Building regulations limit the U-value of walls, roofs and windows. Each layer adds resistance in series – insulation adds the most per centimetre.
How to use the Overall Heat Transfer Coefficient Calculator
- Inside surface coefficient h_i
- Outside surface coefficient h_o
- Layer 1 thickness
- Layer 1 conductivity
- Layer 2 thickness
- Layer 2 conductivity
- Layer 3 thickness
- Layer 3 conductivity
- Layer 4 thickness
- Layer 4 conductivity
- Area
- Temperature difference
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the inside and outside surface coefficients, the thickness and conductivity of each layer (thickness 0 = unused), the area and temperature difference.
Tips
- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.
Common mistakes to avoid
- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).
Inputs explained
| Field | What to enter |
|---|---|
| Inside surface coefficient h_i | Units: W/(m²·K), BTU/(h·ft²·°F) |
| Outside surface coefficient h_o | Units: W/(m²·K), BTU/(h·ft²·°F) |
| Layer 1 thickness | Units: mm, cm, m, in, µm |
| Layer 1 conductivity | Units: W/(m·K), BTU/(h·ft·°F) |
| Layer 2 thickness | Units: mm, cm, m, in, µm |
| Layer 2 conductivity | Units: W/(m·K), BTU/(h·ft·°F) |
| Layer 3 thickness | Units: mm, cm, m, in, µm |
| Layer 3 conductivity | Units: W/(m·K), BTU/(h·ft·°F) |
| Layer 4 thickness | Units: mm, cm, m, in, µm |
| Layer 4 conductivity | Units: W/(m·K), BTU/(h·ft·°F) |
| Area | Units: m², cm², mm², km², in², ft² |
| Temperature difference | Units: K, °C, °F |
What each result means
- Total thermal resistance = 1÷[Inside surface coefficient h_i] + [Layer 1 thickness]÷[Layer 1 conductivity] + [Layer 2 thickness]÷[Layer 2 conductivity] + [Layer 3 thickness]÷[Layer 3 conductivity] + [Layer 4 thickness]÷[Layer 4 conductivity] + 1÷[Outside surface coefficient h_o]
- Overall heat transfer coefficient U = 1÷[Total thermal resistance]
- Heat loss = [Area]×[Temperature difference]÷[Total thermal resistance]
Example values
How the main result changes when Inside surface coefficient h_i changes and everything else stays at the example values above:
| Inside surface coefficient h_i (W/(m²·K)) | Overall heat transfer coefficient U (W/(m²·K)) |
|---|---|
| 3.85 | 0.2904 |
| 5.77 | 0.2979 |
| 7.69 | 0.3018 |
| 9.61 | 0.3042 |
| 11.54 | 0.3058 |
| 15.38 | 0.3078 |
Formula
R_total = 1/h_i + Σ(t_j/k_j) + 1/h_o, U = 1/R_total, q = U·A·ΔTWorked example
Inputs — Inside surface coefficient h_i: 7.69 W/(m²·K); Outside surface coefficient h_o: 25 W/(m²·K); Layer 1 thickness: 102 mm; Layer 1 conductivity: 0.72 W/(m·K); Layer 2 thickness: 100 mm; Layer 2 conductivity: 0.035 W/(m·K); Layer 3 thickness: 100 mm; Layer 3 conductivity: 1.4 W/(m·K); Layer 4 thickness: 12.5 mm; Layer 4 conductivity: 0.17 W/(m·K); Area: 20 m²; Temperature difference: 20 K.
Results — Total thermal resistance = 3.31381 m²·K/W; Overall heat transfer coefficient U = 0.30177 W/(m²·K); Heat loss = 120.70711 W.
Key terms
- U-value
- Heat flow per area per degree difference.
Frequently asked questions
What is a good U-value for a wall?
Below about 0.3 W/(m²·K) in modern buildings.
Why include surface coefficients?
Air films at each surface add resistance.
What is an R-value?
The total thermal resistance, R = 1/U.
Does thermal bridging matter?
Yes – studs and fixings raise the real U-value.
What formula does the Overall Heat Transfer Coefficient Calculator use?
R_total = 1/h_i + Σ(t_j/k_j) + 1/h_o, U = 1/R_total, q = U·A·ΔT.