Convection heat transfer formula
Newton law of cooling gives the heat carried away from a surface by a moving fluid. It is the surface area times the temperature difference times a convection coefficient that captures how vigorously the fluid carries heat.
Variables
| Q | Heat transfer rate | W |
| h | Convection coefficient | W/m²·K |
| A | Surface area | m² |
| ΔT | Surface minus fluid temperature | K or °C |
Rearranged
A = Q / (h × ΔT)
Worked example
A 2 m² surface sits 30 degrees above the air, with h = 25 for moving air.
Multiply through: 25 × 2 × 30 = 1,500.
The convection coefficient h bundles all the messy fluid behavior into one number, and it varies enormously: a few W/m2K for still air, hundreds for forced air, thousands for moving water. Picking the right h for the fluid and flow is the crux of any convection estimate.
Comparing the modes of heat transfer?
See the Thermal Conduction Formula and the Radiation Heat Transfer Formula.
Newton law of cooling
Newton law of cooling says the rate of heat loss from a surface is proportional to the temperature difference between the surface and the surrounding fluid. The proportionality constant, the convection coefficient times area, stays roughly fixed, so a hotter object cools faster, and the cooling slows as it approaches the fluid temperature.
Natural vs forced convection
In natural convection the fluid moves only from buoyancy as it warms, giving a low coefficient, a few to tens of W/m2K for air. In forced convection a fan or pump drives the fluid, raising the coefficient many times over. This is why a fan cools a hot component so much faster than still air, despite the same temperature difference.
FAQ
What is the convection formula?
Newton law of cooling: heat rate equals the convection coefficient times area times the surface-to-fluid temperature difference, Q = h A delta T.
What is the convection coefficient?
A number, h, capturing how effectively a fluid carries heat from a surface. It is small for still air and large for forced liquids.
What is the difference between natural and forced convection?
Natural convection is driven by buoyancy alone and is weak; forced convection uses a fan or pump and has a much higher coefficient.