Heat Transfer Coefficient Calculator
Choose one of six modes to solve for the convective heat transfer coefficient (h), overall heat transfer coefficient (U), heat flux, total heat transfer rate, or the driving temperature difference. The calculator shows every step of the arithmetic using your numbers, and a reference table lists typical h-values for common fluids and flow regimes.
Formula
Worked example
A steel wall (k = 50 W/(m K), 20 mm thick) separates hot water (h_i = 500 W/(m2 K)) from outside air (h_o = 25 W/(m2 K)). 1/U = 1/500 + 0.02/50 + 1/25 = 0.002 + 0.0004 + 0.04 = 0.0424 m2K/W, giving U = 23.6 W/(m2 K). The outer air film, with 94% of the total resistance, is the dominant barrier.
What is the heat transfer coefficient?
The convective heat transfer coefficient (h), sometimes called the film coefficient, quantifies how well a fluid removes or supplies heat at a solid surface. Its definition comes directly from Newton's law of cooling: q = h x deltaT, where q is the heat flux in W/m2 and deltaT is the temperature difference between the surface and the bulk fluid in kelvin. A higher h means the fluid is more effective at carrying heat away from (or into) the surface per degree of temperature difference. The overall heat transfer coefficient (U) extends this idea to a composite system by combining the convective resistances on both sides and the conductive resistance of the wall between them.
How to choose the right calculation mode
Use "h from heat flux and deltaT" when you have measured the heat flux (for example, from a known electrical heater power divided by heated area) and the surface-to-fluid temperature difference; this is the most direct way to determine the film coefficient experimentally. Choose "h from Nusselt number" when you have run a fluid-mechanics correlation (such as the Dittus-Boelter or Churchill-Bernstein equation) to get the Nusselt number; this converts the dimensionless result into a dimensional coefficient using the fluid's thermal conductivity and the characteristic length. Select "Overall U-value" when designing or analysing a heat exchanger or building element: you supply the convection coefficients on both sides and the wall's conductive properties, and the calculator assembles the thermal resistance network. The remaining modes (flux from h, total Q, and deltaT from h) are rearrangements of the same equation for when you already know h and want to find one of the other variables.
Thermal resistance and which resistance dominates
Every heat-transfer path can be described as a series of thermal resistances: 1/h_i for the inner fluid film, delta/k_w for the wall, and 1/h_o for the outer fluid film. The U-value is the reciprocal of their sum. The design rule is straightforward: the resistance that is largest relative to the total controls performance. Doubling h_i when the outer film accounts for 90 percent of total resistance barely changes U, whereas improving h_o in that scenario gives a near-proportional gain. The insight panel identifies the dominant resistance so you can focus effort where it counts. In building elements, the outer air film plus insulation layers usually dominate; in shell-and-tube heat exchangers, the lower of the two convective coefficients typically limits performance.
Practical ranges and common pitfalls
The reference table above lists order-of-magnitude h-values for common cases. Gases in free convection sit at 2-25 W/(m2 K); forced air in a duct or over a circuit board reaches 25-250 W/(m2 K); liquid water under forced flow spans 300-20,000 W/(m2 K); and phase-change processes (boiling, condensation) can exceed 100,000 W/(m2 K). A common design pitfall is treating h as a fixed property of the fluid. It is not: h depends on flow velocity, surface geometry, temperature, and phase. The Nusselt-number route requires selecting the right correlation for the geometry and flow regime (internal pipe flow, external flat plate, crossflow over a cylinder, and so on). Unit confusion is another frequent error: in SI, h is in W/(m2 K); in imperial engineering units it is BTU/(h ft2 degF). The conversion factor is 1 W/(m2 K) = 0.1761 BTU/(h ft2 degF).
Typical convective heat transfer coefficients (h)
| Fluid and regime | Typical h (W/(m²·K)) | Notes |
|---|---|---|
| Free convection - air or gas | 2 - 25 | Very low; large fins or high-area surfaces needed |
| Forced convection - air or gas | 25 - 250 | Fans, blowers, moderate duct flow |
| Free convection - liquid | 50 - 1,000 | Heated/cooled pool, still liquid bath |
| Forced convection - light oil | 50 - 2,000 | Pump-driven tube or plate flow |
| Forced convection - water | 300 - 20,000 | Common in heat exchangers |
| Forced convection - liquid metals | 5,000 - 50,000 | Very high thermal conductivity |
| Boiling - water | 2,500 - 100,000 | Nucleate to film boiling range |
| Condensation - steam | 4,000 - 150,000 | Film condensation on cooled surface |
Order-of-magnitude values for common flow and fluid combinations. Actual h depends on geometry, flow velocity, surface condition, and fluid properties.
Frequently asked questions
What is the difference between h and U?
h (the film or convective heat transfer coefficient) applies to a single fluid-surface interface and represents how well that fluid convects heat at the boundary. U (the overall heat transfer coefficient) applies to a complete wall or heat exchanger surface and accounts for all resistances in series: the inner fluid film, any number of conductive layers in the wall, and the outer fluid film. U is always smaller than the smaller of h_i and h_o.
What are typical values of h for water vs. air?
Water under forced convection in a pipe typically gives h in the range 1,000-20,000 W/(m2 K), while forced air in a duct or over a surface gives 25-250 W/(m2 K). This 10-100x difference is the reason liquid cooling systems are so much more compact than air cooling systems for the same thermal load.
How do I calculate h from the Nusselt number?
Once you have the Nusselt number Nu from a published correlation (for example, Nu = 0.023 Re^0.8 Pr^0.4 for turbulent pipe flow), the film coefficient is h = Nu x k / L, where k is the fluid thermal conductivity in W/(m K) and L is the characteristic length (pipe diameter for internal flow, plate length for external flow) in metres. The Nusselt number is dimensionless, so the dimensions of h come entirely from k/L.
Why does the lowest h control the overall U-value?
Because thermal resistances in a series wall add up, just as electrical resistances in a series circuit do. The reciprocal relationship 1/U = 1/h_i + delta/k_w + 1/h_o means the largest individual resistance has the biggest influence on 1/U and therefore the biggest suppressing effect on U. If one side has very poor convection (small h, large 1/h), it dominates and improving the other side gives diminishing returns until the dominant resistance is addressed.
What units does this calculator use and how do I convert?
In SI mode the calculator uses W/(m2 K) for h and U, W/m2 for heat flux, and m2 K/W for thermal resistance. In imperial mode it converts to BTU/(h ft2 degF) for h and U, BTU/(h ft2) for flux, and h ft2 degF/BTU for resistance. The conversion is 1 W/(m2 K) = 0.17611 BTU/(h ft2 degF). Temperature differences in K and degF are numerically equal (a 1 K difference is exactly 1 degF difference), even though absolute temperatures are not.
Can I use this calculator for multi-layer composite walls?
The built-in U-overall mode handles a single conductive layer plus two convective films. For multi-layer walls, extend the formula by adding one delta/k term per layer: 1/U = 1/h_i + L1/k1 + L2/k2 + ... + 1/h_o. You can do this in stages using the steps panel: compute the total conductive resistance by summing each layer's delta/k, then combine with the two convective resistances to get U.