Use this Ohm's law calculator to find voltage, current, resistance, or power in seconds. Enter any two known values, and the tool solves for the other two. No signup required.
Ohm's law describes the relationship between voltage (V), current (I), and resistance (R) in a circuit. It is one of the most practical formulas in electronics and electrical engineering. Whether you are sizing a resistor for an LED, checking a circuit's current draw, or estimating power dissipated by a component, this calculator gives you a fast, reliable answer.
How this Ohms law calculator tool works
The calculator needs exactly two inputs. Pick any two electrical values you already know:
- Voltage (V) in volts
- Current (I) in amps
- Resistance (R) in ohms
- Power (P) in watts
Enter those two values, then hit calculate. The tool applies the Ohm's law formula and the power formula together to solve for the remaining values.
For example, if you know voltage and resistance, the calculator returns current and power. If you know current and power, it returns voltage and resistance. Every combination of two inputs works.
The Ohm's law formula and equation
Ohm's law states that the voltage across a conductor is directly proportional to the current flowing through it, as long as resistance stays constant. Georg Ohm published this relationship in 1827, and it remains the foundation of circuit analysis.
Mathematically:
V = I × R
Where:
- V = voltage in volts
- I = current in amps
- R = resistance in ohms
Rearrange the equation to solve for any single variable:
- I = V / R (find current)
- R = V / I (find resistance)
That single equation covers most basic DC circuit questions.
Voltage, current, and resistance: how they relate
Think of voltage as electrical pressure pushing current through a circuit. Resistance is what opposes that flow. Increase voltage while resistance stays the same, and current goes up. Increase resistance while voltage stays the same, and current goes down.
This is what "directly proportional" means in practice. Voltage and current move in the same direction when resistance is constant. Current and resistance move in opposite directions when voltage is constant.
Understanding this relationship helps you predict what happens when you change one component in a circuit. It also makes troubleshooting faster, because a measured value that seems off usually points to a specific problem.
Calculate power using the power formula
Power measures how much energy a circuit element uses per second. The unit is the watt (W).
The basic power formula is:
P = V × I
Combine this with Ohm's law and you get two more useful variations:
- P = I² × R (useful when you know current and resistance)
- P = V² / R (useful when you know voltage and resistance)
Power dissipated matters when selecting components. A resistor rated for 0.25 W will overheat if the actual power through it is 0.5 W. This calculator shows the power value alongside voltage, current, and resistance so you can check component ratings immediately.
Calculate voltage, resistance, or current with the calculator
Here is how to use the tool step by step:
- Identify which two values you already know (from a datasheet, measurement, or design spec).
- Enter those two values in the matching fields.
- Leave the other fields empty.
- Click Calculate.
- Read the solved values.
How do you calculate resistance using Ohm's law by hand? Divide voltage by current: R = V / I. The calculator does the same thing, plus it also returns power.
How do you calculate voltage drop? Multiply current by resistance: V = I × R. Enter the current flowing through the component and its resistance, and the calculator returns the voltage across it.
If you need to change units (milliamps to amps, kilohms to ohms), convert before entering values. The calculator expects base units: volts, amps, ohms, and watts.
Ohm's law calculation examples
LED resistor calculation
You want to power a standard red LED from a 9 V battery. The LED has a forward voltage of 2 V and a recommended current of 20 mA (0.02 A).
First, find the voltage the resistor must drop: 9 V − 2 V = 7 V.
Now apply Ohm's law:
R = V / I = 7 / 0.02 = 350 Ω
The nearest standard resistor value is 360 Ω or 390 Ω. Either works safely.
Enter 7 V and 0.02 A into the calculator and it returns 350 Ω along with the power dissipated (0.14 W). A common 0.25 W resistor handles that comfortably.
Circuit voltage calculation
You measure 0.5 A through a 24 Ω resistor and need to know the voltage across it.
V = I × R = 0.5 × 24 = 12 V
What about a question like "What is 20 amps with 12 ohms?" Same formula:
V = 20 × 12 = 240 V
And power in that case: P = 20 × 240 = 4,800 W. That is a significant load, which highlights why checking power is just as important as checking voltage or current.
Ohm's law for AC circuits
Ohm's law in its basic form (V = I × R) applies directly to purely resistive circuits, whether DC or AC. A heating element or incandescent bulb, for example, behaves the same way on AC as on DC (using RMS values).
When a circuit includes capacitors or inductors, simple resistance is replaced by impedance (Z), which accounts for how those components react to alternating current. The generalized form becomes:
V = I × Z
Impedance has both a resistive part and a reactive part, and it changes with frequency. This calculator solves for resistive circuits. If your AC circuit is purely resistive, the results are accurate using RMS voltage and RMS current. For circuits with significant capacitive or inductive elements, you need an impedance calculator that factors in frequency and reactance.
When this Ohm's law calculator is most useful
Electrical engineering and electronics
Engineers and hobbyists reach for Ohm's law constantly:
- Sizing resistors for voltage dividers
- Checking whether a wire gauge can handle expected current
- Estimating power dissipated by a component to choose the right wattage rating
- Verifying measurements against expected values during troubleshooting
Students learning electrical circuits use the calculator to check homework and build intuition. Practicing with real numbers is one of the fastest ways to internalize the relationship between voltage, current, and resistance.
Understanding resistors in a circuit
A resistor limits current flow. Its value in ohms tells you how much it opposes current for a given voltage. Higher resistance means less current at the same voltage.
In practical circuit design, you rarely use just one formula. You might calculate current through a resistor, then check the power it dissipates, then verify the voltage drop is what you expected. This tool handles all of that in a single step because it returns every value at once.
Does Ohm's law apply to semiconductors? Not in the simple linear way. Diodes, transistors, and other semiconductor devices have nonlinear characteristics. Current through a diode does not scale proportionally with voltage. Ohm's law still works for the resistive portions of a semiconductor circuit (like a current limiting resistor in series with an LED), but the semiconductor itself requires its own models.
Limitations of any Ohm's law calculator
This calculator provides estimates for ideal, linear, resistive components. Keep these limitations in mind:
- Temperature effects. Resistance in real conductors changes with temperature. A wire that measures 10 Ω at room temperature may read differently at 80°C.
- Nonlinear devices. Components like diodes, transistors, and varistors do not follow Ohm's law. Their resistance changes with applied voltage or current.
- AC circuits with reactance. Circuits containing inductors or capacitors need impedance calculations, not simple resistance.
- Component tolerances. A resistor labeled 100 Ω with a 5% tolerance could be anywhere from 95 Ω to 105 Ω.
- Real world wiring. Wire resistance, contact resistance, and connector losses add up in long runs or high current applications.
Use results from this Ohm's law calculator as a planning aid and a fast check. For critical electrical work, verify with measured values and consult a qualified electrical engineer.