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Heat Transfer Calculator
Calculate steady-state heat conduction through a slab, wall, window or plate using Fourier’s law. Choose up to four layers in series from 23 materials or enter your own thermal conductivity, and get the heat flow in watts and BTU/h, the total R-value and U-value, and the temperature at every interface.
Last updated:
Layers (warm side first)
Area and temperatures
Temperature input
Enter your values to see the result.
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How to use the Heat Transfer Calculator
- Choose the number of layers and the thickness unit.
- Pick each layer’s material (or Custom with your own k) and enter its thickness, starting from the warm side.
- Tick surface films for building walls and windows exposed to room air and outdoor air.
- Enter the area and either both side temperatures or the temperature difference.
- Read the heat flow, R-value and U-value, and check which layer does most of the insulating in the table and chart.
Conduction formulas
- Single layer (Fourier’s law): Q = k × A × (T_hot − T_cold) ÷ L
- Layer resistance: R = L ÷ k (m²·K/W); for the whole area, R_th = L ÷ (k A) (K/W)
- Layers in series: R_total = R₁ + R₂ + … + R_films; Q = A × ΔT ÷ R_total
- U-value: U = 1 ÷ R_total; heat flux q = U × ΔT (W/m²)
- Interface temperature: the drop across each layer is q × R_layer
- Unit conversions: 1 W = 3.412 BTU/h; RSI × 5.678 = US R-value; 1 BTU·in/h·ft²·°F = 0.1442 W/m·K
Thermal conductivity of common materials
| Material | k (W/m·K) | US R per inch | RSI of 100 mm |
|---|---|---|---|
| Copper | 385 | 0.0004 | 0.0003 |
| Carbon steel | 50 | 0.003 | 0.002 |
| Concrete (dense) | 1.4 | 0.10 | 0.07 |
| Window glass | 1.0 | 0.14 | 0.10 |
| Common brick | 0.72 | 0.20 | 0.14 |
| Softwood (pine) | 0.12 | 1.2 | 0.83 |
| Fiberglass batt | 0.040 | 3.6 | 2.5 |
| EPS foam board | 0.036 | 4.0 | 2.8 |
| XPS foam board | 0.030 | 4.8 | 3.3 |
| Polyiso foam board | 0.023 | 6.3 | 4.3 |
US R per inch = 0.0254 ÷ k × 5.678. Values are typical; real products vary with density, age and moisture.
Tips for good estimates
- Insulation dominates. In a typical wall the insulation provides 80–90% of the resistance; drywall and sheathing add little.
- Framing is a thermal bridge. Wood studs (R about 1.2 per inch) cover 15–25% of a wall, so the whole-wall R-value is lower than the cavity value shown here.
- Use the design temperature difference for sizing heaters (for example the 99% winter design temperature for your area) and average temperatures for seasonal energy estimates.
- Metals barely insulate. A steel or aluminum layer adds almost no resistance; heat flow through bare metal is limited by the surface films.
Frequently asked questions
What is the formula for heat conduction?
Fourier’s law for a flat layer at steady state: Q = k × A × ΔT ÷ L, where k is the thermal conductivity (W/m·K), A the area (m²), ΔT the temperature difference across the layer (K or °C) and L the thickness (m). The result Q is in watts; multiply by 3.412 for BTU/h.
How do I calculate heat transfer through several layers?
Add the thermal resistances in series. Each layer has R = L ÷ k (m²·K/W); the total is R₁ + R₂ + … plus any surface films. Then Q = A × ΔT ÷ R_total. The layer with the largest R takes the largest share of the temperature drop.
What is the difference between R-value and U-value?
R-value is resistance to heat flow (higher is better insulation); U-value is its reciprocal, U = 1 ÷ R, the heat flow per unit area per degree (lower is better). SI R-values (RSI, m²·K/W) times 5.678 give US R-values (h·ft²·°F/BTU).
What are surface air films?
A thin layer of slow-moving air clings to every surface and adds a little resistance. ISO 6946 uses 0.13 m²·K/W on the inside of a wall and 0.04 m²·K/W outside. They matter a lot for thin, conductive layers like a single glass pane, and very little for an insulated wall.
How much heat goes through a single pane of glass?
A 4 mm pane alone has R = 0.004 ÷ 1.0 = 0.004 m²·K/W, almost nothing; with surface films the total is about 0.174 m²·K/W, a U-value near 5.7 W/m²·K. For 1 m² with 20 °C inside and 0 °C outside that is roughly 115 W.
Does this include convection and radiation?
Only through the fixed surface-film resistances, which combine convection and radiation at typical indoor and outdoor conditions. It does not model air leakage, thermal bridges such as studs, moisture, or transient heating. For a whole room heat-loss estimate use a room heat loss calculator.
What thermal conductivity values are used?
Typical published room-temperature values, e.g. copper 385, carbon steel 50, concrete 1.4, glass 1.0, fiberglass 0.040 and XPS 0.030 W/m·K. Real products vary with density, moisture and temperature, so use the manufacturer’s value with the Custom option when you have it.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.
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