Enter any two values to calculate the rest
How This Calculator Works
Enter any two of the four values (voltage, current, resistance, or power), and the calculator will compute the other two. Change the unit dropdown to convert between scales (e.g., milliamperes to amperes, watts to horsepower).
Watts, Volts, Amps, and Ohms Explained
- Voltage (V) is the pressure pushing electricity through a circuit. Think of it as water pressure
in a pipe. Measured in volts. - Current (I) is the flow rate of electricity. Think of it as how much charge passes a point per
second. Measured in amps. - Resistance (R) is how much a material opposes that flow. Think of it like a narrow pipe
restricting water. Measured in ohms (Ω). - Power (P) is the rate at which energy is used or delivered. The end
result of pressure and flow working together. Measured in watts.
These four are locked together by two fundamental laws. If you know any two, the other two are fixed.
Ohm’s Law
Ohm’s Law describes the relationship between voltage, current, and resistance:
V = I × RVoltage equals current times resistance. If you increase the resistance in a circuit while keeping the voltage constant, less current flows. If you increase the voltage while keeping the resistance the same, more current flows.
The Power Equation
The power equation ties power to voltage and current:
P = V × IPower equals voltage times current. A 48V electric motor drawing 10A consumes 480W. An LED bulb running at 120V and 0.1A uses 12W.
All 12 Formulas
| Find | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| Voltage (V) | V = I × R | V = P ÷ I | V = √(P × R) |
| Current (I) | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) |
| Resistance (R) | R = V ÷ I | R = V² ÷ P | R = P ÷ I² |
| Power (P) | P = V × I | P = V² ÷ R | P = I² × R |
Examples
- Household – A standard power outlet in the US runs 120V. A microwave rated at 1,200W draws 10A of current (1200 ÷ 120). A 60W incandescent bulb at 120V draws just 0.5A and has a resistance of 240Ω.
- Electric vehicles – A typical electric scooter has a 500W motor running on a 48V battery, drawing about 10.4A at full load. High-performance dual-motor scooters can pull 5,400W (about 7.2 HP) from a 60V system. That’s 90A of current. EV cars operate on much higher voltages (400–800V) to keep current manageable at power levels of 150kW or more.
- Electronics – A USB charger outputs 5V at up to 2.4A (12W max). USB-C can deliver 20V at 5A for 100W, which is enough to charge a laptop. A typical smartphone draws about 0.5A from a 3.7V battery and uses under 2W during normal use.
Need to convert between Wh and Ah? Use the Battery Capacity Calculator.
A watt (W) is a unit of power that measures the rate of energy transfer. One watt equals one volt times one amp (1W = 1V × 1A). It’s named after James Watt, the Scottish inventor.
Divide watts by 745.7 to get mechanical horsepower. For example, a 3,000W electric bike motor is about 4 HP (3000 ÷ 745.7). Use the power unit dropdown above to convert automatically.
Watts measure instantaneous power (how fast energy is being used right now).
Watt-hours (Wh) measure total energy (how much was used over time). A 100W motor running for 2 hours uses 200Wh.