Biot Number Calculator
Calculate the Biot number to determine whether temperature gradients within a body are significant during heat transfer.
How to Use This Calculator
Enter h (W/m²·K)
Input the convective heat transfer coefficient based on your scenario.
Enter Lc (m)
Provide the characteristic length. For a slab, Lc is typically thickness/2.
Enter k (W/m·K)
Use the material's thermal conductivity.
Calculate
Click Calculate to get Bi. Bi ≪ 0.1 suggests lumped capacitance is valid.
Formula
Bi = (h × Lc) / k
Where: h is W/m²·K, Lc is m, k is W/m·K.
Example: h = 50, Lc = 0.02, k = 200 → Bi = 0.005
About Biot Number
The Biot number compares internal conductive resistance to external convective resistance. Small Bi indicates uniform internal temperature during transient heat transfer.
When to Use
- Assessing lumped capacitance validity
- Estimating transient heating/cooling behavior
- Material and geometry selection in thermal design
Why Use This Tool?
- Instant, accurate Biot number results
- Clear inputs with SI units
- Mobile friendly and free
Frequently Asked Questions
What does a small Biot number mean?
It indicates internal temperature gradients are negligible; lumped analysis is appropriate.
What are typical values of h?
Natural convection: 5–25 W/m²·K; forced air: 25–250; liquids: 100–10,000.
How do I choose Lc?
Often volume/surface area; for a plane wall, thickness/2; for a cylinder, radius/2.