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Physics Tools · Updated October 21, 2026 · Reviewed by FormatFlow Editorial

Ohm's Law Calculator

You can calculate voltage, current, resistance and power by entering any two of the three Ohm's law quantities, with the formula and the substituted working shown beside the result.

Runs in your browserFree · No sign-up · No watermark
V / I / R → PowerRuns in your browser
Loading the tool…

Everything runs locally in this tab. No file is uploaded while the tool loads.

This tool runs in your tab. Nothing is uploaded and no copy is stored.

How files are handled

At a glance

Works with
V / I / R → Power
Processing
In your browser
Price
Free, no account
Updated
October 21, 2026
All Physics Tools

About Ohm's Law Calculator

Ohm's law connects the three quantities in a simple circuit: voltage pushes, current flows and resistance limits. If you know any two, the third follows immediately, and the power dissipated by the component follows from the same pair. This calculator arranges the equation around whichever pair you have, so you do not have to transpose it by hand.

Enter two values and the third plus power appear, with the substituted working underneath and all four quantities in SI units. Division-by-zero and non-positive-resistance cases are reported in plain language instead of showing Infinity. It is a study and quick-check tool for ideal resistors, not a substitute for electrical safety practice.

What this tool does

The details that matter when you use Ohm's Law Calculator for real work.

  • Any pair solves the circuit

    Enter voltage and current, voltage and resistance, or current and resistance; the missing quantity and the power are calculated for you.

  • Power from the same values

    P = V × I, P = I² × R and P = V² ÷ R are all printed, so the power is available without a second calculation.

  • Working shown

    The formula and your substituted numbers appear under the result, including the division that produces the missing value.

  • Clear error messages

    A zero current or a non-positive resistance is explained inline instead of producing Infinity or a misleading number.

  • SI units throughout

    Voltage in volts, current in amperes, resistance in ohms and power in watts, with sensible significant-digit rounding.

  • Copy the answer

    Copy the solved value as a plain number for a lab report, spreadsheet or homework answer.

How to use Ohm's Law Calculator

No account, no installation — the whole job happens in this browser tab.
  1. 01

    Choose the pair you know

    Select voltage and current, voltage and resistance, or current and resistance.

  2. 02

    Enter the two values

    Type the numbers; decimals are accepted, and negative voltage or current is allowed for direction-aware problems.

  3. 03

    Read the solved value

    The headline shows the missing quantity, with the power shown next to it and in the summary cards.

  4. 04

    Check the working

    The formula panel substitutes your values into the equation so the result can be verified by hand.

  5. 05

    Copy or reset

    Copy the solved value to the clipboard, or press Reset to clear all three fields and start again.

How this tool works

Works with
V / I / R → Power
Processing
Client-side, in your browser. Nothing is uploaded.
Requirements
Chrome, Edge, Firefox or Safari · JavaScript enabled

Limitations worth knowing

  • Assumes an ideal resistor with constant resistance; real components change with temperature and manufacturing tolerance.
  • AC behaviour such as reactance and phase shift is not modelled, so AC use is limited to RMS values on a purely resistive load.
  • Power is reported as a positive magnitude, and the direction of conventional current is not interpreted.

Privacy and file handling

Every calculation runs in your browser with plain JavaScript. The values you enter are never uploaded, stored or logged, and no request is made to a server while you use the calculator.

Read the full privacy policy

Ohm's Law Calculator FAQ

Short, direct answers to the questions people ask before using this tool.
What is the difference between voltage, current and resistance?
Voltage is the potential difference that pushes charge around a circuit, current is the rate at which charge flows, and resistance is how strongly the component opposes that flow. Ohm's law links them as V = I × R, so two values determine the third.
How does this calculator work out power as well as the missing value?
Once voltage and current are known, power follows from P = V × I, and the same power can be written as I² × R or V² ÷ R using Ohm's law. The tool shows all three forms and prints the substituted working for the pair you entered.
Why does a zero current give an error in the voltage-and-current mode?
Resistance is voltage divided by current. If the current is zero, the division has no defined result, so the calculator reports the division by zero instead of showing Infinity. Enter a non-zero current, or switch to a mode that takes resistance directly.
Why must resistance be greater than zero?
The calculator models an ordinary resistive component, where resistance is a positive quantity. A zero or negative value would produce a mathematically valid but physically meaningless current, so it is rejected with an explanation rather than calculated silently.
Does this Ohm's law calculator use RMS or peak values?
It does not convert between them. Enter a consistent set of numbers, such as all RMS values for an AC problem or all DC values for a battery circuit, and the result is in the same convention. Mixing a peak voltage with an RMS current gives a power figure that does not mean anything.
Can I use it for alternating current circuits?
Only for a purely resistive load where you supply RMS values, because reactance and phase shift are not modelled. In AC circuits with capacitors or inductors, impedance replaces resistance and the simple V = I × R relationship no longer holds without complex arithmetic.
Why is the power result always positive?
A resistor dissipates energy whatever direction the current is assumed to flow, so the tool reports power as a positive magnitude. Negative voltage or current values are still accepted for solving the ratio; their sign is not treated as a statement about power flow.
Which units does the Ohm's law calculator expect?
Use SI base units: volts for voltage, amperes for current and ohms for resistance. The result is in watts. Convert milliamps, millivolts or kilohms to amps, volts and ohms before entering them, or write them as decimals such as 0.25 A.

Background reading

Format guides that explain the trade-offs behind Ohm's Law Calculator.