An Ohm's Law calculator solves the fundamental electrical equation V = I × R for whichever variable is missing. Pick what you want to find (voltage, current, or resistance), enter the other two values, and the calculator returns the third along with the power dissipated.
Useful for hobbyist electronics, automotive electrical work, basic circuit design, and any situation where you have two of three values in a simple resistive circuit. Ohm's Law is the cornerstone of DC circuit analysis and the starting point for understanding more complex AC and reactive systems.
How it's calculated
Ohm's Law: V = I × R, where:
- V = voltage in volts (V)
- I = current in amperes (A)
- R = resistance in ohms (Ω)
Rearranged for each unknown:
- Voltage:
V = I × R - Current:
I = V ÷ R - Resistance:
R = V ÷ I
Power in a resistive circuit: P = V × I = I² × R = V² ÷ R (all three forms are equivalent). This calculator returns power automatically based on whichever two of V, I, R are known.
A few useful reference points:
- AAA battery: 1.5 V, ~1000 mAh capacity
- USB: 5 V, up to 3 A on USB-C
- US household outlet: 120 V, 15-20 A circuit
- Car 12 V system: 12 V (closer to 14 V running), 60-100 A starter
- Lightning bolt: ~30,000 V, 30,000 A, but lasts only milliseconds
Source: Ohm's law (Georg Ohm, 1827), V = I × R for resistive circuits
Examples
Find voltage: 2 A through 6 Ω
- Solve for Voltage (V)
- Voltage (V) 0
- Current (A) 2
- Resistance (Ω) 6
With 2 A flowing through a 6 Ω resistor, the voltage drop is 12 V by V = I × R. Power dissipated = V × I = 12 × 2 = 24 W, significant heat for a small resistor; would need at least a 0.5 W resistor with derating, or more typically a 25-50 W power resistor for sustained dissipation.
Find current: 12 V across 60 Ω
- Solve for Current (I)
- Voltage (V) 12
- Current (A) 0
- Resistance (Ω) 60
12 V across a 60 Ω load draws 0.2 A (200 mA) by I = V / R. Power = V² / R = 144 / 60 = 2.4 W. Typical of a small 12 V LED panel circuit (a 12 V power supply driving an LED strip with about 60 Ω of total resistance, including the current-limiting resistor and the LED's forward voltage).
Frequently asked questions
What is Ohm's Law?
Ohm's Law states that the current through a resistor is directly proportional to the voltage across it, with resistance as the constant: V = I × R. Discovered by Georg Ohm in 1827, it's the foundational relationship for analyzing DC and simple AC circuits. The law applies to "ohmic" materials, most metals and resistors, but breaks down for non-linear devices like diodes, transistors, and gas-discharge tubes.
How do I calculate power in an electrical circuit?
Three equivalent forms of the power formula: P = V × I, P = I² × R, and P = V² ÷ R. All three give the same answer when V, I, and R obey Ohm's Law. Pick whichever uses the values you already have. Power is in watts (W) when V is in volts, I in amps, R in ohms. Heat dissipation in a resistor follows the I²R form, small current changes have big heat-output consequences.
Does Ohm's Law work for AC circuits?
Yes for purely resistive circuits, with extensions for reactive ones. Pure resistors follow V = IR identically in DC and AC. Capacitors and inductors introduce frequency-dependent behavior that Ohm's Law alone can't model, for AC analysis you replace R with impedance Z, which is a complex number capturing both resistance and reactance: V = I × Z. For practical purposes, Ohm's Law works fine for resistive AC loads like incandescent bulbs and heating elements.
How do I figure out what wire gauge I need?
Calculate the expected current with Ohm's Law and pick a gauge rated above it. A 14 AWG copper wire safely carries ~15 A; 12 AWG ~20 A; 10 AWG ~30 A; 8 AWG ~40 A; 6 AWG ~55 A. Wire ampacity tables are published by NEC (National Electrical Code) in the US, with derating for high-temperature environments and bundled wires. Always use the next gauge up from your calculated minimum for safety margin and to keep voltage drop low over long runs.