Convert between watts, amps, volts, and ohms using Ohm's law with UK 230V mains as default
Understanding the relationship between watts, volts, amps, and ohms is fundamental to electrical work, whether you are wiring a plug, sizing a circuit breaker, or simply trying to work out how much power an appliance draws. These four quantities are linked by Ohm's law and the power equation, and knowing any two allows you to calculate the others. In the UK, mains electricity runs at a nominal 230V and 50Hz. This is the standard voltage used in homes and most commercial premises. Knowing this, you can quickly calculate the power consumption of any device if you know its current draw in amps, or determine the current if you know the wattage. This is particularly useful when checking whether a circuit can handle additional appliances, or when selecting the correct fuse or circuit breaker rating. This calculator supports three calculation modes. The most common is volts and amps (P = V x I), which directly gives you the power in watts. If you know the current and resistance, you can use P = I squared x R. And if you know the voltage and resistance, you can use P = V squared / R. All three modes are derived from the same fundamental laws and will give consistent results.
To calculate watts and other electrical values: 1. Select your calculation mode from the dropdown. Choose "Volts and Amps" if you know the supply voltage and current draw, "Amps and Ohms" if you know the current and resistance, or "Volts and Ohms" if you know the voltage and resistance. 2. Enter the values for your chosen mode. The default voltage is set to 230V (UK mains). For other applications such as car electrics (12V), USB (5V), or industrial three-phase supplies, change the voltage accordingly. 3. The calculator will compute the power in both watts and kilowatts, plus any missing values (voltage or current) depending on the mode selected. 4. Use the results to check circuit capacity, verify fuse ratings, or estimate energy consumption. For example, if an appliance draws 1,150W on a 230V supply, it needs a current of 5A, so a 5A or 13A fuse would be appropriate. 5. Compare common scenarios using the preset buttons to see how different voltage and current combinations affect power output.
The watt calculator uses three formulas derived from Ohm's law (V = I x R) and the power equation (P = V x I): Mode 1, Volts and Amps: Power (W) = Voltage (V) x Current (A). This is the most common calculation. For UK mains at 230V with 5A of current: P = 230 x 5 = 1,150W. Mode 2, Amps and Ohms: Power (W) = Current (A) squared x Resistance (ohms). This also derives voltage using V = I x R. For 10A through 50 ohms: P = 100 x 50 = 5,000W, and V = 10 x 50 = 500V. Mode 3, Volts and Ohms: Power (W) = Voltage (V) squared / Resistance (ohms). This also derives current using I = V / R. For 230V across 46 ohms: P = 52,900 / 46 = 1,150W, and I = 230 / 46 = 5A. Kilowatts are simply watts divided by 1,000. All results are rounded to appropriate decimal places for practical use.
Inputs: Mode: Volts and Amps. Voltage: 230V. Current: 13A.
Inputs: Mode: Volts and Amps. Voltage: 230V. Current needs to be found from known wattage of 100W.
Inputs: Mode: Volts and Ohms. Voltage: 12V. Resistance: 4 ohms.