Ohm's Law Calculator
Solve voltage, current, resistance and power by entering any two known electrical values, with optional energy-use and cost estimates.
- Enter any two of V, A, Ω or W
- Solves the other electrical values automatically
- Optional runtime, kWh and energy-cost estimate
Calculator workspace
Enter the known values, calculate, and review the supporting figures and formula instead of relying on a single unexplained number.
What this Ohm's Law Calculator does
Ohm’s law links voltage, current and resistance, while the electrical power relationships connect those values to watts. Rather than forcing you to choose a separate formula, this calculator accepts any two positive known values among voltage (V), current (I), resistance (R) and power (P), then solves the remaining two. Optional runtime and electricity price fields extend the result into energy use and operating cost for a purely resistive or DC-equivalent load.
The workspace is designed for quick checking as well as repeatable planning. Inputs remain editable, the supporting figures are shown with the headline result, and the calculation runs locally in the browser. That makes it easy to change one assumption at a time, compare scenarios and copy or save a result together with the values that explain it.
Formula and calculation method
V = I×R; P = V×I = I²R = V²/R; energy (kWh) = power (kW) × hours.
The core relationship is V = I×R. Power is P = V×I, which can be rearranged to P = I²R and P = V²/R. The calculator detects which two values you supplied and chooses the appropriate algebraic form. For example, V and R produce I = V/R and P = V²/R; R and P produce I = √(P/R) and V = √(PR). Energy is power in kilowatts multiplied by runtime in hours.
How to use the calculator
- Choose the calculation mode, shape, unit or known-value combination when the page provides one.
- Enter values that belong to the same problem or measurement scenario and keep units consistent with the field labels.
- Select Calculate and review both the main answer and the supporting metrics shown with it.
- Change one assumption at a time when comparing scenarios; this makes the effect of each input easier to understand.
- Use Copy Result, Share Result or Save Result Image when you need a portable record of the calculation.
Worked example
With 12 V and 2 A entered, the calculator finds R = 6 Ω and P = 24 W. At five hours of runtime, that is 0.12 kWh. If the entered energy price is 0.18 per kWh, the theoretical energy cost is about 0.0216 in the selected currency.
How to interpret the result
These equations are exact for ideal DC circuits and purely resistive loads. In real AC circuits, reactive components, power factor, waveform distortion and phase relationships can make the simple P = VI relationship incomplete. Use measured RMS quantities and an appropriate AC power method when working with motors, transformers, capacitors, inductors or switch-mode equipment.
A calculator result is only as useful as the values entered. For measurements, use appropriately precise source data; for costs, use prices that match the same currency and period; for technical work, compare the output with drawings, equipment data or other authoritative information. The supporting metrics on this page are included specifically to make cross-checking easier.
Important assumptions and limitations
The calculator does not check conductor ampacity, component ratings, thermal limits, transient behavior or electrical-code requirements. Energy cost ignores demand charges, tiered tariffs, power-factor penalties and standby behavior. Electrical work can be hazardous; calculations should be verified against equipment data and applicable standards before implementation.
This calculator is provided for planning, checking, education and general informational use. It makes the formula and assumptions visible, but it does not replace project specifications, professional engineering, financial or medical advice, contractual documents, equipment ratings or applicable standards.
Frequently asked questions
Which two values can I enter?
Any two positive values among voltage, current, resistance and power. Leave the other two blank so the calculator knows what to solve.
Does this work for AC circuits?
It can represent simple purely resistive AC loads using compatible RMS values, but it does not model phase angle, reactance or power factor.
How is energy cost calculated?
Solved power is converted to kilowatts and multiplied by runtime in hours, then by the entered price per kWh.