How this calculator works
Biot number compares the temperature drop inside a solid with that across its convecting surface. Use the solid conductivity, not the surrounding fluid conductivity. For the common lumped-capacitance screening, characteristic length is volume divided by exposed surface area. Bi below about 0.1 is a practical screening rule, rather than a guarantee for every geometry or accuracy requirement.
Formula
Bi = hLc/ks; Lc = V/As.
Worked example
With h = 10 W/(m² K), Lc = 0.01 m and ks = 2 W/(m K), Bi = 0.05. A body of V = 0.002 m³ and As = 0.2 m² has the same characteristic length.
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Frequently asked questions
Is this the same as Nusselt number?
No. Biot uses solid conductivity to assess internal gradients; Nusselt uses fluid conductivity to characterize convection.
Which surfaces count?
Include the surfaces actually exposed to the convection represented by h.
Can I use the radius directly?
Only if the chosen model defines it that way. For a sphere, V/As equals radius divided by three.