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

Rth Formula

Rth = Vtest/Itest


  • Rth is the Rth (ohms)
  • Vtest is the test voltage (volts)
  • Itest is the test current (amps)

To calculate Rth, divide the test voltage by the rest 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