Enter the test voltage (volts) and the test current (amps) into the Calculator. The calculator will evaluate Rth (electrical resistance in ohms). 

Rth Calculator

Ohm’s Law (R)
Thevenin Rth
Thermal Rθ

Rth Formula

Rth = Vtest/Itest

Variables:

  • Rth is the equivalent resistance (Thevenin resistance) (ohms, Ω)
  • Vtest is the test voltage (volts)
  • Itest is the test current (amps)

To calculate Rth, divide the test voltage by the test current. (For Thevenin resistance, a common “test” pair is Vtest = Vth (open-circuit voltage) and Itest = Isc (short-circuit current).)

How to Calculate Rth?

The following steps outline how to calculate the Rth.

  1. First, determine the test voltage (volts). 
  2. Next, determine the test current (amps). 
  3. Next, gather the formula from above = Rth = Vtest/Itest.
  4. Finally, calculate the Rth.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

test voltage (volts) = 75

test current (amps) = 100

FAQ

What is Rth in electrical terms?

Rth most commonly refers to Thevenin resistance: the equivalent resistance “seen” looking into a two-terminal network from its output terminals (in ohms, Ω). In thermal calculations, thermal resistance is typically written as Rθ (theta) and has units of °C/W (or K/W).

Why is it important to calculate Rth in electrical circuits?

Thevenin resistance helps you simplify a circuit to a Thevenin equivalent, predict how the terminal voltage changes with different loads, and analyze power transfer and source impedance effects.

Can Rth vary with temperature?

Yes. Since Rth is based on the resistances of the components in the circuit, and many components’ electrical resistance varies with temperature (for example, most metal conductors increase in resistance as temperature rises), the resulting Rth can change with temperature.

How does test voltage and test current affect the calculation of Rth?

Rth is computed from the ratio of a voltage to a current: Rth = Vtest / Itest. In Thevenin terms, this is often calculated as Rth = Vth / Isc, where Vth is the open-circuit voltage and Isc is the short-circuit current.