Ohm’s Law Calculator
Enter any two known circuit values to calculate voltage, current, resistance, and power using Ohm’s Law.
Calculator is for informational purposes only. Terms and Conditions
Choose what to solve for
Use auto mode for any two known values, or choose a specific unknown.
Enter the known values
Use positive magnitude values for a simple resistive circuit.
Visual Check
See how voltage, current, resistance, and power relate in a simple resistive circuit.
Solution
Live result, quick checks, warnings, and full solution steps.
Quick checks
- Voltage—
- Current—
- Resistance—
- Power—
Show solution steps See the equation, substitutions, assumptions, and result path
- Enter at least two known values to see the full solution steps and checks.
Source, Standards, and Assumptions
Calculation basis, constants, assumptions, and limitations.
This calculator uses Ohm’s Law and the electrical power relationship for a simple resistive circuit.
- Assumes a resistive load where \( V = I R \) is valid.
- Uses base SI units internally: volts, amperes, ohms, and watts.
- Use impedance \( Z \), not resistance \( R \), for non-resistive AC circuits.
On this page
Calculator Guide
How to Use the Ohm’s Law Calculator
The Ohm’s Law Calculator above solves voltage, current, resistance, and power from any valid two known circuit values. Enter values such as voltage and resistance, current and resistance, or voltage and current, then use the results below to understand the formula, units, power dissipation, and whether the answer looks reasonable for a simple resistive circuit.
Ohm’s Law is most useful for resistors, simple DC circuits, and resistive loads where resistance stays reasonably constant. It is also a starting point for electronics checks such as current draw, resistor sizing, and power dissipation.
Quick Answer
Ohm’s Law is \(V=IR\), where voltage equals current multiplied by resistance. If voltage and resistance are known, calculate current with \(I=V/R\). If voltage and current are known, calculate resistance with \(R=V/I\). Electrical power can be calculated with \(P=VI\), \(P=I^2R\), or \(P=V^2/R\).
Do not rely on a simplified Ohm’s Law result when…
Do not use this simplified calculation alone for LEDs, motors, batteries under heavy load, capacitors, inductors, AC circuits with reactance, or code-based electrical design. Those cases may involve nonlinear behavior, startup current, impedance, heat rise, manufacturer limits, or electrical code requirements.
Ohm’s Law Calculator Inputs and Outputs
The calculator accepts any valid pair of known values and calculates the remaining circuit quantities. The most common workflow is entering voltage and resistance to calculate current and power.
| Type | Value | What It Means | Common Unit |
|---|---|---|---|
| Input or Output | Voltage | Electrical potential difference across the resistor or load. | V, mV, kV |
| Input or Output | Current | Electrical flow through the circuit branch or load. | A, mA, μA |
| Input or Output | Resistance | Opposition to current flow in an ohmic load. | Ω, kΩ, MΩ |
| Input or Output | Power | Electrical energy converted per unit time, often as heat in a resistor. | W, mW, kW |
| Check | Recommended wattage margin | A practical resistor power rating check above the calculated dissipation. | W |
Useful calculator behavior
If you enter more than two values, the values should agree with each other. For example, \(12\,V\), \(20\,mA\), and \(600\,\Omega\) are consistent because \(12/600=0.02\,A\). If one value conflicts, clear the questionable field and solve from the two values you trust most.
Ohm’s Law Formula
The main formula relates voltage, current, and resistance. Power formulas are derived from the same relationship and are needed whenever resistor heating, wattage, or electrical load size matters.
Main Formula
Use \(V=IR\) when current and resistance are known and you want voltage.
Rearranged Forms
Use \(I=V/R\) to calculate current from voltage and resistance. Use \(R=V/I\) to calculate resistance from voltage and current.
Power Formulas
These forms calculate electrical power from different known pairs. Power is critical because it tells you whether a resistor, wire, supply, or load may overheat.
Power-Based Reverse Formulas
These are useful when power is one of the known values, such as checking a load rating or resistor wattage.
