Heat Transfer Calculator (Q = mcΔT)

Heat transfer is the movement of thermal energy from a hotter region to a cooler one. It occurs through three mechanisms: conduction (through a solid material), convection (through a fluid), and radiation (through electromagnetic waves). The calculator focuses on conductive heat transfer through walls, insulation, and building materials.

Fourier’s Law of Heat Conduction: Q = k × A × ΔT / L Q/t = (A × ΔT) / R_total

What each variable means:

• Q: heat transferred (Joules or BTU)
• k: thermal conductivity of the material (W/m·K or BTU·in/hr·ft²·°F)
• A: cross-sectional area perpendicular to heat flow (m² or ft²)
• ΔT: temperature difference between hot and cold sides (°C or °F)
• L: thickness of the material (meters or inches)
• R_total: total thermal resistance (R-value); for layered materials, R-values add: R_total = R₁ + R₂ + R₃ + …
• R-value: L / k; higher R = better insulation; US units: hr·ft²·°F/BTU

Worked example: An exterior wall (10 ft × 8 ft = 80 ft²) consists of:

• Exterior sheathing: R-3
• Fiberglass batt insulation: R-19
• Drywall: R-0.5 Total R = 22.5

Indoor temperature: 70°F. Outdoor: 25°F. ΔT = 45°F.

Heat loss = A × ΔT / R = 80 × 45 / 22.5 = 160 BTU/hour

Over 24 hours: 160 × 24 = 3,840 BTU/day lost through this one wall.

Thermal conductivity reference (k values):

• Copper: 385 W/m·K (excellent conductor)
• Steel: 50 W/m·K
• Concrete: 1.7 W/m·K
• Glass: 1.0 W/m·K
• Wood: 0.1–0.2 W/m·K
• Fiberglass insulation: 0.04 W/m·K
• Air gap: 0.025 W/m·K (surprisingly good insulator)

Practical use: Doubling wall thickness doubles R-value and halves heat loss. Spray foam (R-6 per inch) outperforms fiberglass batts (R-3.5 per inch), making it preferable where space is limited.