Ohm's Law Calculator
Enter any two of voltage, current and resistance to get the third plus power.
Formula and sources updated · Data sources · 1View algorithm, constants and default inputs (JSON)
About this calculator
Choose the two known values. The selected pair must determine a positive resistance without dividing by zero; conductance is the reciprocal of resistance.
Worked example
calculated with the default values below
What do you know Voltage and resistance · Voltage 12 V · Current 0.5 A · Resistance 24 Ω Current 0.500000 A · Voltage 12.0000 V · Resistance 24.0000 Ω · Power 6.0000 W · Power 6,000.00 mW · Conductance 0.04166667 S
Method and assumptions
Formula basis and scope
V = IR; electrical power = VI; conductance = 1/R.
Ideal ohmic DC component; excludes AC phase, reactive impedance and temperature-dependent resistance.
Data sources
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
V = I × R; P = V × I
- V
- voltage, V
- I
- current, A
- R
- resistance, Ω
- P
- power, W
The mode sets the unknown: I = V/R, R = V/I or V = I×R; division by zero yields 0; conductance = 1/R; power in mW = 1000×P.
Frequently asked questions
Why does the calculator also give conductance when I only entered voltage and resistance?
Once you pick 'voltage and resistance' the tool computes current as 12 ÷ 24 = 0.5 A, then derives power as 12 × 0.5 = 6 W. Conductance is simply the reciprocal of resistance, 1 ÷ 24 ≈ 0.0417 siemens, so it appears automatically. The three known-pair modes (V–R, V–I, I–R) each solve the missing quantity the same way through V = I × R.