Soil Thermal Calculator

• Volumetric heat capacity: C = ρ cₚ (J/m³·K)
• Thermal diffusivity: α = k / (ρ cₚ) (m²/s)
• Temperature gradient: dT/L = (T2 − T1)/L (K/m)
• Steady 1D heat flux: q = −k (dT/L) (W/m²)
• Thermal resistance: R = L/k (m²·K/W)
• Moisture estimate (optional): k = k₍dry₎ (1 + b·w), with w = moisture%/100
• Sinusoidal attenuation: A(z)/A(0) = exp(−z·m), m = √(ω/(2α)), ω = 2π/P
• Sinusoidal time lag: tₗₐg = z·√(1/(2αω))

1. Select Use measured conductivity if you have k from testing.
2. Or select Estimate from moisture and enter k₍dry₎, moisture %, and factor b.
3. Enter soil density and specific heat for diffusivity and lag metrics.
4. Set layer thickness L and boundary temperatures T1 and T2 for heat flux.
5. Optional: set depth z and period P to approximate damping and time lag.
6. Press Calculate, then download CSV or PDF if needed.

1) Why soil thermal performance matters

Soil governs heat transfer around foundations, buried services, and slab edges. Higher conductivity drives faster heat loss or gain, while higher volumetric heat capacity moderates temperature swings. For frost protection, heat-traced lines, or energy storage backfill, documenting k, ρ, c_p, and thickness supports consistent design decisions.

2) Typical parameter ranges used in estimates

For preliminary checks, teams often use k ≈ 0.6–1.0 W/m·K for drier sands and k ≈ 1.2–2.0 W/m·K for moist granular soils. Bulk density commonly falls near 1500–2000 kg/m³, while effective specific heat is often 750–1000 J/kg·K. Values vary with grading, compaction, mineralogy, and water content.

3) Worked data example using this calculator

Assume k = 1.50 W/m·K, ρ = 1800 kg/m³, c_p = 850 J/kg·K, and L = 0.50 m. With T1 = 28 °C and T2 = 18 °C, the gradient is −20 K/m and the heat flux is about +30 W/m² toward boundary 1. The tool also reports C ≈ 1,530,000 J/m³·K and α ≈ 9.8×10⁻⁷ m²/s.

4) Diffusivity, attenuation, and time lag

Diffusivity α controls how quickly temperature changes propagate. For a daily cycle (P = 24 h) and α near 1×10⁻⁶ m²/s, temperature amplitude decreases with depth. At z = 1.2 m, the amplitude ratio can drop to a small fraction of the surface swing, and the peak can lag by several hours, informing burial depth decisions.

5) Practical reporting notes

Use measured k where possible, then run a moisture sensitivity scenario to bracket uncertainty. Keep units consistent: k in W/m·K, ρ in kg/m³, c_p in J/kg·K, and L in meters. Export CSV for calculation logs and PDF for submittals, noting boundary temperatures, thickness, and the period used for lag estimates.

1) What does a higher conductivity mean on site?

Higher conductivity moves heat faster through soil, increasing heat loss in cold seasons and heat gain near warm sources. It can raise freezing risk for shallow utilities.

2) Why does moisture increase conductivity in many soils?

Water bridges air gaps between particles. Because air is a poor conductor, replacing air with water typically increases the effective heat-transfer path.

3) What is volumetric heat capacity used for?

C = ρc_p indicates how much energy a soil volume stores per degree change. Higher C buffers temperature swings and affects time-lag predictions.

4) Why is my heat flux negative?

The sign follows q = −k(dT/L). If T2 is warmer than T1, the gradient is positive and q becomes negative, indicating heat flow toward boundary 2.

5) How should I choose the moisture factor b?

Use b as a sensitivity control when lab data is unavailable. Start near 0.7, then run low and high cases (for example 0.3 and 1.5) to bracket outcomes.

6) When should I use 24 hours versus 8760 hours?

Use 24 hours for daily surface swings. Use 8760 hours for seasonal cycles when checking deeper temperature lag and attenuation for long-term ground response.

7) Does this replace a full geothermal or frost analysis?

No. It is a transparent screening and documentation tool. For critical designs, use measured properties, layered models, groundwater effects, and local climate data.