Voltage, current, resistance, and power
Voltage, current, and resistance satisfy V = IR. Power is the rate of electrical energy transfer, expressed in watts. The results use positive magnitudes and do not describe current direction or polarity.
To estimate operating hours from usable battery energy and load power, use the Battery Runtime Calculator .
The formula
The selected pair supplies two independent positive values. The third quantity follows Ohm’s law and power is calculated from voltage times current. Inputs use base units: volts, amperes, and ohms; convert milliamperes or kilo-ohms before entering them.
Worked example
A 12 V source across a 6 Ω ohmic resistance gives current 2 A and power 24 W. The same result follows from 2 A and 6 Ω, or from 12 V and 2 A.
How to use this calculator
- Select the two quantities you know.
- Enter positive values in the labeled base units; 250 mA is 0.25 A and 2 kΩ is 2,000 Ω.
- Calculate and compare the resulting power with your circuit’s actual operating conditions.
Ohmic behavior is an assumption
An ohmic component has a proportional voltage-current relationship at the conditions being modeled. Real resistance can change with temperature. Diodes, transistors, batteries, and many other devices do not follow a single fixed-resistance model across all operating points. This calculator solves the stated relation; it does not identify the type of component.
Power is different from energy
Watts describe how quickly energy is transferred. Watt-hours describe an accumulated amount over time. A 24 W load running for two hours uses 48 Wh under a constant-power assumption. Use the electricity-cost or battery-runtime tools when duration, energy prices, or usable battery capacity matter to the next calculation.
Direct current and alternating current
The displayed power formula suits a DC resistive model. AC circuits can require RMS quantities, impedance, phase angle, and power factor. Entering a peak voltage as though it were RMS produces a different result. The tool does not choose component ratings, wire sizes, protective devices, or a safe installation design.
Assumptions & limitations
What this calculation assumes
- Positive steady-state magnitudes in an ohmic DC model.
What to keep in mind
- Zero resistance or current is excluded; no AC impedance, power factor, non-ohmic behavior, or component-rating assessment.
Common questions
Why must resistance be greater than zero?
With known voltage, current calculation divides by resistance. A zero-resistance ideal short circuit is outside this finite model.
Can I enter milliamperes directly?
Convert them to amperes first. Divide milliamperes by 1,000, so 500 mA becomes 0.5 A.