What is the Bi (Biot Number)?
The Biot number compares heat transfer at a body's surface to conduction within it. Bi<0.1 means the body heats almost uniformly (lumped-capacitance valid); larger Bi means strong internal gradients.
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Biot Number Calculator
Compute the Biot number from heat transfer coefficient, characteristic length and solid thermal conductivity — surface dissipation vs internal conduction.
Bi = h·L ÷ kₛ
Frequently asked questions
Can you show a worked example?
h=50, L=0.05, kₛ=50 → Bi = (50·0.05)/50 = 0.05 → below 0.1, lumped-capacitance valid.
Where is this used in real life?
Transient heating/cooling: quenching, food processing and deciding if lumped-capacitance applies.
What are the limits or edge cases?
All inputs must be physically valid; a zero in the denominator (e.g. zero viscosity, velocity or conductivity) is rejected rather than producing infinity. Regime thresholds are standard textbook values and can shift with geometry and conditions.
What decision does the Biot number actually drive?
It tells you whether you can treat an object as a single uniform temperature. If Bi < 0.1, internal conduction keeps up with the surface and the simple lumped-capacitance model is valid. If Bi > 1, the core lags the surface badly and you must model temperature varying through the body.