Natural Convection Thermal Resistance

Formula

Description

The surface-to-ambient thermal resistance for natural (free) convection depends on the heat transfer coefficient h and the effective surface area. For natural convection in still air, h is typically 5-25 W/(m²·K) depending on surface orientation, geometry, and temperature difference. Vertical surfaces with large temperature differences have higher h. This formula is fundamental for estimating cooling performance of heatsinks, PCBs, and enclosures without forced airflow. In many embedded systems, natural convection is the only cooling mechanism available.

Variables

• R_sa — Surface-to-ambient thermal resistance (°C/W)
• h — Convection heat transfer coefficient (W/(m²·K), typically 5-25 for natural convection)
• A — Effective cooling surface area (m²)

Practical Notes

Typical h values in W/(m²·K): natural convection vertical plate 5-10, horizontal plate (hot side up) 6-12, horizontal plate (hot side down) 3-6, forced convection 25-250 depending on air velocity. Radiation also contributes significantly at moderate temperatures (adds 5-10 W/(m²·K) effective). For rough estimates in natural convection, h ≈ 10 W/(m²·K) is a common starting point.