Skip to content

Ohm's Law Calculator

Calculate voltage, current, resistance, and power using Ohm's Law. Enter any 2 values to solve for the other 2.

Enter any 2 values to calculate the other 2.

volts
amps
ohms
watts
Results
R = V / I, P = V × I
Voltage
12 V
Current
500.000 mA
Resistance
24 Ω
Power
6 W
Solve ForFormulaValue
Voltage (V)V = I × R12 V
Current (I)I = V / R500.000 mA
Resistance (R)R = V / I24 Ω
Power (P)P = V × I6 W
VIR

Ohm's Law

Ohm's Law is the fundamental relationship in electrical circuits. It states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to the resistance.

Core Formulas

FindFormulaVariables
VoltageV = I × RV (volts), I (amps), R (ohms)
CurrentI = V / R
ResistanceR = V / I

Electrical Power

Power is the rate of energy transfer. In DC circuits, power equals voltage times current. Combined with Ohm's Law, power can be expressed three ways:

FormulaKnown ValuesUse When
P = V × IVoltage, CurrentBoth V and I are known
P = I² × RCurrent, ResistanceVoltage unknown
P = V² / RVoltage, ResistanceCurrent unknown

Common Examples

ScenarioVIRP
LED with resistor5V20mA250Ω0.1W
Car headlight12V4.6A2.6Ω55W
USB charging5V2.4A2.08Ω12W
Space heater120V12.5A9.6Ω1,500W
EV charger (L2)240V30A8Ω7,200W

SI Prefixes for Electrical Units

PrefixSymbolFactorExample
MegaM10⁶1 MΩ = 1,000,000 Ω
Kilok10³4.7 kΩ = 4,700 Ω
Millim10⁻³20 mA = 0.020 A
Microμ10⁻⁶100 μA = 0.0001 A

Safety Notes

  • Household circuits: US standard is 120V/15A (1,800W max) or 120V/20A (2,400W max) per circuit
  • Wire sizing: Higher current requires thicker wire — use our Voltage Drop Calculator for wire sizing
  • Circuit breakers: Should be rated for 80% of max continuous load (e.g., 15A breaker → 12A continuous)
  • Never exceed ratings: Overloading circuits causes overheating, wire damage, and fire risk

Related Calculators

Age Calculator

Calculate your exact age in years, months, and days. Find the time between any two dates.

How Old Was I Calculator

Find out how old you were on any date. Calculate age in years, months, days, hours, minutes, and seconds.

Add Times Calculator

Add multiple time durations together in hours, minutes, and seconds. Get results in HH:MM:SS and decimal hours.

Time Until Calculator

Calculate the time remaining until a specific time and date. Live countdown in days, hours, minutes, and seconds.

End of School Countdown Calculator

Calculate the time remaining until a specific time and date. Live countdown in days, hours, minutes, and seconds.

Weeks Between Dates Calculator

Calculate the number of weeks between two dates. Also shows total days, months, weekdays, and weekend days.

Frequently Asked Questions

What is Ohm's Law?

Ohm's Law states that voltage (V) equals current (I) times resistance (R): V = I × R. It describes the linear relationship between voltage, current, and resistance in an electrical circuit with resistive elements.

How do I use this calculator?

Enter any 2 of the 4 values (voltage, current, resistance, power) and the calculator solves for the other 2. Leave the unknown fields blank.

What is the power formula?

Electrical power P = V × I (watts = volts × amps). It can also be expressed as P = I²R or P = V²/R. Power is the rate of energy transfer in a circuit.

What units are used?

Voltage in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W). The calculator auto-formats with SI prefixes (kΩ, mA, kW, etc.).

Does Ohm's Law work for AC circuits?

Ohm's Law applies to AC circuits using impedance (Z) instead of resistance (R). For purely resistive AC loads (heaters, incandescent bulbs), R and Z are the same. For capacitive or inductive loads, impedance includes reactance.

What is a practical example?

A 12V car battery powering a 24Ω resistor: I = 12/24 = 0.5A, P = 12 × 0.5 = 6W. Or: a 120V outlet with a 1500W heater draws I = 1500/120 = 12.5A.