Enter new numbers and see the remaining output value change. Floating point format ("1.1E-6") works; engineering units ("1.1u", etc.) do not.

Note that the units are simply ratios, so their actual units do not matter (as long as the same units are used for all steps). They're labeled in V and Ω for convenience.

This calculation arises when you want to find a Thevenin equivalent source, and cannot perform the trivial test (open circuit voltage, short circuit current; the resistance is simply the ratio). Reasons might include: the source is nonlinear and exhibits a different equivalent when shorted, or cannot sustain a shorted load; or you don't have a current probe handy to test a very low resistance. Here, a load resistor is connected to the source, or the source is left open. The open voltage is given (V_{2} = V_{1}, when open circuit), and the loaded voltage is V_{2L}. R_{s} is solved using the voltage divider equation.

When the open-circuit test is not available either, this method has to be used. An existing load (known, not removable) produces some known output voltage V_{2}. A known resistor is connected in parallel with the existing load resistor, giving a loaded output voltage V_{2L}. Examples might include: testing a power supply's regulation without having to unplug or adjust many resistors; determining the output impedance of an amplifier driving a fixed termination resistance; etc.