Ohm’s Law Triangle Explained with Units and Worked Examples

Read the triangle as a memory aid

The Ohm's law triangle places voltage V above current I and resistance R. Covering V leaves I and R beside each other, suggesting multiplication: V = IR. Covering I leaves V above R, suggesting I = V/R. Covering R similarly gives R = V/I. This arrangement is a convenient way to remember three rearrangements of the same equation. It is not a geometric relationship involving angles or side lengths, and it does not supply a physical model for every electrical component that happens to have a voltage and current.

       V
      ───
     I × R

V = I × R
I = V ÷ R
R = V ÷ I

Connect the picture to ordinary algebra

Starting from V = IR, divide both sides by nonzero R to obtain I = V/R. Divide by nonzero I to obtain R = V/I. The divisions explain the conditions that the picture alone can hide. If the denominator is zero, covering a letter in the triangle does not make the resulting division meaningful. You can use the memory aid when learning, but practise the algebra as well. Rearranging a formula correctly becomes especially important when later circuit equations contain several terms, powers or more than three quantities.

Use multiplication when voltage is unknown

Suppose a current of 25 mA flows through a 220 Ω resistor. Convert 25 mA to 0.025 A, then use V = 0.025 × 220 = 5.5 V. The units also check: amperes multiplied by ohms give volts. In the linked calculator, select Voltage and enter current 0.025 and resistance 220. Covering the top V in the diagram reminds you to multiply, but it does not remind you to divide the milliampere value by 1000; that conversion must still be made explicitly.

Use division when current or resistance is unknown

With 9 V across 1.5 kΩ, first write resistance as 1500 Ω. Current is 9/1500 = 0.006 A, equivalent to 6 mA. For a different example with voltage 3.3 V and current 11 mA, resistance is 3.3/0.011 = 300 Ω. The numerical division looks similar in both cases, so label the unknown and the denominator before calculating. The local form changes its input labels with the selected mode. Read those labels rather than assuming that the same pair of numbers has the same meaning after changing modes.

Use units to catch an inverted fraction

Resistance has units V/A, while conductance has units A/V. If the task asks for resistance but you calculate I/V, the result is conductance, the reciprocal of resistance for the simple model. For the 300 Ω example, conductance is 1/300 S, approximately 0.003333 S. A unit check catches the inversion even if the resulting decimal seems plausible. Similarly, voltage divided by resistance has units A, not Ω. Write the desired output unit beside the unknown before using the triangle to choose an operation.

Keep signs and zero values outside the mnemonic

Using a consistent passive convention, −12 V divided by −3 A gives +4 Ω. The two negative signs describe the chosen reference directions and cancel in the resistance ratio. The triangle does not decide those directions for you. A current of zero with a finite positive resistance gives zero voltage, while zero voltage divided by a positive resistance gives zero current. However, zero voltage divided by zero current cannot determine a resistance. The local calculator rejects zero divisors and negative passive resistance, and accepts consistent signed voltage and current cases.

Recognize what the triangle cannot answer

The triangle does not calculate power, energy, component temperature, alternating-current impedance or the equivalent resistance of an arbitrary network. Power requires another relationship, such as P = VI; at 5.5 V and 0.025 A, the value is 0.1375 W. It also does not establish that resistance remains constant when operating conditions change. Use it as a prompt for a supported Ohm's law calculation, then check units, the component model and any additional requirements separately. The calculator returns one selected V, I or R value, while the reasoning about the circuit stays part of your solution.

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