Heat Transfer Rate Calculator

• Conduction (Fourier): Q = k · A · (ΔT / L)
• Convection (Newton): Q = h · A · ΔT
• Thermal radiation: Q = ε · σ · A · (T1⁴ − T2⁴), where σ = 5.670374419×10⁻⁸ W/(m²·K⁴)
• Heat exchanger (LMTD): Q = U · A · ΔT_lm, and ΔT_lm = (ΔT1 − ΔT2) / ln(ΔT1/ΔT2)

Sign matters: a negative Q indicates heat leaving the surface toward the surroundings in the chosen direction.

1. Select a heat transfer model that matches your situation.
2. Enter the heat transfer area and choose the correct units.
3. Provide the required inputs for the selected model.
4. Press Calculate to view results above the form.
5. Use the CSV or PDF buttons to export the calculation.

1) What the heat transfer rate represents

Heat transfer rate Q is energy flow per time, reported in watts (W). A positive result indicates heat moving in the direction implied by your temperature inputs. In design work, Q connects thermal resistances, surface area, and the driving temperature difference. Typical engineering calculations target steady conditions, where temperatures do not change rapidly with time.

2) Conduction through solids

Conduction is common in walls, plates, pipes, and insulation layers. The calculator uses Q = k·A·(ΔT/L), where k is thermal conductivity. Metals often show high conductivity (aluminum ≈ 205 W/(m·K), stainless steel ≈ 15 W/(m·K)), while insulation can be 0.02–0.05 W/(m·K). Reducing thickness L or increasing area increases Q linearly.

3) Convection at fluid–surface boundaries

Convection is modeled with Q = h·A·ΔT. The coefficient h depends strongly on flow regime and fluid properties. Natural convection in air is often 2–10 W/(m²·K), forced air may reach 10–100 W/(m²·K), and water convection is commonly 100–10,000 W/(m²·K). Because h is uncertain, engineers often bracket calculations with low and high estimates.

4) Thermal radiation exchange

Radiation uses the Stefan–Boltzmann relation with σ = 5.670374419×10⁻⁸ W/(m²·K⁴). The net term (T1⁴ − T2⁴) can dominate at high temperatures. Emissivity ε ranges from about 0.03–0.10 for polished metals to 0.80–0.95 for painted or oxidized surfaces. Always input absolute temperatures for accurate results.

5) Heat exchanger rate using LMTD

For heat exchangers, the calculator applies Q = U·A·ΔT_lm. The log mean temperature difference uses two end temperature gaps, and it changes with flow arrangement. Counterflow typically provides a larger effective driving force than parallel flow for the same terminal temperatures, improving thermal duty for a given area.

6) Unit handling and reporting

Inputs are converted internally to SI units so models remain consistent. Area converts to m², lengths to meters, and temperature differences to kelvin. Results are provided in W, kW, and Btu/hr for quick comparison with equipment ratings and datasheets. Consistent units reduce hidden errors in multi-step thermal calculations.

7) Interpreting heat flux

The calculator also reports heat flux q'' = Q/A in W/m². Heat flux is useful for surface limits such as boiling, thermal stress, or coating performance. For example, electronics cooling may aim for tens of kW/m², while large-area building envelopes are typically far lower under normal conditions.

8) Practical accuracy tips

Match the model to the dominant mechanism. Many real systems involve combined modes (e.g., conduction through a wall plus convection outside). If you have multiple layers or films, consider using an overall resistance approach to compute an effective U. When uncertain, perform sensitivity checks by varying k, h, and temperatures within realistic ranges.

1) Which model should I choose?

Pick conduction for heat through solids, convection for heat between a surface and moving fluid, radiation for thermal emission to surroundings, and LMTD for exchanger duty using inlet and outlet temperatures.

2) Why can radiation results be negative?

If T2 is higher than T1, the net term (T1⁴ − T2⁴) becomes negative, meaning heat flows from the surroundings to the surface.

3) Do I need absolute temperature for every model?

Absolute temperature is required for radiation because of the fourth-power term. Conduction and convection use temperature difference, so ΔT in °C equals ΔT in K.

4) What if I only know one of the temperatures?

This calculator needs a driving difference: either ΔT for conduction/convection or both temperatures for radiation and LMTD. If one value is unknown, estimate it from measurements, energy balance, or manufacturer data.

5) How do I choose a convection coefficient?

Use published ranges based on fluid and flow type, then run low and high cases. Natural air is usually single digits W/(m²·K), forced air is higher, and water can be hundreds to thousands.

6) Why does LMTD fail when ΔT1 and ΔT2 have opposite signs?

Opposite signs indicate a temperature cross that breaks the standard LMTD assumption. Re-check inlet/outlet assignments, flow arrangement, and whether the exchanger operates with a true temperature crossover.

7) Can I export results without recalculating?

Yes. After you calculate once, the page stores the last result in the session. Use the CSV or PDF buttons to download the saved calculation summary.