Ohm's Law Calculator

Enter any two known values among Voltage (V), Current (I), and Resistance (R). This calculator instantly computes the missing quantity and Power (P), with unit conversions and precision control. Built for engineers, technicians, and students who need fast, trustworthy results.

Inputs

Provide any two of V, I, R. Validation occurs on blur; results update as you type.

Results

Voltage (V) —
Current (I) —
Resistance (R) —
Power (P) —

Enter any two inputs to compute remaining values.

Data Source and Methodology

Authoritative reference: IEC 60050 — International Electrotechnical Vocabulary (Electropedia), entry on Ohm’s law, latest consolidated edition (accessed 2025). https://www.electropedia.org/. For SI units and prefixes, see NIST Special Publication 330 (The International System of Units). https://www.nist.gov/pml/owm/metric-si/si-units.

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

Ohm’s law: $$V = I \cdot R$$
Equivalent forms: $$I = \frac{V}{R}, \quad R = \frac{V}{I}$$
Power: $$P = V \cdot I = I^2 \cdot R = \frac{V^2}{R}$$

The calculator converts your inputs to SI units internally, applies the relevant equation based on which two inputs are present, then formats results according to your selected units and decimals.

Glossary of Variables

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  • Voltage (V): Electric potential difference, measured in volts (V).
    • -
  • Current (I): Flow of electric charge, measured in amperes (A).
    • -
  • Resistance (R): Opposition to current, measured in ohms (Ω).
    • -
  • Power (P): Rate of energy transfer, measured in watts (W).
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  • SI prefixes: m (10⁻³), μ (10⁻⁶), k (10³), M (10⁶).

How It Works: A Step-by-Step Example

Scenario: A 12 V supply drives 2 A through a load. What are the resistance and power?

  1. Known: V = 12 V, I = 2 A; Unknown: R, P
  2. Compute resistance using Ohm’s law: R = V / I = 12 / 2 = 6 Ω.
  3. Compute power: P = V · I = 12 · 2 = 24 W.

Answer: R = 6 Ω, P = 24 W.

Frequently Asked Questions (FAQ)

Do I have to use SI units?

No. You can select mV, kV, mA, μA, kΩ, or MΩ. The tool converts to SI internally and displays results in your chosen units.

Can resistance be zero?

Ideal conductors model R = 0, but dividing by zero is undefined. If a computation requires division by R and you entered R = 0, the calculator will flag an error.

Why does the calculator require at least two inputs?

Ohm’s law relates three variables (V, I, R). You need two to determine the third; power is then derived from V and I.

How are rounding and precision handled?

Use the “Decimals” control to set the number of decimal places (0–10). Internally, calculations use full double precision.

Is temperature dependence accounted for?

No. Resistance can vary with temperature and material. This calculator assumes linear, ohmic behavior at a given temperature.

Can I paste values with units (e.g., 3.3k)?

For accuracy and accessibility, use the numeric field plus the unit dropdown (e.g., 3.3 and kΩ). This avoids parsing ambiguities.

Is negative current or voltage supported?

Yes. Sign depends on your defined reference direction. The power displayed is computed as P = V × I and therefore may be negative (indicating power delivered by the element).

Tool developed by Ugo Candido. Content verified by the CalcDomain Engineering Editorial Team.
Last reviewed for accuracy on: .