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Ohm's Law Calculator

Solve voltage, current, resistance, and power from any two known values with SI-prefix conversion, formula steps, resistor rating context, and tolerance sensitivity.

  • ohms law
  • v ir
  • volts amps ohms watts
  • voltage current resistance
  • resistor power

Only the two values named here are used to solve the circuit.

Optional context is calculated from this entered rating; it is not a component recommendation.

A user-entered comparison threshold, not a universal derating rule.

Status: initial

Results

Awaiting calculation

Calculator guide

Solve the complete V-I-R-P relationship, not just one rearrangement

The Ohm's Law Calculator finds voltage, current, resistance, and power from any selected pair of known values. It accepts practical SI prefixes, exposes the equation and substitution used, compares dissipation with an entered component rating and design target, and evaluates all four low/high tolerance corners instead of hiding the calculation behind a single answer.

Use it for ideal resistive-circuit checks and learning

Use the page to check a resistor, heater element, resistive lamp, laboratory exercise, or measurement where the device behaves approximately as an ohmic load. The result is a model of positive magnitudes. Reactive AC loads, semiconductors, motors, switching supplies, temperature-dependent elements, wiring protection, and energized work require additional data and qualified methods.

Ohm's Law and Joule power equations share four quantities

Ohm's Law is V = I × R. Electrical power for a resistive element is P = V × I. Combining these relationships produces twelve useful formula-wheel forms, so any two positive values among voltage, current, resistance, and power determine the other two. Inputs are converted to volts, amperes, ohms, and watts before solving.

Variable explanations

Understand what each input and result means before calculating.

⚡ Known-value pair

Choose exactly which two values are authoritative. The other visible fields remain available for switching modes but are ignored until selected.

🔋 Voltage (V)

Potential difference across the same element being analyzed. Select microvolts, millivolts, volts, or kilovolts.

〰️ Current (I)

Current through that element, not a separate branch or whole-circuit current unless those are physically the same.

Ω Resistance (R)

The ideal or measured resistance under the relevant operating conditions. Real resistance may change with temperature, frequency, voltage, age, and construction.

🔥 Power (P)

Real power dissipated by an ideal resistive element. Apparent and reactive power require an AC impedance and phase model.

↔️ SI-prefix units

Every known input is normalized to V, A, Ω, and W. This prevents common errors such as treating 20 mA as 20 A or 2 kΩ as 2 Ω.

± Input tolerance

Enter tolerances for the first and second quantities named in the selected pair. The range table solves all four endpoint combinations and reports the true minimum and maximum among those corners.

🧯 Nominal power rating

Enter the component rating you want to compare. A rating depends on manufacturer test conditions, pulse duration, ambient temperature, mounting, enclosure, airflow, and voltage limits.

🎯 Design utilization target

This is your own comparison threshold. The calculator deliberately does not claim that one percentage is safe for every resistor or application.

🔢 Significant digits

Choose display precision that matches the quality of the source values. More digits do not make a nominal component or measurement more accurate.

Reviewed by the Calculator.org.in Editorial Team

Formula behavior, validation cases, explanatory examples, and cited sources are checked before publication. This review supports educational accuracy and is not a substitute for qualified professional advice.

Last reviewed: 2026-08-10

Review process

Formula guide

See the calculation logic, variable definitions, and practical meaning.

Voltage from current and resistance

V = I × R

  • V is potential difference in volts
  • I is current in amperes
  • R is resistance in ohms

Current through a resistance produces a voltage drop.

Current from voltage and resistance

I = V ÷ R

  • Use consistent RMS values only for a purely resistive AC case

At fixed resistance, current rises in direct proportion to voltage.

Resistance from voltage and current

R = V ÷ I

  • The ratio is constant only when the element is ohmic under the stated conditions

Resistance describes opposition to current in this ideal model.

Power from voltage and current

P = V × I

  • P is real power dissipated by the resistive element in watts

Multiply the voltage across the element by the current through it.

Power from voltage and resistance

P = V² ÷ R

  • Derived by substituting I=V/R into P=VI

Useful when a voltage is applied to a known resistor.

Power from current and resistance

P = I² × R

  • Derived by substituting V=IR into P=VI

The squared-current term makes dissipation sensitive to current increases.

Voltage from power and resistance

V = √(P × R)

  • This page returns the positive magnitude

One of the two square-root forms used when power and resistance are known.

Current from power and resistance

I = √(P ÷ R)

  • This page returns the positive magnitude

The matching current for the entered resistive power and resistance.

Conductance

G = 1 ÷ R

  • G is conductance in siemens

Conductance is the reciprocal of resistance.

Rating utilization

Utilization = Pcalculated ÷ Prating × 100%

  • The entered rating and target do not replace manufacturer derating data

A comparison aid, not a safety certification or component-selection rule.

Worked examples

Follow realistic inputs through the calculation step by step.

