Geothermal Heat Pump Sizing Calculator
A ground-source (geothermal) heat pump is sized from the building's peak load and the heat its ground loop can exchange per foot of bore or trench. This calculator converts a peak load in BTU per hour into a capacity in tons (one ton equals 12,000 BTU per hour) and a total ground loop length using the heat exchange rate you supply in BTU per hour per linear foot. Because that exchange rate depends on local soil conductivity, moisture, and loop design, it is a user-editable input rather than a fixed assumption. The result is a transparent first-pass planning figure; a licensed designer must run the full load calculation and soil assessment before installation.
Geothermal sizing formula
Capacity (tons) = load BTU/h / 12,000
Capacity (kW thermal) = load BTU/h * 0.00029307107
Total loop length = load BTU/h / exchange rate per foot
Length per loop = total loop length / number of loops
One ton equals 12,000 BTU/h. The watt-to-BTU/h conversion factor is 0.00029307107 kW per BTU/h. Loop length divides the load by the per-foot exchange rate you supply for your soil and loop type.
Ground-source sizing context
- Size to a calculated peak load (such as a Manual J result), not floor area or a rule of thumb.
- The per-foot exchange rate rises in wet, conductive soils and falls in dry sandy ground.
- Oversizing causes short-cycling and wasted capital; undersizing fails to meet peak demand.
- Vertical bores and horizontal trenches have different per-foot rates; use the value for your design.
- A licensed designer must confirm soil conductivity and final loop layout before installation.
Geothermal sizing: frequently asked questions
How is a geothermal heat pump sized?
Sizing starts from the building's peak heating or cooling load in BTU per hour. Divide by 12,000 to get the size in tons (1 ton equals 12,000 BTU/h). The ground loop length comes from dividing the load by the ground heat exchange rate in BTU per hour per foot of bore or trench, a value that depends on soil conductivity and loop design.
What ground heat exchange rate should I use?
The heat a loop can move per linear foot depends on soil type, moisture, and loop configuration, so it is a user-editable input. Wetter, more conductive soils support higher rates per foot; dry sandy soils support less. Your loop designer or a local thermal-conductivity test gives the correct figure.
What is a ton of cooling?
One ton of heating or cooling capacity equals 12,000 BTU per hour, historically the rate at which one short ton of ice melts over 24 hours. Residential ground-source systems are often in the 2 to 6 ton range; the right size comes from a proper load calculation, not floor area alone.
Does a bigger heat pump always work better?
No. An oversized unit short-cycles, runs inefficiently, and costs more to install, while an undersized one cannot meet peak load. Size to a calculated load (such as a Manual J calculation) rather than rounding up. This tool converts a load you supply into capacity and loop length.
Is this calculator a substitute for a professional design?
No. It gives a transparent first-pass estimate from the load and exchange rate you enter. A licensed designer must perform the full load calculation, soil assessment, and loop layout. Treat the output as a planning figure, not a construction specification.
Official sources
- U.S. Department of Energy: Geothermal Heat Pumps.
- ENERGY STAR: geothermal heat pump program.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.