Ohm's Law Calculator

Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. It states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to the resistance. The core formula is V = I × R, where V is voltage in volts, I is current in amps, and R is resistance in ohms.

The Ohm's Law wheel contains 12 rearranged formulas derived from two base relationships (V = I × R and P = V × I). For Voltage: V = I × R, V = P / I, V = √(P × R). For Current: I = V / R, I = P / V, I = √(P / R). For Resistance: R = V / I, R = P / I², R = V² / P. For Power: P = V × I, P = I² × R, P = V² / R.

Ohm's Law in its basic form (V = IR) applies directly to purely resistive AC loads like heating elements, kettles, and incandescent globes. For inductive loads like motors, compressors, and fluorescent ballasts, the relationship becomes more complex because impedance (Z) replaces simple resistance (R), and power factor must be accounted for. Use V = I × Z for AC impedance calculations.

Resistance is measured in ohms, represented by the Greek symbol Ω (omega). Common multiples used in electrical work include kilohms (kΩ, equal to 1,000 Ω) for higher-resistance components and megohms (MΩ, equal to 1,000,000 Ω) for insulation resistance testing. A standard Australian 230V / 10A GPO circuit has an effective load resistance of 23Ω when fully loaded.

Set the multimeter to the Ω (resistance) range. Make sure the circuit is completely de-energised and isolated — never measure resistance on a live circuit. Disconnect at least one end of the component you're testing to avoid parallel paths giving false low readings. Touch the probes across the component and read the display. Compare the reading against the calculated expected value.

OL (Over Limit or Open Loop) means the resistance is higher than the meter's selected range can measure. In practical terms, it usually indicates an open circuit — the conductor is broken, a fuse is blown, or an element has failed. If you're testing a heating element and get OL, the element's internal wire has burnt through and it needs replacing.

Several factors cause discrepancies. The supply voltage at the outlet may differ from the nominal 230V — it can range from 216V to 253V under Australian standards. The component's actual resistance changes with temperature (hot elements have higher resistance than cold ones). For motor loads, the clamp meter reads apparent current including reactive components, while Ohm's Law calculates resistive current only.

Yes, but only for DC resistance or very short AC runs. Conductor resistance depends on the material (copper or aluminium), cross-sectional area (mm²), and route length. For example, 2.5 mm² copper cable has a resistance of approximately 7.41 Ω per kilometre. For longer AC cable runs where voltage drop matters, use the voltage drop calculator which accounts for both resistance and reactance.

Resistance (R) is the opposition to current flow in a DC circuit or a purely resistive AC circuit, measured in ohms. Impedance (Z) is the total opposition to current flow in an AC circuit, also measured in ohms, but it combines resistance (R) and reactance (X) from inductors and capacitors. In a purely resistive circuit, impedance equals resistance. In circuits with coils or capacitors, impedance is always higher than resistance alone.

Use P = I² × R or P = V² / R to find the power dissipated by the resistor in watts. Then select a resistor with a wattage rating at least double the calculated dissipation to provide a safety margin and prevent overheating. A resistor running at its maximum rated wattage gets extremely hot and degrades quickly. Standard through-hole resistors come in 0.25W, 0.5W, 1W, 2W, 5W, and 10W ratings.

Near-zero resistance means the circuit is effectively short-circuited. According to Ohm's Law (I = V / R), as resistance approaches zero, current spikes towards infinity. In practice, the current surges to hundreds or thousands of amps in milliseconds. This extreme current triggers the magnetic trip element in the circuit breaker, disconnecting the supply almost instantly to prevent conductor meltdown and fire.

The base formula V = I × R is the same for both DC and purely resistive AC circuits. The difference arises with reactive AC loads. In DC circuits, resistance is the only opposition. In AC circuits with inductors or capacitors, you must use impedance (Z) instead of resistance (R), giving V = I × Z. Impedance accounts for the phase angle between voltage and current waveforms, which basic Ohm's Law does not.