Ohm's LawCalculator

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across those points and inversely proportional to the resistance. Expressed as V = I × R (voltage equals current multiplied by resistance). It was formulated by German physicist Georg Ohm in 1827 and forms the foundation of all electrical circuit analysis. Every electrician uses it daily to verify circuit designs, check test results, and diagnose faults.

Ohm's Law underpins every calculation an electrician makes on site. When you measure a voltage drop under load, you can calculate the circuit resistance. When you know the load resistance and supply voltage, you can predict the current — and therefore check whether the protective device will operate. It is also the basis for volt drop calculations under BS 7671 Appendix 4, where maximum permissible volt drop for lighting is 3% and for other circuits is 5% of the nominal supply voltage.

Power factor is the ratio of real power (kW) to apparent power (kVA) in an AC circuit containing reactive components such as motors, transformers, or capacitors. A purely resistive load (like a heater or incandescent bulb) has a power factor of 1.0 — all apparent power is real power. Ohm's Law as shown in this calculator applies to DC circuits and purely resistive AC loads. For inductive or capacitive AC loads you need impedance (Z = V/I) rather than resistance, which adds a phase angle component.

Resistance (Ω) is the opposition to current flow in DC circuits and purely resistive AC circuits. Impedance (Z, also in Ω) is the total opposition in AC circuits and includes resistance (R), inductive reactance (XL = 2πfL) and capacitive reactance (XC = 1/(2πfC)). For a motor, transformer, or any inductive load the impedance will be higher than the measured DC resistance. For fault calculations under BS 7671, electricians use impedance figures from IEE Wiring Regulations Appendix 14 rather than DC resistance.

UK mains supply is 230V AC single-phase (between live and neutral) and 400V AC three-phase (between any two line conductors, formerly called 415V before harmonisation in 1995). Single-phase is used for domestic supplies and most light commercial loads. Three-phase is used for industrial machinery, commercial premises, EV charge points above 7kW, and any load over approximately 7–10kW where balanced loading is important. The phase-to-neutral voltage on three-phase is also 230V — the 400V is the line-to-line voltage, which is 230 × √3.

Set your multimeter to the resistance (Ω) function. Ensure the circuit is ISOLATED — dead and proved dead with an approved voltage indicator before connecting. Connect the test leads across the component or cable run you want to measure. For cable continuity, short the far end and measure from the near end; for earth loop path resistance, use a dedicated loop impedance tester rather than a standard multimeter. When measuring in-circuit, be aware that parallel paths will give a lower reading than the true component resistance. Always zero the leads first by touching them together and noting the residual lead resistance.

The most common mistakes are: (1) measuring voltage at the board rather than at the load, so the meter sees supply voltage not the actual volt drop; (2) forgetting that volt drop compounds over a long circuit — each metre of cable adds resistance; (3) using the wrong temperature coefficient — cable resistance increases approximately 0.4% per °C above 20°C, so a cable at 70°C operating temperature has about 20% more resistance than its cold value; (4) ignoring the return path — volt drop is calculated over the full circuit length (go and return), not just the outgoing cable.