There are two limiting cases of the Butler–Volmer equation:
* the low overpotential region (called "polarization resistance", i.e., when E ≈ E), where the Butler–Volmer equation simplifies to:
* the high overpotential region, where the Butler–Volmer equation simplifies to the Tafel equation. When , the first term dominates, and when , the second term dominates.
: for a cathodic reaction, when E , or
: for an anodic reaction, when E >> E
where  and  are constants (for a given reaction and temperature) and are called the Tafel equation constants. The theoretical values of the Tafel equation constants are different for the cathodic and anodic processes. However, the Tafel slope  can be defined as:
where  is the faradaic current, expressed as , being  and  the cathodic and anodic partial currents, respectively.