Ohm's Law Calculator
Solve for Voltage, Current, or Resistance. Enter circuit values to calculate the selected electrical quantity.
Mode
Known Values
Electronics guide · Electrical power, energy, and wiring
About Ohm's Law Calculator
Ohm's law relates voltage, current, and resistance in a simple electrical model. This calculator uses known circuit values to find the remaining quantity and may show related power. It is useful for basic DC or resistive calculations, not every real circuit.
Power and energy calculations relate quantities such as voltage, current, resistance, time, and load. AC and DC systems can require different assumptions, and a calculated estimate is not a substitute for checking source capability, wire ratings, protection, efficiency, and equipment specifications.
How to Use This Tool
- 1.Enter the known voltage, current, or resistance values in the fields provided.
- 2.Check that the entered units are correct and that the values describe the same circuit element or operating point.
- 3.Calculate and review the missing value and any power result. Compare it with component ratings before building a circuit.
Technical Details to Check
1.Check AC or DC assumptions
In AC calculations, voltage and current may refer to RMS values, and phase relationships can affect real power. Do not apply a DC relationship to an AC system unless the tool's stated assumptions fit.
2.Power is not the same as energy
Watts describe a rate of energy use; watt-hours and joules describe an amount of energy over time. A runtime or consumption estimate needs both a load and a duration or duty profile.
3.Real systems lose energy
Inverters, batteries, wiring, motors, and power supplies are not perfectly efficient. Confirm whether the selected calculation includes efficiency, power factor, voltage drop, or only ideal values.
4.Ratings and protection are essential
An estimated current or power value does not determine a safe wire size, fuse, battery connection, or supply. Follow applicable standards, product ratings, and qualified design guidance.
How the Calculation Works
Ohm's law is V=I×R. Rearranging gives I=V/R and R=V/I. Electrical power for a resistive DC case can be found with P=V×I. The simple relationships assume compatible units and an appropriate linear resistive model.
Worked Example
A 12 V source across a 6 Ω resistor gives I=12/6=2 A. The resistor would dissipate 24 W under this ideal calculation, so its power rating matters.
Where This Tool Can Help
Check elementary circuit values, estimate current or voltage in resistive paths, and understand relationships before selecting components.
Estimating a load
Use known voltage and current or power figures to understand a device's approximate demand before checking its label and startup behavior.
Planning energy use
Combine a power estimate with realistic operating hours to compare possible consumption, while accounting separately for standby use and efficiency.
Checking a wiring estimate
Use current or voltage-drop results as an early check, then verify conductor size and protection with the correct installation rules and environment.
Comparing supply options
Compare candidate sources on the same voltage, current, power, and duration basis, and include headroom where the equipment specification requires it.
Tips for More Reliable Results
- Confirm whether voltage and current are RMS, peak, nominal, or measured under load.
- Keep watts, watt-hours, joules, and time units distinct when entering values.
- Include efficiency or power factor only when the tool provides that input and you have a suitable value.
- Check conductor, connector, battery, and supply ratings before using an estimate in a real installation.
Limitations and Assumptions
Many components are nonlinear, and AC circuits need impedance and phase rather than resistance alone. This calculation does not include wire losses, transients, heat, or safety margins. Use a suitable circuit model and ratings.
Electrical power tools estimate values from entered figures and stated assumptions. They do not measure a circuit, model every transient or loss, or certify wiring and equipment as safe. Confirm important designs with manufacturer data and applicable electrical requirements.
Frequently Asked Questions
What is the basic Ohm's law formula?
Voltage equals current multiplied by resistance: V=I×R.
Can this solve an AC circuit with a capacitor?
Not accurately using resistance alone; AC analysis may require impedance and phase.
Does the result tell me which resistor wattage to buy?
It estimates ideal power; select a rated part with an appropriate margin and thermal conditions.
Are watts and watt-hours interchangeable?
No. Watts are a rate; watt-hours measure energy used over time.
Does a calculated value make a circuit safe to build?
No. Safety depends on ratings, protection, installation conditions, and applicable standards as well as the arithmetic.