Heat Transfer Calculator – Conduction, Convection & Radiation
Analyze thermal performance with our heat transfer calculator. Compute heat flow rates for conduction, convection, and radiation in HVAC, manufacturing, and engineering design.
Q = kAΔT / L
Step 1: Select the heat transfer mode - conduction (through solids), convection (through fluids), or radiation (electromagnetic).
Step 2: Enter the required parameters for your selected mode (thermal conductivity, area, temperatures, etc.).
Step 3: Click Calculate to see the heat transfer rate in watts.
Conduction - Heat Through Solids
Conduction is heat flowing through a material from hot to cold:
Q = kAΔT / L
k is thermal conductivity (how well the material conducts heat), A is area, ΔT is temperature difference, L is thickness. Metals have high k (copper = 400 W/m·K). Insulators have low k (wood = 0.1 W/m·K).
Convection - Heat Through Fluids
Convection is heat carried by moving fluids (air, water, oil):
Q = hA(T_s - T_∞)
h is the heat transfer coefficient (depends on fluid and flow), A is surface area, T_s is surface temperature, T_∞ is fluid temperature. Forced convection (fans, pumps) has much higher h than natural convection.
Radiation - Heat Through Empty Space
Radiation is heat transfer via electromagnetic waves - no medium needed:
Q = εσA(T₁⁴ - T₂⁴)
ε is emissivity (0-1, blackbody = 1), σ is Stefan-Boltzmann constant, A is area, T is absolute temperature (Kelvin). Radiation dominates at high temperatures - it scales with T⁴.
| Material | Thermal Conductivity (W/m·K) | Use Case |
|---|---|---|
| Copper | 400 | Heat sinks, cookware |
| Aluminum | 237 | Heat exchangers, fins |
| Steel (carbon) | 50 | Structural, pipes |
| Glass | 1.0 | Windows, insulation |
| Water | 0.6 | Coolant, heating |
| Wood (oak) | 0.17 | Building, handles |
| Air (still) | 0.026 | Insulation (double glazing) |
| Styrofoam | 0.033 | Insulation, packaging |
Higher thermal conductivity means better heat transfer. Insulators have low k values. Metals are excellent conductors.
Conduction Example: House Wall
A brick wall (k = 0.7 W/m·K) that's 10 m² area and 0.2 m thick, with 20°C inside and 0°C outside: Q = 0.7 × 10 × 20 / 0.2 = 700 W. That's 700 joules per second leaking through the wall. Add insulation and you can cut this by 80%.
Convection Example: CPU Cooler
A CPU heatsink with 0.01 m² surface area, h = 50 W/m²·K (forced air), CPU at 70°C, air at 25°C: Q = 50 × 0.01 × 45 = 22.5 W. That's why high-performance CPUs need bigger heatsinks and faster fans - more area and higher h.
Radiation Example: Sun to Earth
The Sun (5,800 K) radiates energy across space. Earth receives about 1,360 W/m² at the top of atmosphere. This is pure radiation - no air in space. The T⁴ term means doubling temperature increases radiation by 16x.
Which heat transfer mode is fastest?
Depends on the situation. In solids, conduction dominates. In fluids with flow, convection is usually faster. At high temperatures (above 500°C), radiation becomes dominant. Most real situations involve all three modes simultaneously.
Why does metal feel colder than wood at the same temperature?
Metal conducts heat away from your hand much faster than wood. Your skin senses heat flow rate, not absolute temperature. A 20°C metal block feels cold because it's pulling heat from your hand. A 20°C wood block feels neutral because heat flows slowly.
How do I reduce heat loss in my home?
Add insulation (low k materials like fiberglass, foam). Seal air leaks (reduces convection). Use double/triple glazing (traps air between panes). Reflective barriers work for radiation - that's why radiant barriers go in attics.
What is the R-value of insulation?
R-value is thermal resistance - the inverse of conductivity, adjusted for thickness. R = L/k. Higher R means better insulation. Typical walls need R-13 to R-21. Attics need R-38 to R-60. Double-pane windows are about R-2 to R-3.
Does paint color affect heat transfer?
Yes, for radiation. Dark colors have high emissivity and absorptivity - they radiate and absorb heat well. Light colors reflect radiation. That's why white roofs stay cooler in summer. But for conduction and convection, color doesn't matter.
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