Ohm’s Law Formula Wheel Table
A formula wheel is just a shortcut for choosing the right rearranged formula. Use this table when you know which values are available but are not sure which equation to use.
| If You Know | Solve For | Use This Formula |
|---|---|---|
| Current and resistance | Voltage | \(V=IR\) |
| Voltage and resistance | Current | \(I=V/R\) |
| Voltage and current | Resistance | \(R=V/I\) |
| Voltage and current | Power | \(P=VI\) |
| Current and resistance | Power | \(P=I^2R\) |
| Voltage and resistance | Power | \(P=V^2/R\) |
| Power and current | Resistance | \(R=P/I^2\) |
| Power and voltage | Current | \(I=P/V\) |
What the Variables Mean
Each variable has a specific electrical meaning. The formula is simple, but the answer can be wrong by a factor of 1,000 if units are entered incorrectly.
| Symbol | Meaning | How to Enter It |
|---|---|---|
| \(V\) | Voltage across the load or resistor. | Enter in volts, millivolts, or kilovolts. For most electronics examples, volts are common. |
| \(I\) | Current through the circuit branch. | Enter in amperes, milliamperes, or microamperes. Be careful not to enter 20 mA as 20 A. |
| \(R\) | Resistance of the load or resistor. | Enter in ohms, kilo-ohms, or mega-ohms. A 10 kΩ resistor is 10,000 Ω, not 10 Ω. |
| \(P\) | Electrical power dissipated or consumed by the load. | Enter or read in watts, milliwatts, or kilowatts. Resistor heating depends on this value. |
| \(Z\) | Impedance in an AC circuit. | Use impedance instead of simple resistance when inductance, capacitance, or phase angle matters. |
How to Use the Calculator
Use the solve mode that matches the value you want, or leave the calculator in auto mode and enter any two known values. The calculator uses the selected units to convert values internally before solving.
Choose auto mode or a specific solve mode
Auto mode is best for most users. Select voltage, current, resistance, or power only when you want to hide the unknown field and focus on one result.
Enter two known values
Common pairs include voltage and resistance, voltage and current, current and resistance, or power and one other value.
Check the unit selectors
Verify whether the value is in \(A\), \(mA\), \(Ω\), \(kΩ\), \(W\), or \(mW\). Unit mistakes are the most common source of incorrect Ohm’s Law results.
Review power and warnings
Do not stop at current or resistance. Check the power result to see whether the load, resistor, or power supply rating is reasonable.
Positive magnitude note
The calculator uses positive magnitude values for simple resistive checks. In full circuit analysis, negative signs may describe current direction or voltage polarity rather than an invalid physical value.
How to Interpret the Result
The result tells you both the electrical relationship and the practical stress on the circuit. Current affects wiring and supply size, while power affects heat and component rating.
| Result Pattern | What It May Mean | What to Check Next |
|---|---|---|
| Very small current | Resistance may be high, voltage may be low, or the circuit may be a signal/reference branch. | Check whether the current should be in μA or mA. |
| Very large current | Resistance may be too low for the voltage source. | Check power supply rating, wiring, fusing, and heat. |
| High power | The load or resistor may dissipate significant heat. | Check resistor wattage, enclosure temperature, airflow, and derating. |
| Very low resistance | Small contact, wire, or source resistance may affect the result. | Confirm measurement method and conductor resistance. |
| Conflicting values | More than two entered values do not agree with \(V=IR\). | Clear one input or correct the units. |
What to do with the result
Use the current result to check source and wire capacity. Use the resistance result to select or verify a load. Use the power result to check heating, resistor wattage, power supply size, and whether a safety margin is needed.
What changes the result most?
Current is directly proportional to voltage and inversely proportional to resistance. Doubling voltage doubles current if resistance stays constant. Doubling resistance cuts current in half if voltage stays constant. Power can change even faster because \(P=V^2/R\) and \(P=I^2R\).
Quick sanity check
For a 12 V circuit with a 600 Ω resistor, current should be around \(20\,mA\). If the result shows 20 A, the unit was probably entered incorrectly. For electronics work, this type of 1,000× error usually comes from mixing up \(A\) and \(mA\) or \(Ω\) and \(kΩ\).
Input Quality Checklist
Before trusting the output, verify the values represent the same circuit condition and the same branch of the circuit.
Use the right branch
Ohm’s Law applies across a specific resistor or load. Do not mix total circuit voltage with current from a different branch.
