q=−λΔxΔT

q is the heat flux (W/m²).
λ is the thermal conductivity of the material (W/m·K).
ΔT is the temperature difference (T1−T2T1−T2) (K).
Δx is the distance between the two surfaces (m).

Fourier’s Law

  T1 (Hot)      Δx       T2 (Cold)
  ┌────────────┐──────────┐
  │            │          │
  │            │          │
  │            │          │
  └────────────┘──────────┘
      ↑ q                       ↑ q
  Heat Flux (q)              Heat Flux (q)
  
  q = -λ (ΔT / Δx)