Voltage, current and resistance in a simple circuit are related by one equation. Knowing any two determines the third — and from there, the power.
Key terms
- Voltage (V) — the electrical “push”, in volts.
- Current (I) — the flow of charge, in amps.
- Resistance (R) — how much the circuit opposes that flow, in ohms.
- Power (P) — the rate of energy use, in watts.
How it works
Ohm’s law and power
V = I R · P = V I
Pick which one is unknown; supply the other two. Power follows automatically from whichever pair of V, I and R you have.
Why this is a “solve for” calculator, not a converter
Voltage, current and resistance are not the same quantity measured differently, the way a length in inches and the same length in centimetres are. They are three different physical quantities linked by one equation — so typing a voltage does not “convert” to a current the way a unit conversion would; it only produces a current once a resistance is also known. That is why this calculator asks which one is unknown rather than filling every field from a single typed value.
How to use this calculator
- Choose which quantity you want solved — voltage, current or resistance.
- Enter the other two.
- Read the solved value and the resulting power together.
Frequently asked questions
What happens if resistance is zero?
Solving for current with zero resistance is declined here rather than shown as an infinite current — a genuine short circuit does draw a very large current in practice, limited by real-world factors this simple equation does not model (wire resistance, source limits), which is exactly why “infinite” is not a meaningful answer to display.
How is power related to voltage and current?
P = V × I always. It can also be written as I²R or V²/R using Ohm’s law to substitute for whichever variable is missing, but all three forms describe the same power.
Does Ohm’s law apply to every electrical component?
No — it describes “ohmic” components, where resistance stays constant regardless of voltage or current. Many real components (diodes, transistors, light bulbs at high temperature) are non-ohmic, meaning their resistance changes with the conditions, and this simple linear equation no longer applies cleanly.
Why is current measured in amps and not some other unit?
The ampere is one of the seven SI base units, defined via the elementary charge since the 2019 SI redefinition — it is a fundamental unit of the system, not derived from volts and ohms the way this calculator’s third value is derived from the other two.
What’s a typical household voltage and resistance?
Household mains run at roughly 120 V (US) or 230 V (UK/EU); a typical incandescent bulb’s resistance at operating temperature might be in the tens to low hundreds of ohms range, though it varies by wattage and design.
