Heat Transfer Calculator
A five-mode thermal suite: conduction through a wall, thermal resistance, LED / heat-sink junction temperature, convection, and overall U-value heat loss — with a built-in material library that fills in thermal conductivity for you.
A Five-Mode Heat Transfer Suite
Heat moves three ways — conduction, convection and radiation — and most engineering problems come down to one of a few standard calculations. This suite bundles the five you actually reach for, each with automatic Watts and BTU/hr output and a material library that supplies thermal conductivity so you do not have to look it up.
Conduction Through a Wall
The steady-state heat flow through a flat layer:
where k is thermal conductivity, A the area, ΔT the temperature difference across the layer and L its thickness. Bigger area and hotter difference push more heat; thicker or more insulating material slows it down.
Thermal Resistance
Often it is cleaner to think in resistance, especially for electronics and stack-ups:
Resistance adds in series just like electrical resistance, and the temperature drop across a layer is ΔT = Q × R.
Heat Sink / LED Junction Temperature
The mode that sets this tool apart. Given a device power, the ambient temperature and the thermal resistances from junction to case and case to ambient (the heat sink and interface), the junction temperature is:
This answers the question that sells the design review: I have a 30 W LED on this heat sink at 40°C ambient — what is my junction temperature, and does it stay under the device limit? The calculator flags any result above the ~125°C maximum common to many LEDs and power semiconductors.
Convection
Newton’s law of cooling. The heat transfer coefficient h captures the fluid and flow: roughly 5–25 W/m²K for natural air, 25–250 for forced air, and far higher for liquids.
Overall Heat Transfer (U-Value)
The building and HVAC workhorse. The U-value rolls every layer of an assembly into one number, and the suite turns the heat loss into daily and annual energy so you can see the running cost of a temperature difference.
Thermal Conductivity Reference
The material library auto-fills these typical values of k (W/m·K):
| Material | k (W/m·K) |
|---|---|
| Copper | 401 |
| Aluminum 6061 | 167 |
| Brass | 109 |
| Steel | 50 |
| Stainless steel | 16 |
| Concrete | 1.4 |
| FR4 (PCB) | 0.3 |
| Wood | 0.15 |
| Fiberglass | 0.04 |
| Air | 0.026 |
Frequently Asked Questions
Watts or BTU/hr?
They measure the same thing — rate of heat flow. The calculator shows both (1 W = 3.412 BTU/hr) so you can hand the result to either an SI or an imperial audience.
What is a good Rca for an LED?
It depends entirely on the heat sink and airflow. A small natural-convection sink might be 5–15°C/W; a large finned sink with a fan can be under 1°C/W. Lowering it is usually the cheapest way to drop junction temperature.
Does this include radiation?
Not directly. At moderate temperatures conduction and convection dominate; radiation matters more at high surface temperatures and can be added as a parallel path.
Helpful thermal references
- Thermal conductivity table (above)
- R-value chart for insulation
- Common material properties
- Heat transfer coefficient guide
- LED thermal design guide
Related calculators
- Thermal Expansion Calculator — coming next to the Engineering hub.
- Pipe Flow Calculator — flow, velocity and pressure drop for fluids.
- Reynolds Number Calculator — laminar or turbulent flow.