Ohm’s Law Calculator
Solve voltage, current, resistance, or electrical power using Ohm’s Law and the basic DC power relationships.
Calculator is for informational purposes only. Terms and Conditions
Choose what to solve for
Select the unknown quantity, known-value pair, and preferred unit setup.
Enter the known values
Fill in only the visible fields. Hidden fields are ignored for the active solve mode.
Circuit Visual
Use the diagram to connect voltage, current, resistance, and power in a simple circuit.
Solution
Live result, quick checks, warnings, and full solution steps.
Quick checks
- Power check—
- Conductance—
- Energy for 1 hour—
Show solution steps See the equation, substitutions, unit conversions, assumptions, and checks
- Enter values to see the full solution steps and checks.
Source, Standards, and Assumptions
Calculation basis, constants, assumptions, and limitations.
Based on Ohm’s Law and the standard DC power equations. This is an educational circuit calculation, not a code-compliance or component-rating check.
- Assumes steady-state DC or RMS-equivalent values for simple resistive AC loads.
- Assumes an ideal resistor and does not model temperature rise, tolerance, wire losses, transients, or reactive impedance.
- Uses standard unit conversion constants only.
How to Use an Ohm’s Law Calculator
An Ohm’s Law Calculator helps you solve for voltage, current, resistance, and power when at least two electrical values are known. It is most useful for simple DC circuits, resistor checks, electronics troubleshooting, and quick electrical design estimates.
The calculator above is designed around the values users most often need: volts, amps, ohms, and watts. Enter any two known values, choose the units, and the calculator solves the remaining quantities using Ohm’s Law and the electric power equation.
Direct answer
To use Ohm’s Law, enter any two known values from voltage, current, resistance, and power. The calculator converts the units, solves the missing values, and shows the relationship between electrical pressure, charge flow, circuit resistance, and heat-producing power.
Ohm’s Law Formula
Ohm’s Law describes the relationship between voltage, current, and resistance in a resistive electrical circuit. It is usually written as:
Main Ohm’s Law equation
Voltage equals current multiplied by resistance. This form is useful when current and resistance are known.
Rearranged to solve for current
Current equals voltage divided by resistance. This is one of the most common uses of an Ohm’s Law calculator.
Rearranged to solve for resistance
Resistance equals voltage divided by current. This is useful when selecting or checking a resistor value.
The calculator also uses the electrical power relationship:
Power equals voltage multiplied by current. This matters because electrical power becomes heat in resistive components.
What the Variables Mean
A correct Ohm’s Law calculation depends on using the right electrical quantity in the right unit. The table below shows what each value means and what users should enter into the calculator.
| Symbol | Quantity | Base Unit | What It Means |
|---|---|---|---|
| V | Voltage | Volt (V) | Electrical potential difference, or the “push” that drives current through a circuit. |
| I | Current | Ampere (A) | The rate of electric charge flow through a component or circuit path. |
| R | Resistance | Ohm (Ω) | Opposition to current flow. Higher resistance reduces current for the same voltage. |
| P | Power | Watt (W) | The rate electrical energy is used or dissipated, often as heat in a resistor. |
Important unit note
The calculator can display millivolts, volts, milliamps, amps, ohms, kilohms, milliwatts, and watts, but the calculation itself is performed internally using volts, amps, ohms, and watts.
How to Use the Ohm’s Law Calculator
Most users searching for an Ohm’s Law calculator want a fast answer, but the most accurate workflow is to identify which two values are truly known and leave the unknown values blank.
Choose the result focus
Select Auto if you want the calculator to solve the missing values automatically. Choose Voltage, Current, Resistance, or Power if you want one result highlighted.
Enter any two known values
For example, enter voltage and resistance to solve current and power, or enter current and resistance to solve voltage and power.
Select the correct units
Use V, mA, Ω, kΩ, W, or mW as needed. The calculator handles conversions, but the entered value still needs to represent the correct physical quantity.
Review all four outputs
Do not stop at the highlighted answer. Check voltage, current, resistance, and power together to make sure the circuit makes practical sense.
Check power and safety warnings
If the result shows high current, high voltage, or high resistor power, verify component ratings before using the value in a real circuit.