1

Worked example

12 V across a 24 Ω resistor

  1. 1Choose Voltage + resistance.
  2. 2Enter 12 V and 24 Ω.
  3. 3The result is 0.5 A and 6 W.
2

Worked example

Find resistance from a 5 V, 200 mA device

  1. 1Choose Voltage + current.
  2. 2Enter 5 V and 200 mA.
  3. 3The ideal equivalent resistance is 25 Ω and power is 1 W.
3

Worked example

Find a resistive load from voltage and power

  1. 1Choose Voltage + power.
  2. 2Enter 120 V and 60 W.
  3. 3The ideal result is 0.5 A and 240 Ω.
4

Worked example

Current and resistance determine voltage

  1. 1Choose Current + resistance.
  2. 2Enter 3 A and 4 Ω.
  3. 3The result is 12 V and 36 W.
5

Worked example

Current and power determine the remaining pair

  1. 1Choose Current + power.
  2. 2Enter 2 A and 18 W.
  3. 3The result is 9 V and 4.5 Ω.
6

Worked example

Resistance and power square-root case

  1. 1Choose Resistance + power.
  2. 2Enter 100 Ω and 4 W.
  3. 3The result is 0.2 A and 20 V.
7

Worked example

Review a nominal resistor rating

  1. 1Calculate the circuit power.
  2. 2Enter the actual component's nominal rating and your design target.
  3. 3Review utilization, then consult the component data sheet and derating curve.
8

Worked example

Explore input tolerance

  1. 1Enter tolerances for both known quantities.
  2. 2The calculator evaluates all four endpoint combinations.
  3. 3Use the range as sensitivity context, not as a complete uncertainty or safety analysis.

Common mistakes

Avoid these common input and interpretation errors.

Mixing milliamps and amps

1 mA is 0.001 A. Select the unit that matches the entered number rather than manually guessing the factor.

Mixing kilo-ohms and ohms

1 kΩ is 1,000 Ω. A missed prefix can change current and power by three orders of magnitude.

Using values from different circuit locations

V, I, R, and P must refer to the same element or equivalent resistive network.

Treating every device as ohmic

Diodes, LEDs, transistors, lamps during warm-up, batteries, and many electronic loads do not have one constant V/I resistance.

Using resistance instead of impedance

Capacitors and inductors introduce frequency-dependent reactance and phase. Complex impedance, not this scalar resistance model, is required.

Mixing peak and RMS AC values

For a purely resistive sinusoidal AC load, use RMS voltage and RMS current consistently when calculating average real power.

Assuming the supply can deliver the result

Ohm's Law does not check source current limit, internal resistance, voltage sag, protection, or battery chemistry.

Treating a nominal wattage as a safe operating target

Manufacturer ratings depend on conditions and other limits. Apply the actual data sheet's derating, pulse, temperature, and voltage information.

Ignoring temperature coefficient and self-heating

Power dissipation can heat a component and change its resistance, so a room-temperature nominal value may not remain constant.

Measuring resistance on an energized circuit

Resistance measurements are normally made with power removed and stored energy safely discharged, following the instrument and equipment procedures.

Confusing power with energy

Watts describe a rate. Energy requires multiplying power by elapsed time and is expressed in joules or watt-hours.

Reporting false precision

Tolerance, meter accuracy, leads, contacts, temperature, and model limitations usually matter more than extra calculator digits.

Frequently asked questions

Quick answers to the questions users ask most often.

What is Ohm's Law?
For an ohmic element under stable conditions, voltage equals current multiplied by resistance: V=IR.
What values can this calculator solve?
It solves the complete positive-magnitude set of voltage, current, resistance, and resistive power from any selected pair.
How do I calculate current?
Use I=V/R, I=P/V, or I=√(P/R), depending on which two values are known.
How do I calculate resistance?
Use R=V/I, R=V²/P, or R=P/I².
How do I calculate voltage?
Use V=IR, V=P/I, or V=√(PR).
How do I calculate power?
For a resistive element, use P=VI, P=I²R, or P=V²/R.
Can I enter milliamps and kilo-ohms?
Yes. Unit selectors convert µA, mA, A, kA and mΩ, Ω, kΩ, MΩ through base SI units.
Does Ohm's Law work for AC?
The scalar form works directly for a purely resistive AC load when consistent RMS values are used. Reactive loads require complex impedance and phase relationships.
What is the difference between resistance and impedance?
Resistance dissipates real power and is the real part of opposition to current. Impedance also includes capacitive or inductive reactance and can depend on frequency.
Does it work for LEDs and diodes?
Not as a constant-resistance device model. Their current-voltage relationships are nonlinear and normally require device-specific data.
What does the power-rating percentage mean?
It is calculated power divided by the rating you entered. It is only a comparison and does not account for the manufacturer's full thermal, pulse, voltage, or environmental limits.
Is a 50% design target always safe?
No. The target is user-entered because no single percentage fits every technology, mounting method, ambient temperature, pulse shape, or reliability requirement.
How does tolerance sensitivity work?
The calculator varies both known values to their entered low and high endpoints, solves all four combinations, then reports the minimum and maximum of each result.
Is the tolerance range a full uncertainty analysis?
No. It excludes probability distributions, correlation, calibration bias, temperature behavior, nonlinear effects, and model uncertainty.
Why is conductance shown?
Conductance G=1/R expresses how readily current flows and is measured in siemens.
What happens when resistance approaches zero?
The ideal equations predict very high current for nonzero voltage, but real sources, conductors, protection, and parasitic resistance limit it. Fault-current analysis is outside this calculator.
Can this calculator choose a resistor for mains voltage?
No. Mains design requires working-voltage, insulation, creepage, clearance, surge, thermal, fault, protection, and code considerations beyond Ohm's Law.

References

Sources used to support the calculator guidance.

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Version history

A transparent record of calculator content updates.

Updated 2026-08-10
  • 1.0.0 · 2026-08-10

    Initial standalone release with six known-pair modes, SI-prefix conversion, equation audit, four-corner tolerance sensitivity, conductance, and user-defined resistor power-rating context.