Ohm's Law states that the voltage (V) across a conductor is directly proportional to the current (I) flowing through it, provided temperature remains constant. The formula is V = I × R, where R is resistance in ohms (Ω). It can be rearranged to find current (I = V/R) or resistance (R = V/I). Georg Ohm published this relationship in 1827.

What unit is resistance measured in?

Resistance is measured in ohms, with the symbol Ω (uppercase Greek letter omega), named after Georg Simon Ohm. Common multiples include kilohms (kΩ = 1,000 Ω) and megohms (MΩ = 1,000,000 Ω). One ohm is defined as the resistance that allows one ampere of current to flow when one volt is applied across it.

How do you calculate watts from volts and amps?

Power in watts (W) equals voltage in volts (V) multiplied by current in amps (A): P = V × I. For example, a device drawing 5 A at 240 V uses 240 × 5 = 1,200 W (1.2 kW). You can also calculate power using only resistance: P = I² × R or P = V² / R. These three power formulas are all equivalent via Ohm's Law.

What fuse should I use for a 1200W appliance on 240V?

First calculate the current: I = P/V = 1200/240 = 5 A. UK plug fuses are available in 3 A, 5 A and 13 A ratings. You should select the fuse rated just above the calculated current — so a 5 A fuse for a 1200W appliance at 240V. A 13 A fuse would also protect the wiring but offers less protection to the appliance. Never fit a fuse rated below the calculated current or the fuse will blow under normal operation.

Does Ohm's Law apply to AC circuits?

Ohm's Law applies to purely resistive AC circuits: V = IR holds true where V and I are the RMS (root mean square) values. However, AC circuits often contain capacitors and inductors that introduce reactance (frequency-dependent resistance). In these cases, the generalised form uses impedance Z instead of R: V = IZ, where Z is a complex number combining resistance R and reactance X. For GCSE physics, pure resistors in AC circuits obey Ohm's Law with RMS values.

What is the difference between voltage and current?

Voltage (measured in volts, V) is the electrical potential difference — it is the 'push' or pressure that drives charge through a circuit, analogous to water pressure in a pipe. Current (measured in amperes or amps, A) is the rate of flow of electric charge, analogous to the flow rate of water. Resistance (ohms, Ω) is what opposes the current. A higher voltage with the same resistance produces more current. Ohm's Law (V = IR) describes exactly how these three quantities relate.

Ohm's Law Calculator

Enter any two values below and calculate the remaining voltage, current, resistance and power using Ohm's Law. The calculator only runs when you click calculate.

V = I x R

Power: P = V x I, P = I^2 x R, P = V^2 / R

Voltage (V)

Current (A)

Resistance (Ω)

Power (W)

Enter exactly two values. The calculator will solve the other two and show a quick UK fuse note.

About the Ohm's Law Calculator

Ohm's Law Calculator lets you find voltage, current or resistance in electrical circuits using the fundamental relationship between these quantities.

Formula: V = I × R (Voltage = Current × Resistance)
Voltage (V) in Volts, Current (I) in Amps, Resistance (R) in Ohms (Ω)

Frequently Asked Questions

What is Ohm's Law?

Ohm's Law states that voltage across a conductor equals the current flowing through it multiplied by its resistance: V = I × R. Named after Georg Ohm (1827).

How do I calculate current?

Current (I) = Voltage (V) ÷ Resistance (R). For a 12V circuit with 4Ω resistance: I = 12 ÷ 4 = 3 Amps.

How do I calculate resistance?

Resistance (R) = Voltage (V) ÷ Current (I). If voltage is 240V and current is 2A: R = 240 ÷ 2 = 120 Ohms.

What is power in electrical circuits?

Power (P) = V × I = I² × R = V² ÷ R. Power is measured in Watts. UK mains voltage is 230V AC at 50Hz.

Ohm's Law Reference: Common Household Appliances

Appliance	Voltage (V)	Power (W)	Current (A)
LED bulb	230	10	0.043
Phone charger	230	20	0.087
Laptop	230	65	0.28
Television (50")	230	100	0.43
Microwave	230	800	3.48
Kettle	230	2,000	8.70
Electric shower	230	9,500	41.3
Immersion heater	230	3,000	13.0

What is electrical resistance and how is it measured?

Electrical resistance is the opposition a conductor offers to the flow of electric current. It is measured in Ohms (Ω), named after Georg Simon Ohm. Factors that increase resistance include longer wire length, smaller cross-sectional area, higher temperature (for most metals), and the intrinsic resistivity of the material. Copper has very low resistivity (1.68 × 10⁻⁸ Ω·m), which is why it is widely used for electrical wiring in UK homes and commercial buildings.

How does Ohm's Law apply to UK household circuits?

UK household mains voltage is 230V AC at 50Hz. Using Ohm's Law (I = V/R), a 13-amp fuse protects circuits where total resistance is at least 230 ÷ 13 = 17.7 Ω. UK ring main circuits are rated at 30A for sockets and 6A for lighting circuits. When connecting multiple appliances, their combined power must not exceed the circuit's rated capacity — a 30A circuit at 230V can safely supply 6,900W (6.9 kW) of appliances.

What are resistors in series vs parallel?

In series circuits, resistances add up: R_total = R1 + R2 + R3. Current is the same through all components, but voltage divides across them. In parallel circuits, the reciprocal of total resistance equals the sum of reciprocals: 1/R_total = 1/R1 + 1/R2 + 1/R3.

Voltage is the same across all parallel branches, but current divides. UK house wiring uses parallel circuits so that switching off one appliance does not affect others on the same circuit.

How do I calculate the cost of running an appliance?

First calculate power using Ohm's Law: P = V × I or P = V²/R. Then multiply by hours used to get energy in Watt-hours, divide by 1,000 to get kWh, and multiply by your electricity unit rate. For example, a 2,500W electric heater running 4 hours per day uses 10 kWh/day.

At the UK average rate of approximately 24.5p/kWh (2025 price cap), that costs around £2.45 per day or about £74 per month. Use our electricity cost calculator for a full breakdown.

Understanding Ohm's Law

The Three Key Quantities

Practical Applications