Ohm’s Law Formula Wheel
The formula wheel is a compact way to remember how voltage, current, resistance, and power relate to each other. The calculator uses the same relationships behind the scenes.
| Solve For | Use This Formula | When It Applies |
|---|---|---|
| Voltage | V = I × R | Current and resistance are known |
| Voltage | V = P / I | Power and current are known |
| Voltage | V = √(P × R) | Power and resistance are known |
| Current | I = V / R | Voltage and resistance are known |
| Current | I = P / V | Power and voltage are known |
| Current | I = √(P / R) | Power and resistance are known |
| Resistance | R = V / I | Voltage and current are known |
| Resistance | R = V² / P | Voltage and power are known |
| Resistance | R = P / I² | Power and current are known |
| Power | P = V × I | Voltage and current are known |
| Power | P = V² / R | Voltage and resistance are known |
| Power | P = I² × R | Current and resistance are known |
The most common beginner mistake is memorizing only V = I × R and forgetting that power must also be checked. A circuit can have the correct voltage and resistance but still overheat a resistor if the wattage rating is too low.
Step-by-Step Worked Example
Suppose you have a 12 V source connected across a 220 Ω resistor. You want to find the current and the power dissipated by the resistor.
Step 1: Solve current
Step 2: Convert current to milliamps
Step 3: Solve power
Result
The current is approximately 54.5 mA, and the resistor dissipates approximately 0.654 W.
How to interpret the result
A 1/4 W resistor would be too small for this example because the calculated power is about 0.654 W. A designer would typically select a resistor with a higher wattage rating and suitable safety margin.
Power and Heat Check
Power is one of the most important outputs in an Ohm’s Law calculation because power dissipation becomes heat in resistive components. A circuit may look electrically correct but still fail if the component cannot handle the heat.
Low power
Usually easier to manage, but still verify component ratings for small parts.
Moderate power
Check whether a 1/4 W, 1/2 W, or 1 W resistor is appropriate.
High power
Requires careful review of heat rise, spacing, enclosure temperature, and safety margin.
The calculator’s resistor wattage check is useful because it compares the calculated circuit power to the resistor rating you enter. If the calculated power is higher than the rating, the component is undersized.
How to Size a Resistor with Ohm’s Law
Resistor sizing usually requires two checks: the resistance value and the power rating. The resistance controls current. The wattage rating controls how much power the resistor can dissipate without overheating.
Calculate the required resistance
Use R = V / I when voltage and target current are known.
Calculate power dissipation
Use P = V × I, P = I²R, or P = V²/R to find the expected heat-producing power.
Select a resistor wattage rating
Choose a resistor rated above the calculated power. For many basic checks, a margin of at least 2× is a practical starting point.
Resistor wattage is not optional
If a resistor is rated for 0.25 W and the circuit dissipates 0.65 W, the resistance value may be correct but the component is still not acceptable.
Common Unit Conversions
Users often enter values in milliamps, millivolts, kilohms, or milliwatts. The calculator handles these conversions automatically, but it helps to understand what the units mean.
| Quantity | Conversion | Example |
|---|---|---|
| Millivolts to volts | 1 mV = 0.001 V | 500 mV = 0.5 V |
| Kilovolts to volts | 1 kV = 1000 V | 2 kV = 2000 V |
| Microamps to amps | 1 µA = 0.000001 A | 250 µA = 0.00025 A |
| Milliamps to amps | 1 mA = 0.001 A | 20 mA = 0.02 A |
| Kilohms to ohms | 1 kΩ = 1000 Ω | 4.7 kΩ = 4700 Ω |
| Megohms to ohms | 1 MΩ = 1,000,000 Ω | 2 MΩ = 2,000,000 Ω |
| Milliwatts to watts | 1 mW = 0.001 W | 250 mW = 0.25 W |
| Kilowatts to watts | 1 kW = 1000 W | 3 kW = 3000 W |
Ohm’s Law for DC Circuits vs AC Circuits
This calculator is best for resistive circuits where voltage, current, resistance, and power are directly related. That is usually a DC circuit or an AC circuit where the load can be treated as purely resistive.
Use this calculator for
DC resistor circuits, simple electronics checks, voltage-current-resistance relationships, and resistor power estimates.