Check prefixes
Confirm \(mA\), \(A\), \(Ω\), \(kΩ\), \(mW\), and \(W\). Prefix errors are the fastest way to get a believable but wrong answer.
Use positive magnitudes
For this simplified calculator, enter positive magnitude values. Direction and polarity are circuit-analysis details handled separately.
Confirm the load is resistive
Resistors are straightforward. LEDs, motors, capacitors, inductors, and lamps may not behave like fixed resistors.
Step-by-Step Worked Example
The most common Ohm’s Law calculator use case is finding current and power from a known voltage source and resistor value.
Calculate Current
Substitute Values
Calculate Power
Result
Current: \(20\,mA\). Power: \(0.24\,W\). A resistor rated at \(0.25\,W\) would be very close to the calculated dissipation, so a \(0.5\,W\) or larger resistor is often a better first-pass choice before manufacturer derating and heat review.
Why this result is reasonable
A 600 Ω resistor on a 12 V source should draw a modest electronics-level current. The power is not huge, but it is high enough that resistor wattage matters. This is exactly why the power result should not be ignored.
Quick Mini Examples
These short examples target common solve-for questions that come up when using Ohm’s Law by hand.
Calculate voltage from current and resistance
Calculate resistance from voltage and current
Calculate power from voltage and resistance
Ohm’s Law Circuit Relationship
In a simple resistive circuit, voltage pushes current through resistance. The same current that flows through the resistor creates power dissipation in the load.
Voltage \(V\)
Voltage is the electrical potential difference across the load. More voltage pushes more current through the same resistance.
Resistance \(R\)
Resistance limits current. For a fixed voltage, higher resistance means lower current.
Power \(P\)
Power shows the energy conversion rate. In a resistor, this usually appears as heat.
Conceptual relationship
Voltage source → pushes current → through resistance → producing power dissipation.
| Change | Effect on Current | Effect on Power |
|---|---|---|
| Increase voltage while resistance stays fixed | Current increases. | Power increases by the square of voltage using \(P=V^2/R\). |
| Increase resistance while voltage stays fixed | Current decreases. | Power decreases using \(P=V^2/R\). |
| Increase current while resistance stays fixed | Current is the direct input. | Power increases by the square of current using \(P=I^2R\). |
Typical Reference Values
Ohm’s Law applies over a wide range, but typical values differ greatly between microelectronics, hobby electronics, and power circuits.
| Application | Common Voltage | Common Current | Common Resistance Range |
|---|---|---|---|
| Small signal electronics | millivolts to a few volts | μA to mA | kΩ to MΩ |
| Hobby DC circuits | 3.3 V, 5 V, 9 V, 12 V | mA to a few A | Ω to kΩ |
| Power resistors and loads | 12 V, 24 V, 48 V, or higher | A-level currents possible | fractions of an Ω to hundreds of Ω |
| High-voltage work | 50 V and above may be hazardous | depends on source and load | requires qualified safety review |
Common Resistor Wattage Checks
Resistor wattage ratings are not a guarantee that a resistor will run cool in every enclosure or ambient temperature. Use these only as first-pass checks before manufacturer derating.
| Calculated Power | First-Pass Rating Check | Practical Note |
|---|---|---|
| About \(0.05\,W\) | \(0.125\,W\) may be acceptable in many small-signal uses. | Still check tolerance, ambient temperature, and component package. |
| About \(0.12\,W\) | \(0.25\,W\) is commonly safer than \(0.125\,W\). | Useful when a basic 2× margin is desired. |
| About \(0.24\,W\) | \(0.5\,W\) is often a better first-pass choice than \(0.25\,W\). | A \(0.25\,W\) part would be near its nominal rating. |
| \(1\,W\) or more | Use a power resistor and check derating carefully. | Spacing, airflow, surface temperature, and mounting matter. |
Safety reference point
Treat voltage and current hazards seriously. The exact safety threshold depends on conditions, body contact, energy source, environment, and applicable standards. This calculator does not determine whether a circuit is safe to touch or work on.
Design Ranges and Practical Checks
A mathematically correct result is not always enough for a reliable design. Heat, tolerance, supply limits, wiring, and component ratings often matter as much as the formula.
