Ohm's Law Calculator

Last updated: Jul 22, 2026 | Reviewed by Calkulator Team | 2 min read

Science

Solve voltage, current, and resistance using Ohm's law

V = I × R. A 12V battery driving 2A through a circuit means the resistance is 6Ω. Whether you are designing circuits, troubleshooting electronics, or studying for exams — Ohm's law is foundational.

Power (in watts) = V × I = I²R = V²/R — all four forms are useful depending on what values you know.

Solve For

Voltage (V)

Ohm's Law Triangle

V = I × R
I = V / R
R = V / I
P = V × I = I² × R = V² / R

All Values

Voltage

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Current

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Resistance

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Power

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Live Result Illustration

Visual summary — updates instantly as you enter values above

LIVE

Real-Life Guide to Using the Ohm's Law Calculator

Voltage, current, resistance. Use the examples and checks below to turn the number into a practical decision.

When this calculator is useful

Used when wiring or troubleshooting a circuit — sizing a current-limiting resistor, checking how much current a component will draw, or working out an unknown resistance from a measured voltage and current.

For most people, the best way to use the Ohm's Law Calculator is to try the real case first, then change one input at a time. That makes the trade-off visible. For example, with a loan calculator you can change tenure while keeping the same rate; with an investment calculator you can change return assumption while keeping the same monthly contribution; with a health, education or measurement calculator you can check how much one input changes the final category.

The result should answer a practical question: Can I afford this? How much should I save? Is this score enough? Is this measurement within range? What is the safer or cheaper option? If the output does not answer the decision clearly, adjust the inputs until the scenario matches your real situation.

Real-Life Example

Sizing an LED resistor: A hobbyist wants to know how much current will flow through a resistor connected across a 9V battery.

1With V = 9 V and R = 470 Ω: I = V / R = 9 / 470 ≈ 0.0191 A, or about 19.1 mA.

2Now change one input, such as rate, time, quantity, unit or score, and compare the new result with the first one.

At 19.1 mA the circuit is right at the edge of a typical LED's 20 mA rating, so bumping the resistor up to 560 Ω or higher gives a safer margin.

Practical Advice

Use the Ohm's Law Calculator as a planning tool, not just a number generator. Write down the inputs you used, because the final answer is meaningful only when you remember the assumptions behind it.

If the decision affects money, health, tax, safety, academics or legal compliance, keep a second check ready. That second check may be a bank quote, payslip, official rule, prescription, site measurement, mark sheet or invoice.

Common Mistakes

Leaving resistance in kilo-ohms while expecting the current answer in amps — a 4.7 kΩ resistor entered as '4.7' instead of '4700' understates the resistance by 1000x.
Confusing V = IR with the power formula P = IV, and typing a wattage value into the voltage field.
Ignoring the internal resistance of a battery or source, which matters when precise current values are needed for sensitive components.
Applying the calculator's simple V = IR relationship to an AC circuit with reactance, where impedance rather than plain resistance governs current flow.
Losing track of which two of the three values (V, I, R) are actually known, and solving for the wrong quantity.

How to Interpret Results

Compare the computed current against the safe operating current of your component — for instance, a standard LED is typically rated around 20 mA, so a result near or above that signals you need a larger resistor.

A good interpretation looks at both the main result and the supporting values. If a page shows totals, ratios, categories, schedules or warnings, read those together instead of focusing only on the biggest number.

Ohm's Law Calculator FAQs

Useful answers for interpreting the output, avoiding mistakes and using the result responsibly.

What relationship does Ohm's law describe?

It states V = I × R — voltage equals current multiplied by resistance — and the calculator rearranges this to solve for whichever of the three values you leave blank.

How do I handle resistor values given in kilo-ohms or mega-ohms?

Convert to plain ohms before entering: 4.7 kΩ is 4700 Ω, and 1 MΩ is 1,000,000 Ω. Skipping this conversion is the single most common source of wildly wrong current values.

Can I use this for multiple resistors in a circuit?

Only after first combining them into a single equivalent resistance — add resistances directly for a series circuit, or use the reciprocal sum for a parallel circuit, then plug that total into the calculator.

How do I find power once I have voltage and current?

Multiply them together: P = V × I. For the LED example above, 9 V × 0.0191 A ≈ 0.172 W dissipated across the whole circuit.

Does this work for AC circuits?

It works for pure resistive AC loads using RMS voltage and current, but circuits with capacitors or inductors involve impedance and phase shift, which plain Ohm's law does not capture.

What does a resistance of zero imply?

Zero resistance in an otherwise powered circuit represents a short circuit, where current would theoretically become infinite — in practice this trips a fuse or damages the source.

Why is this calculator useful for electronics work?

It is the starting point for nearly every circuit design decision, from choosing resistor values for LEDs and sensors to estimating current draw on a battery-powered project.

What should I check next after finding the current?

Verify it against your component's datasheet rating, and if you are unsure of the exact resistor to use, cross-reference with the resistor color code calculator to confirm the part you have on hand.

Ohm's Law Explained

Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to the resistance: V = I × R. It's the foundational relationship in electronics, named after German physicist Georg Ohm (1827).

The power formula P = V × I extends Ohm's Law. Combining with V = IR gives P = I²R (for heating calculations, like how much heat a resistor dissipates) and P = V²/R (for power loss in wires). These are used in circuit design, electrical safety, and energy efficiency calculations.

Example

Car battery: 12V, 60W headlight:

1I = P/V = 60/12 = 5 A

2R = V/I = 12/5 = 2.4 Ω

✓ Two headlights draw 10A from a 12V battery

Frequently Asked Questions

What is Ohm's Law?

Ohm's Law states that the voltage (V) across a resistor equals the current (I) flowing through it multiplied by its resistance (R): V = IR. It applies to linear components in DC circuits and as an approximation for AC circuits. Most conductive materials follow Ohm's Law over a wide range of voltages and temperatures.

What is the difference between AC and DC resistance?

In DC circuits, resistance (R, in ohms) is the only opposition to current flow. In AC circuits, the total opposition is impedance (Z), which combines resistance, capacitive reactance, and inductive reactance. Ohm's Law generalises to V = IZ for AC. Impedance changes with frequency; resistance does not.

How do I calculate the resistor needed for an LED circuit?

Use Ohm's Law: R = (Vsupply − VLED) ÷ ILED. Example: 5V supply, red LED with 2V forward voltage and 20 mA current: R = (5 − 2) ÷ 0.02 = 150 ohms. Always use the nearest standard resistor value at or above the calculated value to protect the LED from over-current damage.