Use impedance instead for
AC circuits with capacitors, inductors, motors, transformers, phase angle, reactance, or frequency-dependent behavior.
In AC circuits, resistance may not be enough. You often need impedance, which includes both resistance and reactance. For AC circuit checks, use an Impedance Calculator instead.
Common Ohm’s Law Mistakes
Ohm’s Law is simple, but the wrong assumptions can still produce a misleading answer. These are the most common mistakes to avoid.
Common Don’ts
- Enter milliamps as amps without converting units
- Use Ohm’s Law for non-ohmic devices without checking their behavior
- Ignore resistor wattage and heat dissipation
- Use simple resistance for reactive AC circuits
- Assume calculated current is safe just because the formula works
- Enter more than two values that do not agree with each other
Better Checks
- Use the unit dropdowns carefully
- Check the power result every time
- Use impedance for capacitors, inductors, and AC loads
- Compare calculated values to component ratings
- Use the known pair selector when entering extra values
- Verify the result against the actual circuit conditions
Practical Uses for an Ohm’s Law Calculator
Ohm’s Law is one of the most widely used relationships in basic electrical engineering because it connects the values needed for real troubleshooting and design decisions.
Electronics
Estimate resistor values, LED currents, sensor currents, and small DC load behavior.
Troubleshooting
Check whether measured voltage, current, and resistance are consistent with each other.
Power checks
Estimate watts, heat, and component loading before selecting a part.
If you are learning the fundamentals of circuits, also review Basic Electronic Components to understand how resistors, capacitors, diodes, and other components behave in circuits.
Limitations of Ohm’s Law
Ohm’s Law is powerful, but it is not a complete model for every electrical device. It works best when the component behaves like a stable resistor.
Non-ohmic devices
Diodes, LEDs, transistors, and many semiconductors do not follow a simple fixed resistance relationship.
Temperature changes
Resistance can change as components heat up, especially in lamps, heating elements, and precision applications.
Reactive AC circuits
Inductors and capacitors require impedance, not just resistance, because phase angle and frequency matter.
Real source limits
Batteries and power supplies have internal resistance, current limits, voltage sag, and protection behavior.
Engineering judgment still matters
The calculator provides the mathematical result for an idealized resistive relationship. For real circuits, verify ratings, operating conditions, wiring limits, heat dissipation, and safety requirements.
Frequently Asked Questions
What is an Ohm’s Law Calculator?
An Ohm’s Law Calculator is a tool that solves voltage, current, resistance, and power when at least two values are known. It uses V = I × R and P = V × I.
What two values do I need to use Ohm’s Law?
You need any two related values from voltage, current, resistance, and power. For example, voltage and resistance can be used to calculate current and power.
How do I calculate current with Ohm’s Law?
Use I = V / R. Divide voltage by resistance to calculate current. For example, 12 V divided by 220 Ω equals about 0.0545 A, or 54.5 mA.
How do I calculate resistance?
Use R = V / I. Divide voltage by current to calculate resistance. Make sure current is entered in amps, not milliamps, unless the calculator is handling the unit conversion.
How do I calculate electrical power?
Use P = V × I when voltage and current are known. You can also use P = V² / R or P = I² × R when resistance is part of the known information.
Why does resistor wattage matter?
Resistor wattage matters because electrical power becomes heat. If the calculated power is higher than the resistor’s rating, the resistor can overheat or fail.
Can I use Ohm’s Law for LEDs?
You can use Ohm’s Law to size a series resistor for an LED, but the LED itself is not a simple fixed resistor. You need the LED forward voltage and target current.
Does Ohm’s Law work for AC circuits?
It works for purely resistive AC loads, but many AC circuits require impedance instead of resistance. Capacitors, inductors, motors, and transformers are not fully described by simple resistance.
What is the difference between resistance and impedance?
Resistance opposes current in a simple resistive way. Impedance includes resistance plus reactance from capacitors and inductors, so it depends on frequency and phase angle.
Why are my entered values inconsistent?
If you enter more than two values, they must agree with Ohm’s Law. For example, 12 V and 220 Ω imply about 54.5 mA. If you also enter 100 mA, one of the values is inconsistent.