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Ohm’s Law Calculator

Updated Sep 2026 Used 1 times
Choose the known pairThe interface shows only the two quantities required for that formula.
Choose the two measured or specified quantities; the remaining two are solved.
Enter measured or specified valuesSelect SI prefixes directly; all calculations normalize to volts, amperes, ohms, and watts.
Voltage across the same ideal resistive element.
Current through the same ideal resistive element.
Positive resistance of the ideal resistive element.
Power dissipated by the same ideal resistive element.

Guest calculations stay on this device. Signed-in results sync privately.

Your result

Calculated voltage

Enter your values, then calculate to see a verified result.

Inputs usedReview the information used for this result.
Full calculation and sourcesOpen Full calculation and sources to audit the formula, breakdown, assumptions, and source version.

Versioned calculationFormula v1.0.0

Method reviewed 2026-09-26NIST SI relationshipsFormula v1.0.0

All six known pairs

Solve voltage, current, resistance, and power from V-I, V-R, I-R, V-P, I-P, or R-P.

Transparent calculation

Inputs, formula path, output bridge, limitations, and sources remain visible.

Private by default

Guest calculations run in the browser and are not sent to a lead form.

Result actions
1
Choose what is knownSelect the two values from the same element and operating point.Voltage and resistance
2
Select the prefixesEnter V or mV, A or mA, ohms or kiloohms, and W or mW directly.20 mA is 0.02 A
3
Read the derivationUse the full calculation to see the selected formula path.V = I x R and P = V x I

Interpretation

Voltage, current, resistance, and power refer to the same element.

Power is ideal dissipation; component temperature and rating require margin and manufacturer data.

For nonlinear or reactive devices, use a device model or impedance analysis instead.

Use this result

Public verification recordFormula v1.0.0
Review scope
Ideal-resistive-ohm-and-power-law
Review team
CalculatorGeek Algorithmic Team
Verified
2026-09-26
Next source review
2027-09-26
Automated fixtures
6 cases

Review method

All six valid V/I/R/P input pairs were independently solved, SI-prefix normalization was round-tripped, and invalid zero or negative denominators were rejected in JavaScript and PHP.

Known limitations

  • The model represents one ideal linear resistive element at one operating point.
  • It does not calculate series or parallel networks, impedance, phase, transient behavior, temperature rise, tolerance, or component derating.
  • A mathematically valid power result is not proof that a component or conductor is safe.

Primary sources

Published calculation checks

CaseInputsExpected result
Voltage and resistance solve current and powerSee the versioned calculator fixture.{"current_amps":0.5,"power_watts":6,"pair_code":2}
Voltage and current solve resistanceSee the versioned calculator fixture.{"resistance_ohms":25,"power_watts":1,"pair_code":1}
Current and resistance solve voltageSee the versioned calculator fixture.{"voltage_volts":9.4,"power_watts":0.188,"pair_code":3}
On this page

What the Ohm's Law Calculator calculates

This Ohm Law Calculator solves voltage, current, resistance, and power from any two positive known values. Choose V and I, V and R, I and R, V and P, I and P, or R and P; the calculator normalizes the selected SI prefixes and displays all four base-unit results.

The result describes one ideal linear resistive element. It is not a complete circuit simulator or electrical-safety approval.

Continue with Verify the SI relationships for the next calculation.

How to use this calculator

  1. Select the two quantities you know.
  2. Enter their values and choose the matching unit prefixes.
  3. Calculate to see all four quantities and their common alternate units.
  4. Open the full calculation to confirm the formula path before using the result.

Use a dedicated series, parallel, LED-resistor, voltage-drop, impedance, or thermal calculator when the circuit contains more than one ideal resistive relationship.

Method and formula

The base relationship is V = I x R. Electrical power is P = V x I, producing the derived forms P = I squared x R and P = V squared / R. Voltage and power solve I = P / V and R = V squared / P. Current and power solve V = P / I and R = P / I squared. Resistance and power solve V = square root of P x R and I = square root of P / R.

Every entered prefix is converted to the coherent SI base used by the equations, then the display produces both base and convenient alternate units.

Worked example

With 12 V across 24 ohms, current is 12 / 24 = 0.5 A and power is 12 x 0.5 = 6 W. The same state can be displayed as 500 mA, 0.024 kiloohm, and 6,000 mW without changing the underlying quantities.

How to interpret the result

Confirm that both known values apply across and through the same element. A computed wattage describes ideal electrical dissipation; choose real components from manufacturer ratings with appropriate tolerance, temperature, voltage, transient, and safety margins.

Assumptions and limitations

Ohm law in this form assumes a linear resistive relationship. Diodes, LEDs, batteries, motors, capacitors, inductors, semiconductors, and many loads can be nonlinear, time-varying, or reactive. AC power can require RMS values, impedance, phase, and power factor. Never use this page as a substitute for qualified electrical design or safe isolation.

See the CalculatorGeek calculation methodology for formula sourcing, boundary testing, and correction handling.

Inputs, outputs and result meaning

Uses an explicit known-pair workflow, SI prefixes, all six pair equations, full derivation, and clear nonlinear, thermal, mains, and reactive-circuit boundaries.

Inputs

Two quantities you know
Choose the two measured or specified quantities; the remaining two are solved.
Known voltage
Voltage across the same ideal resistive element.
Known current
Current through the same ideal resistive element.
Known resistance
Positive resistance of the ideal resistive element.
Known power
Power dissipated by the same ideal resistive element.

Outputs

Calculated voltage
Primary result. Calculated from the displayed inputs without intermediate display rounding.
Calculated current
Calculated from the displayed inputs without intermediate display rounding.
Calculated resistance
Calculated from the displayed inputs without intermediate display rounding.
Calculated power
Calculated from the displayed inputs without intermediate display rounding.
Voltage in millivolts
Calculated from the displayed inputs without intermediate display rounding.
Current in milliamps
Calculated from the displayed inputs without intermediate display rounding.
Resistance in kiloohms
Calculated from the displayed inputs without intermediate display rounding.
Power in milliwatts
Calculated from the displayed inputs without intermediate display rounding.

Frequently asked questions

Which two values can I enter?

Any positive pair among voltage, current, resistance, and power determines the other two in the ideal model.

Can I use milliamps and kiloohms?

Yes. Select the prefixes and the calculator normalizes them before applying the equations.

Why can power be calculated three ways?

Substituting V = I x R into P = V x I gives P = I squared x R and P = V squared / R.

Does this work for an LED?

Not by itself. An LED has a nonlinear current-voltage curve and needs forward-voltage and current-limit data.

Does it prove a resistor rating is safe?

No. Real design also needs rating margin, ambient temperature, enclosure, pulse behavior, tolerance, and manufacturer data.

Primary sources and review record

Method and sources reviewed 2026-09-26. The calculator shows its assumptions and does not replace an official eligibility decision, professional advice, or a measurement made under controlled conditions.