Low-Power Check
If power is only a few milliwatts, heating is usually minor, but precision, leakage, and noise may matter.
Resistor Wattage Check
If calculated power is near the resistor rating, choose a higher wattage or review derating, airflow, and temperature rise.
High-Current Check
If current is several amps or more, review wire size, connector rating, fuse sizing, power supply capacity, and heat.
Practical resistor margin
A common first-pass approach is to choose a resistor rated above the calculated power, often with a margin such as 2×. Final selection should still account for ambient temperature, enclosure conditions, mounting, pulse loads, and manufacturer derating curves.
Resistor tolerance matters
A \(600\,\Omega\) resistor with ±5% tolerance may actually be between \(570\,\Omega\) and \(630\,\Omega\). In a 12 V circuit, that range changes the current and power slightly, so precision circuits should account for tolerance rather than assuming the printed value is exact.
Unit Conversion Notes
Unit errors are the biggest source of wrong Ohm’s Law answers. Always confirm the unit selector before interpreting the result.
| Quantity | Conversion | Common Mistake |
|---|---|---|
| Current | \(1\,A=1000\,mA=1{,}000{,}000\,\mu A\) | Entering 20 mA as 20 A. |
| Resistance | \(1\,k\Omega=1000\,\Omega\), \(1\,M\Omega=1{,}000{,}000\,\Omega\) | Entering 10 kΩ as 10 Ω. |
| Power | \(1\,W=1000\,mW\), \(1\,kW=1000\,W\) | Reading 0.25 W as 0.25 mW. |
| Voltage | \(1\,V=1000\,mV\), \(1\,kV=1000\,V\) | Using mV when the source is actually in volts. |
Ohm’s Law vs. Power Law vs. Impedance
Ohm’s Law is the core relationship for voltage, current, and resistance. Power formulas add heat and energy-rate checks. Impedance is the broader AC concept used when capacitance, inductance, and phase angle matter.
| Method | Formula | Best For | Main Caution |
|---|---|---|---|
| Ohm’s Law | \(V=IR\) | Resistors and simple resistive loads. | Assumes resistance is reasonably constant. |
| Power Law | \(P=VI\) | Heat, wattage, and load power checks. | Power rating needs derating and temperature review. |
| AC Impedance | \(V=IZ\) | AC circuits with inductors, capacitors, and phase angle. | Resistance alone is not enough when reactance matters. |
| Voltage Drop | Depends on conductor resistance and current. | Wire runs, feeders, branch circuits, and long conductors. | Conductor size, length, temperature, and code rules matter. |
When Ohm’s Law Is Not Enough
Ohm’s Law is accurate for ideal resistive relationships, but many real devices are not fixed resistors. In those cases, \(V=IR\) may still be useful for one part of the circuit, but it should not be treated as the full device model.
LEDs
LEDs are nonlinear devices with a forward voltage and target current. Use Ohm’s Law for the series resistor, not for the LED as if it were a fixed resistor.
Motors
Motors can have startup current, back EMF, changing load, and winding heating effects that a simple resistance model does not capture.
Bulbs and heating elements
Some loads change resistance as they heat. A cold resistance measurement may not match operating resistance.
Capacitors and inductors
Reactive components require time-domain or frequency-domain analysis. Use impedance \(Z\), not just resistance \(R\).
LED Resistor Note
For a basic LED circuit, subtract the LED forward voltage from the supply voltage, then use Ohm’s Law to size the series resistor.
In this formula, \(V_f\) is the LED forward voltage and \(I\) is the target LED current. Final LED design should still check the LED datasheet, resistor power, current limits, and temperature.
AC circuit note
For a purely resistive AC load, Ohm’s Law can be applied using RMS voltage and RMS current. For circuits with capacitance or inductance, use impedance \(Z\) instead of resistance \(R\).
Common Mistakes That Cause Wrong Results
Most incorrect Ohm’s Law results come from wrong units, wrong circuit assumptions, or ignoring the power result.
Common Mistakes
- Entering milliamps as amps.
- Entering kilo-ohms as ohms.
- Using total circuit voltage across only one component.
- Ignoring resistor wattage after calculating current.
