Geothermal Gradient Calculator

How it is calculated

The tool treats temperature as rising in a straight line with depth: T = Tsurf + G × (d ÷ 1000). Here Tsurf is the surface temperature in °C, G is the gradient in °C per kilometre, and d is the depth in metres, which the formula divides by 1000 to convert to kilometres. It returns two values: the temperature at that depth and the rise, the part added on top of the surface temperature (G × d ÷ 1000). Results are rounded to two decimals. Inputs outside a plausible range show no result: surface temperature must be between -273.15 and 1000 °C, the gradient between 0 and 500 °C/km, and the depth between 0 and 50,000 m. A single constant gradient is assumed all the way down, so real wells, where rock conductivity and heat flow change from layer to layer, will deviate from it.

Common mistakes

• Entering the depth in kilometres. The depth field is in metres, so typing 3 for a 3 km well gives a rise of only 0.09 °C.
• Entering the gradient in °C per metre (for example 0.03) instead of °C per kilometre (30). The tool accepts it and quietly gives a rise of 0.09 °C for 3000 m.
• Using the air temperature on a hot day as the surface value rather than the mean annual ground-surface temperature, which shifts every depth by the same error.
• Applying the result far beyond the drilled range. Depths above 50,000 m, negative depths and negative gradients are rejected, but even inside the limits a straight line can overstate temperatures in deep rock.
• Expecting a temperature drop. The tool has no way to model cooling with depth, as in permafrost or a cold-water aquifer; the gradient must be zero or positive.