How is Power related to Voltage, Current, and Resistance?

Power (in watts) equals Voltage × Current (P = VI). Substituting Ohm's Law gives two equivalent forms: P = I²R and P = V²/R, all algebraically identical.

Why do I need to enter exactly two values, not just one?

Ohm's Law relates three quantities (V, I, R); with only one known, there are infinitely many combinations of the other two that would satisfy the equation. Two knowns pin down a unique answer for the rest.

What happens if I enter three or four values?

The circuit becomes over-constrained — the calculator asks you to clear values down to exactly two, since entering extra values risks an internally inconsistent set of numbers (e.g., ones that don't actually satisfy V=IR together).

Does this work for AC circuits?

No — AC circuits involve impedance (resistance plus reactance from capacitors/inductors, which depends on frequency), not just simple resistance. This calculator models DC (or purely resistive) circuits only.

What's a real-world example of using Ohm's Law?

Sizing a resistor for an LED: given the supply voltage, the LED's forward voltage drop, and your target current, Ohm's Law tells you the resistor value needed to limit current to a safe level.

Why does resistance have to be zero or positive?

Physical resistance in a real circuit can't be negative — a negative sign in circuit analysis usually indicates current/voltage direction relative to a reference, which this calculator doesn't model separately.

Is Ohm's Law exact for every material?

No — it's exact for 'ohmic' materials/components (resistors, most conductors over a reasonable range), where resistance stays constant regardless of voltage or current. Non-ohmic components like diodes, transistors, and light bulbs (whose resistance changes with temperature) don't follow it exactly.

Ohm's Law Calculator

About the Ohm's Law Calculator

Solves for any two unknowns among voltage, current, resistance, and power in a simple DC circuit, using Ohm's Law and the power identities derived from it — enter any two known values and it computes the other two.

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How to use it

Enter any two of Voltage, Current, Resistance, or Power.
Leave the other two fields blank.
Read the two solved values in the result panel, alongside the two you entered.

Formula

Ohm's Law: V = I × R (Voltage = Current × Resistance).
Power identities derived from it: P = V × I = I²R = V²/R.
Given any two of {V, I, R, P}, the other two follow algebraically — e.g. given V and R: I = V/R and P = V²/R.

Worked example

With Voltage = 12V and Current = 2A given: Resistance = 12 ÷ 2 = 6Ω, and Power = 12 × 2 = 24W.

Ohm's Law quick-reference — solving for each variable

Ohm's Law quick-reference — solving for each variable
Known pair	Voltage	Current	Resistance	Power
V, I	—	—	V/I	V×I
V, R	—	V/R	—	V²/R
I, R	I×R	—	—	I²×R
V, P	—	P/V	V²/P	—
I, P	P/I	—	P/I²	—
R, P	√(P×R)	√(P/R)	—	—

Interpreting your result

This models a simple resistive DC circuit only — it doesn't account for AC impedance/reactance, temperature-dependent resistance, or non-ohmic components (like diodes or transistors) whose resistance isn't constant. Enter exactly two known values; entering fewer leaves the circuit underdetermined, and entering more than two over-constrains it (and could be inconsistent).

Recommendations

• If Current is 0, Resistance and Power can't be solved from Voltage and Current alone (division by zero) — that's an open circuit, not a valid two-value input for this pair.
• Resistance of 0Ω models a short circuit — Current and Power become undefined from Voltage and Resistance in that case too.
• Power scales with the square of both Current (P=I²R) and Voltage (P=V²/R) — doubling either one quadruples power dissipated, not just doubles it.

Frequently asked questions

Ohm's Law states that voltage equals current times resistance (V = I × R) in a simple resistive circuit — named after Georg Ohm, who published the relationship in 1827.

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