- Applying simple resistance formulas to LEDs, motors, capacitors, or inductors without additional checks.
Better Practice
- Match every value with the correct unit selector.
- Use the voltage across the exact load being analyzed.
- Check current and power together.
- Use a wattage margin for resistors and heat-producing loads.
- Use impedance or device-specific analysis when the load is not a simple resistor.
Troubleshooting Unexpected Results
If the result looks impossible or much larger than expected, check units first. A unit prefix mistake can make the result wrong by 1,000× or more.
| Problem | Likely Cause | Fix |
|---|---|---|
| Current is far too high | Resistance entered too low, kΩ entered as Ω, or voltage too high for the load. | Check resistance units and confirm the load is correct. |
| Power is much higher than expected | Current or voltage may be entered in the wrong unit, or resistance may be too low. | Check \(P=VI\), then verify component wattage. |
| Resistance result seems impossible | Current may be nearly zero, or the measured values may be from different circuit states. | Confirm current measurement and use values from the same operating condition. |
| Entered values conflict | Three or four entered values do not satisfy \(V=IR\) and \(P=VI\). | Clear one value and solve from the two most reliable measurements. |
| Measured current differs from calculated current | The load may not be a fixed resistor, the supply voltage may sag, resistor tolerance may be wide, or the meter may be measuring a different branch. | Measure voltage across the load under operating conditions and check the actual resistance and tolerance. |
| Real circuit does not match calculation | The load may be nonlinear, temperature-dependent, reactive, or changing over time. | Use a more detailed model or measured operating data. |
Assumptions, Sources, and Limitations
This calculator is intended for educational use, quick checks, and preliminary engineering estimates for simple resistive circuits.
Formula Assumption
The calculation assumes an ohmic load where resistance is constant and \(V=IR\) is a valid approximation.
Unit Assumption
Values are converted to base SI units internally: volts, amperes, ohms, and watts.
Application Limit
The calculator does not model transient behavior, semiconductor curves, motor startup current, battery sag, impedance, or temperature-dependent resistance.
Final Design Note
For final electrical design, verify conductor size, overcurrent protection, equipment ratings, heat rise, enclosure conditions, manufacturer data, and applicable code requirements.
Calculation basis
The formulas on this page use the standard voltage-current-resistance relationship \(V=IR\) and the power relationship \(P=VI\). For additional educational background on voltage, current, and resistance, see the All About Circuits explanation of voltage, current, and resistance.
Glossary of Terms
These terms help connect the calculator result to the physical circuit.
Voltage
Electrical potential difference across a component or load, measured in volts.
Current
The flow of electric charge through a circuit branch, measured in amperes.
Resistance
Opposition to current flow in a resistive element, measured in ohms.
Power Dissipation
The rate at which electrical energy is converted, often into heat, measured in watts.
Ohmic Load
A load whose voltage-current relationship is approximately linear, so resistance stays reasonably constant.
Impedance
The AC opposition to current that includes resistance and reactance, usually written as \(Z\).
Frequently Asked Questions
What does the Ohm’s Law Calculator calculate?
The calculator calculates voltage, current, resistance, and power when any valid two circuit values are known. It is most useful for simple resistive circuits and resistor/load checks.
What is the basic Ohm’s Law formula?
The basic formula is \(V=IR\), where \(V\) is voltage, \(I\) is current, and \(R\) is resistance.
How do I calculate current from voltage and resistance?
Use \(I=V/R\). For example, \(12\,V\) across \(600\,\Omega\) gives \(I=12/600=0.02\,A\), or \(20\,mA\).
How do I calculate resistor power?
Use \(P=VI\) if voltage and current are known, \(P=I^2R\) if current and resistance are known, or \(P=V^2/R\) if voltage and resistance are known.
Does Ohm’s Law work for LEDs?
Not by itself. LEDs are nonlinear devices with a forward voltage and current rating. Ohm’s Law is commonly used to size the series resistor after the LED forward voltage and target current are known.
Can I use this calculator for AC circuits?
For purely resistive AC loads, you can use RMS voltage and RMS current. For AC circuits with capacitors, inductors, or phase angle, use impedance \(Z\) instead of simple resistance \(R\).