperature-versus-depth chart.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Typical Continental Borehole
T0 = 15°C, T1 = 65°C at D1 = 2000 m, extrapolate to D2 = 3000 m
Example 2 — Active Geothermal Field
T0 = 20°C, T1 = 120°C at D1 = 1000 m, extrapolate to D2 = 1500 m
Example 3 — Stable Continental Craton
T0 = 10°C, T1 = 40°C at D1 = 3000 m, extrapolate to D2 = 5000 m
❓ Frequently Asked Questions
🔗 Related Calculators
How do you calculate geothermal gradient?
Geothermal gradient G equals the temperature difference between a measured depth and the surface, divided by that depth in kilometers: G = (T1 - T0) / (D1/1000), in degrees C per km. It measures how quickly temperature rises with depth below the surface.
What is a typical geothermal gradient value?
Typical continental geothermal gradients are about 25 to 30 degrees C per kilometer. Active volcanic or geothermal areas can show gradients of 100 degrees C per kilometer or more, while old, stable continental cratons often show gradients well below the continental average.
How do you extrapolate temperature to a different depth?
Once the local geothermal gradient G is known, temperature at any other depth D2 is T2 = T0 + G x (D2/1000), assuming the geotherm stays approximately linear between the measured depth and the target depth.
Why does geothermal gradient vary by location?
Geothermal gradient depends on local heat flow from below and the thermal conductivity of the rock above. Tectonically active regions (volcanic zones, spreading ridges, subduction-related settings) generally have higher gradients, while old, thick, stable continental crust generally has lower gradients.
Is the geothermal gradient the same at all depths?
Not exactly. Near-surface gradients are often approximately linear over a few kilometers, which is why this calculator uses a simple linear extrapolation, but at greater depths conductivity changes, radiogenic heat production, and mantle convection cause the true geotherm to curve rather than stay perfectly straight.
What causes Earth's internal heat that drives the geothermal gradient?
Earth's internal heat comes primarily from the decay of radioactive isotopes (uranium, thorium, potassium) in the crust and mantle, along with residual primordial heat left over from the planet's formation and core differentiation.
Why is geothermal gradient important for geothermal energy?
A higher geothermal gradient means useful temperatures for power generation or direct heating are reached at shallower, cheaper-to-drill depths. Regions with above-average gradients, such as volcanic arcs and rift zones, are prime targets for geothermal energy development.
Can this calculator be used for oil and gas exploration?
Yes, borehole temperature gradients like the one this calculator computes are routinely used in petroleum geology to estimate subsurface temperatures at drilling depths, which affect hydrocarbon maturation, drilling fluid properties, and equipment specifications.
What units does this calculator use?
Temperatures are in degrees Celsius and depths are in meters, with the gradient reported in degrees Celsius per kilometer, the standard convention for reporting geothermal gradients.
Does this calculator account for changes in rock type with depth?
No, this calculator uses a single locally linear gradient derived from one surface and one measured-depth temperature. It does not model changing thermal conductivity across different rock layers, which is why extrapolations are most reliable over depth ranges reasonably close to the measured interval.