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Ohm’s Law Calculator

Enter any two values—voltage, current, resistance, or power—to calculate the other two using V=IR and P=VI.

Use this voltage calculator, current calculator, resistance calculator, and power calculator for an ideal resistive load. Private by design: entries stay in your browser.

Enter exactly two known values

Leave both unknown fields blank. A result appears automatically once two valid values are present.

Symbol: V
Symbol: I
Symbol: R
Symbol: P
Output units

Hints: Enter calculates · Esc clears · Values auto-scale (e.g., 0.002 A → 2 mA).

Results will appear here.

Complete formula wheel

Every useful rearrangement of Ohm’s Law and Watt’s Law is shown as readable text.

Ohm's Law and power formulas grouped by value to calculate
Voltage (V)Current (I)Resistance (R)Power (P)
V = I × RI = V ÷ RR = V ÷ IP = V × I
V = P ÷ II = P ÷ VR = V² ÷ PP = V² ÷ R
V = √(P × R)I = √(P ÷ R)R = P ÷ I²P = I² × R

How to use this calculator

  1. Enter exactly two known quantities; leave unknown fields blank.
  2. Select the units beside each entry.
  3. Calculate automatically or press Calculate.
  4. Review the normalized SI substitution and converted result.

Example: enter 9 V and 1.5 kΩ; the calculator converts 1.5 kΩ to 1500 Ω and finds 6 mA and 54 mW.

Understanding Ohm’s Law

Ohm’s Law relates voltage, current, and resistance as V = I × R. Joule’s power relationship is P = V × I. This calculator converts all entries to volts, amperes, ohms, and watts before applying those equations.

Applicability: this is an ideal, steady, resistive-load model—not a model of every circuit. Resistance may change with temperature. Capacitors and inductors in AC circuits require frequency-dependent impedance and phase; LEDs and other semiconductors require nonlinear device curves or datasheets.

Resistance versus impedance: resistance is the real opposition modeled here. Impedance combines resistance and reactance and may introduce a phase difference between AC voltage and current.

Worked examples

Current and power from V and R

9 V and 1.5 kΩ (1500 Ω) give 0.006 A (6 mA) and 0.054 W (54 mW).

Resistance from V and I

12 V and 6 mA (0.006 A) give 2000 Ω (2 kΩ) and 72 mW.

Voltage from P and R

0.25 W and 100 Ω give √(0.25 × 100) = 5 V and 50 mA.

Select a resistor power rating

12 V across 1 kΩ dissipates 0.144 W. Choose a rating above the calculated dissipation with suitable design margin and verify the datasheet.

Practical limits and common mistakes

  • Units: convert kΩ to Ω and mA to A before hand calculations.
  • Temperature and tolerance: real resistance can differ from its nominal value and change as a component warms.
  • AC: purely resistive RMS calculations can use this model; reactive circuits need impedance, frequency, and phase.
  • Non-ohmic devices: LEDs, diodes, transistors, lamps, and other nonlinear devices do not have one fixed resistance across all operating points.
  • Power rating: calculated dissipation is not the rating to buy. Use an appropriately higher rated part after checking ambient temperature, derating, transients, and the manufacturer’s datasheet.

Frequently asked questions

What is Ohm’s Law?

Ohm’s Law states that voltage equals current multiplied by resistance: V = IR, for an ideal ohmic resistive load under steady conditions.

How do I calculate voltage, current, resistance, and power?

Use V = IR and P = VI. Rearranging these gives V = IR, I = V/R, R = V/I, and power forms P = VI = V²/R = I²R.

What values must I enter?

Enter exactly two known values from voltage, current, resistance, and power, with their units. Leave the other two fields blank.

Does Ohm’s Law apply to AC and DC?

The simple real-number form works directly for steady DC resistive loads and purely resistive AC values. General AC circuits require impedance and phase, usually represented with complex quantities.

What is the difference between resistance and impedance?

Resistance opposes current without a phase model. Impedance includes resistance and frequency-dependent reactance from capacitance or inductance, and can shift phase in AC circuits.

Why might a real measurement differ from the calculation?

Component tolerance, temperature, source and meter resistance, wiring losses, noise, and changing or nonlinear behavior can make measured values differ from this ideal estimate.

Can Ohm’s Law be used for LEDs or semiconductors?

Not as a fixed-resistance model across all operating points. LEDs and other semiconductors are nonlinear; use their datasheets or characteristic curves and apply Ohm’s Law only to appropriate circuit elements such as a series resistor.

Methodology and review

Prepared by: Starlight Tools technical editorial team. Technical review: Starlight Robotics engineering team. Reviewed: 12 July 2026. Updated: 12 July 2026.

Method: inputs are converted to SI units, solved using standard Ohm’s Law and Joule’s Law equations, checked for a finite physical result, and converted to the chosen display units.

References: NIST: SI Units—Electric Current; All About Circuits: nonlinear conduction; All About Circuits: resistance, reactance, and impedance; OSHA electrical requirements.

Educational estimate only. It does not replace component datasheets, manufacturer ratings, applicable electrical codes, qualified design review, or electrical safety requirements.

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