Geothermal Gradient Calculator
Calculate temperature at depth from the geothermal gradient.
About this calculator
Rock temperature rises steadily with depth, and this calculator applies that relationship with a simple linear model: temperature at depth equals surface temperature plus the geothermal gradient multiplied by depth. The global average gradient is about 25-30°C per kilometer, but it varies enormously by tectonic setting — active volcanic and rift zones can exceed 50°C/km, while stable continental interiors ("cratons") can run below 20°C/km. If you'd rather work from measured heat flow and rock thermal conductivity instead of an assumed gradient, the calculator derives an equivalent gradient from Fourier's law of heat conduction (gradient equals heat flow divided by conductivity) and reports the resulting temperature alongside the direct estimate. It also back-solves the linear equation to report the depths where the reservoir would cross 150°C and 200°C, the rough thresholds separating direct-use applications (heating, greenhouses) from binary and flash-steam power generation.
A resource grade of 1 (high, gradient ≥50°C/km), 2 (moderate, ≥30°C/km), or 3 (low) gives a quick read on prospectivity. The biggest caveat: real crust is not thermally uniform. Groundwater convection, fault zones, and layered rock with different conductivities all bend the temperature-depth curve away from a straight line, so this is a first-pass screening estimate, not a substitute for an actual borehole temperature log or a detailed conductive-convective subsurface model.
Inputs
Results
Temperature at Depth
105 °C
Depth for 150°C
4.5 km
≈ 14 Eiffel Towers
How to Use This Calculator
- Enter the surface temperature at your location in °C.
- Set the target depth of interest in kilometers (km).
- Input the local geothermal gradient in °C/km (global average ~25–30 °C/km), or provide heat flow and rock thermal conductivity to have the gradient calculated from them instead.
- Review the estimated temperature at depth and the depths required to reach 150 °C and 200 °C.
- Check the resource grade — deep direct-use and EGS power projects typically need temperatures above 150 °C, so a higher-grade gradient favors development.
How the result changes with Geothermal Gradient
| Geothermal Gradient | Temperature at Depth | Depth for 150°C |
|---|---|---|
| 15 | 60 °C | 9 km |
| 23 | 84 °C | 5.9 km |
| 45 | 150 °C | 3 km |
| 75 | 240 °C | 1.8 km |
What each input means
- Surface Temperature
- Mean annual surface temperature.
- Geothermal Gradient
- Temperature increase per kilometer of depth (25-30 is typical).
- Target Depth
- Depth of interest in kilometers.
- Heat Flow
- Surface heat flow (65 mW/m² is average continental).
- Rock Conductivity
- Thermal conductivity of the rock formation.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersSurface Temperature = 15, Geothermal Gradient = 30, Target Depth = 3, Heat Flow = 65 = 5 input(s) provided
- Calculate Temperature at DepthTemperature at Depth105 = 105
- Calculate Depth for 150°CDepth for 150°C4.5 = 4.5
- Calculate Gradient from Heat FlowGradient from Heat Flow26 = 26
- Calculate Depth for 200°CDepth for 200°C6.2 = 6.2
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator show two different temperature estimates?
One comes directly from your entered gradient (surface temperature plus gradient times depth), and the other, "Temp From Heat Flow," is derived independently from your heat flow and rock conductivity inputs using Fourier's law. They'll only match exactly if the gradient you typed happens to equal heat flow divided by conductivity — comparing the two is a useful sanity check on whether your assumed gradient is consistent with measured heat flow data for that rock type.
What do the depths for 150°C and 200°C actually tell me?
The calculator solves the same linear equation backward, finding the depth at which your gradient would bring the rock to those two temperatures. 150°C is roughly the floor for economical binary-cycle power generation, and 200°C opens up flash-steam plants, so these depths give a quick feel for how deep you'd need to drill to reach power-grade heat at this location.
How is the resource grade determined?
It's a simple threshold on your entered gradient: 1 (high grade) for 50°C/km or above, 2 (moderate) for 30 up to 50°C/km, and 3 (low grade) below that. It's a coarse screening label, not a substitute for a full resource assessment that would factor in reservoir permeability, fluid availability, and depth economics.
Why might my real borehole temperature not match this calculator's estimate?
The model assumes a perfectly uniform, purely conductive rock column, but actual crust rarely behaves that way — groundwater moving through fractures carries heat convectively, fault zones can channel or block flow, and rock layers with different thermal conductivities each bend the temperature-depth line differently. Treat this as a first-pass linear estimate to guide where to look, not a stand-in for an actual logged temperature profile.
Related Calculators
The questions that sit next to this one — chosen by subject, including calculators filed under a different category.
Geothermal Well Design Calculator
Estimate well depth, casing, flow rate, and cost for a geothermal well.
Geothermal EnergyGround Temperature Calculator
Estimate undisturbed ground temperature at any depth and time of year.
Geothermal EnergyGeothermal Payback Calculator
Calculate payback period from utility savings vs conventional HVAC.
More in Energy & Utilities.