Watt Calculator: Watts, Volts, Amps and Ohms
Enter any two of the four electrical quantities - power (W), voltage (V), current (A), or resistance (R) - and this calculator solves for the remaining two using Ohm's Law and Watt's Law. Switch between DC and AC single-phase mode. Each input accepts SI prefixes from microwatts to megawatts. Results update instantly with a step-by-step breakdown of the math.
Formula
Worked example
A 120 V household circuit powers a 1200 W hair dryer. Current = P / V = 1200 / 120 = 10 A. Resistance = V / I = 120 / 10 = 12 ohms. Cross-check: P = V x I = 120 x 10 = 1200 W. As an AC appliance with power factor 1.0, apparent power equals real power at 1200 VA.
Ohm's Law and Watt's Law explained
Two fundamental equations govern every DC electrical circuit. Ohm's Law states that voltage equals current multiplied by resistance: V = I x R. Named after physicist Georg Simon Ohm, it tells you how voltage, current and resistance relate in a resistive circuit. Watt's Law states that power equals voltage multiplied by current: P = V x I. Named after James Watt, it connects power to the other two quantities. Combining both laws produces twelve equivalent formulas - any single unknown can be found from any two knowns. In AC circuits a third factor, the power factor (PF), appears because reactive loads (motors, transformers) store and return energy each cycle; real power becomes P = V x I x PF, and the apparent power (volt-amperes) is P / PF.
How to use this calculator
Select 'Solve for' to choose which quantity you want to find. Then choose which two quantities you know using the 'Known values' selector. Enter those two values with their SI prefix (microwatts, millivolts, kiloamperes, etc.). For AC circuits, switch the circuit type to 'AC single-phase' and enter the power factor (0 for purely reactive, 1 for purely resistive). The calculator instantly returns all four quantities - power, voltage, current and resistance - plus a kilowatt reading, apparent power (AC), and the exact formula it applied. The 'Show your work' panel reproduces every arithmetic step with your actual numbers.
Reading your result - practical thresholds
Milliwatt and microwatt results are typical for sensors, microcontrollers and signal-level circuits. 1-100 W covers lighting (LED bulbs are 8-20 W, incandescent 40-100 W), laptop chargers and small appliances. 100-1000 W spans desktop computers, kitchen appliances and power tools. 1-10 kW covers electric ovens, EV chargers and small industrial motors. Residential breakers in North America are typically 15 A or 20 A on 120 V circuits (1800 W or 2400 W), and 30-50 A on 240 V circuits (7200-12000 W). European 230 V circuits are similarly rated. If your calculated current exceeds the breaker rating for a circuit, the load must be split or upgraded.
Power factor and AC apparent power
In an AC circuit, resistive loads (heaters, incandescent bulbs) have a power factor of 1 - all current drawn does useful work. Inductive loads (motors, transformers, fluorescent ballasts) and capacitive loads store energy each half-cycle, so the current waveform is phase-shifted from the voltage. The ratio of useful (real) power to total (apparent) power is the power factor. A PF of 0.8 means 20 percent of the current the supply must provide produces no useful work. Utility companies penalise large industrial customers with poor power factor because it wastes distribution capacity. This calculator's AC mode lets you enter PF to see both real power and apparent power (VA).
Ohm's Law and Watt's Law - all 12 formulas
| Solve for | Known: V + I | Known: V + R | Known: I + R |
|---|---|---|---|
| Power (P) | P = V × I | P = V² / R | P = I² × R |
| Voltage (V) | V = P / I | V = sqrt(P × R) | V = I × R |
| Current (I) | I = P / V | I = V / R | I = sqrt(P / R) |
| Resistance (R) | R = V / I | R = V² / P | R = P / I² |
Any quantity can be derived from two known values. P = power (W), V = voltage (V), I = current (A), R = resistance (ohms).
Frequently asked questions
How do I calculate watts from volts and amps?
Multiply voltage by current: W = V x A. For example, a device drawing 5 A on a 12 V supply consumes 5 x 12 = 60 W. In an AC circuit with a power factor less than 1, multiply by the power factor too: W = V x A x PF.
How do I calculate amps from watts and volts?
Divide power by voltage: A = W / V. A 100 W bulb on a 120 V circuit draws 100 / 120 = 0.833 A. On a 240 V circuit the same 100 W draws only 0.417 A, which is why high-voltage distribution loses less energy in the wires.
How do I calculate volts from watts and amps?
Divide power by current: V = W / A. A solar panel delivering 300 W at 8.33 A operates at 300 / 8.33 = 36 V. This is the standard way to find the operating voltage of a source or load when you know its power and current rating.
What is the difference between watts and volt-amperes?
Watts (W) measure real power - the energy doing useful work per second. Volt-amperes (VA) measure apparent power - the total power the supply must deliver, including the reactive portion that is returned each cycle. They are equal for purely resistive loads (PF = 1). For inductive or capacitive loads PF is less than 1, so VA is larger than W. UPS units and generators are often rated in VA rather than W because their output capacity depends on the total current, not just the real power.
How do I convert watts to kilowatts?
Divide by 1000: 1500 W = 1.5 kW. Kilowatts are the standard billing unit; your electricity meter measures kilowatt-hours (kWh), which is power in kW multiplied by time in hours. A 1.5 kW heater running for 4 hours uses 6 kWh.
What does power factor mean in Ohm's Law?
Power factor (PF) is a number from 0 to 1 that describes what fraction of the apparent power in an AC circuit is real power. It only appears in AC circuits - Ohm's Law and Watt's Law in DC are PF = 1 by definition. In AC: real power (W) = apparent power (VA) x PF. A motor with PF = 0.85 means 85 percent of the current it draws does useful mechanical work; the remaining 15 percent oscillates back and forth as reactive power.
Why does resistance affect how much power a device uses?
From P = V² / R, a lower resistance draws more power at a given voltage, and from P = I² x R, a higher resistance dissipates more power at a given current. These two facts seem to contradict each other but they apply in different circuit configurations: constant voltage (parallel circuits) and constant current (series circuits) respectively. In most household wiring, load resistance determines how much current is drawn from a fixed-voltage supply.