Calculate body heat loss in watts from surface area, skin-to-air temperature difference and air conditions, or solve for area or temperature difference.
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Body Heat Loss Formula
The calculator estimates dry heat loss from bare skin by convection and radiation. Heat loss depends on body surface area, the skin-to-air temperature difference and a heat transfer coefficient h that rises with air movement. It can solve for heat loss, surface area, or temperature difference when you enter the other two values.
- Q = body heat loss, in watts (W)
- h = combined convective and radiative heat transfer coefficient, in W/m²·K (about 8 in still air, 13 in a light breeze, 21 in a 3 m/s wind)
- A = surface area, in square meters (m²)
- ΔT = skin temperature minus air temperature, in degrees Celsius (°C)
The calculator first converts your entered units into base units: watts, square meters, and degrees Celsius. It then applies the matching formula for the value you choose to find, and converts the answer into the selected unit. Sweating, breathing and clothing are not included.
- To calculate heat loss: enter surface area (1.8 m² is prefilled) and temperature difference, and pick the air conditions.
- To calculate surface area: enter heat loss and temperature difference.
- To calculate temperature difference: enter heat loss and surface area.
Unit Conversions Used by the Calculator
| Quantity | Unit | Conversion to base unit | Base unit |
|---|---|---|---|
| Heat loss | W | 1 W = 1 W | W |
| Heat loss | kW | 1 kW = 1000 W | W |
| Heat loss | BTU/hr | 1 BTU/hr = 0.293071 W | W |
| Surface area | sq ft | 1 sq ft = 0.092903 m² | m² |
| Surface area | sq yd | 1 sq yd = 0.836127 m² | m² |
| Temperature difference | °F difference | 1 °F difference = 5/9 °C difference | °C |
Typical Adult Body Surface Area Reference
| Person size | Approximate body surface area | Equivalent in square feet |
|---|---|---|
| Smaller adult | 1.5 m² | 16.1 sq ft |
| Average adult | 1.7 to 1.9 m² | 18.3 to 20.5 sq ft |
| Larger adult | 2.0 to 2.3 m² | 21.5 to 24.8 sq ft |
Example Calculations
Example 1: Calculate heat loss
You enter a surface area of 1.8 m² and a temperature difference of 12°C in still air (h = 8 W/m²·K).
The estimated heat loss is 172.8 W (about 590 BTU/hr).
Example 2: Calculate surface area
You enter a heat loss of 200 W and a temperature difference of 20°C in still air (h = 8 W/m²·K).
The required surface area is 1.25 m².
FAQs
Why do I need to enter two values?
The formula has three main variables: heat loss, surface area, and temperature difference. Choose the one to find and enter the other two; the air conditions supply h.
Is a Fahrenheit temperature difference converted the same way as a Fahrenheit temperature reading?
No. This calculator treats the input as a temperature difference, not an absolute temperature reading. A difference of 1°F equals a difference of 5/9°C. Because it is a difference, you do not subtract 32 during the conversion.
How accurate is the estimate?
It is an estimate of dry heat loss from bare skin. Real heat loss also depends on clothing, humidity, sweating, breathing, water immersion and metabolic heat production; use a custom h for other conditions.
