Calculate voltage (V), current (I), resistance (R), and power (P) with Ohm's Law and Joule's Law. Free calculator with formulas, unit steps, and circuit examples.
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In any electrical conductor, voltage (V) acts as the electrostatic pressure driving electric charges, while current (I) measures the volume rate of charge flowing through the cross-section per unit time. Electrical resistance (R) represents the material friction and collision obstacles encountered by charge carriers.
Ohm's Law (V = I × R) proves that under constant physical temperature, current scales in exact linear proportion to applied voltage. Complementing this, Joule's Law proves that electrical work converted to thermal energy is directly proportional to both voltage and current (P = V × I), or alternatively to the square of current multiplied by resistance (P = I²R).
The classic Ohm's Law Wheel combines Ohm's Law and Joule's Law into twelve algebraic equations that permit solving for any parameter from any two knowns:
To solve for Voltage: V = I × R, V = P / I, or V = √(P × R). To solve for Current: I = V / R, I = P / V, or I = √(P / R). To solve for Resistance: R = V / I, R = V² / P, or R = P / I². To solve for Power: P = V × I, P = I² × R, or P = V² / R.
Ohm's law holds true for 'ohmic' conductors, which include copper and aluminum wiring, carbon film and metal film resistors, and wound alloy heating elements, provided temperature remains relatively stable.
In non-ohmic components—such as diodes, LEDs, transistors, thermistors, and vacuum tubes—the relationship between voltage and current is non-linear. For example, a diode conducts negligible current until the threshold forward voltage (typically 0.7V for silicon) is reached, after which current increases exponentially.
The reference table below illustrates standard voltage, current, resistance, and power ratings found in everyday electrical and electronic systems.
| Application / System | Supply Voltage (V) | Operating Current (I) | Effective Resistance (R) | Power Dissipated (P) |
|---|---|---|---|---|
| Microcontroller Logic (3.3V) | 3.3 V | 20 mA (0.02 A) | 165 Ω | 0.066 W (66 mW) |
| USB 2.0 Port Charging (5V) | 5.0 V | 1.0 A | 5.0 Ω | 5.0 W |
| Automotive Sensor Shunt | 12.0 V | 0.25 A (250 mA) | 48.0 Ω | 3.0 W |
| Automotive Headlight (12V) | 12.0 V | 5.0 A | 2.40 Ω | 60.0 W |
| Industrial Control Circuit (24V) | 24.0 V | 1.5 A | 16.0 Ω | 36.0 W |
| 120V US Space Heater (1500W) | 120.0 V | 12.5 A | 9.60 Ω | 1,500 W (1.5 kW) |
| 230V EU Electric Kettle (2200W) | 230.0 V | 9.57 A | 24.04 Ω | 2,200 W (2.2 kW) |
For full industrial and domestic electrical design, pair your Ohm’s Law calculations with our dedicated power conversion and current sizing engines.
Need to compute active power dissipation, daily kilowatt-hour energy usage, or convert 3-phase current to kilowatts? Explore our Electrical Power Calculator or our Amps to kW Calculator for balanced phase calculations.
Launch Electrical Power Calculator →A 12-volt automotive DC battery powers a halogen headlight with a filament resistance of 2.4 ohms. Calculate the circuit current and total wattage.
The headlight draws 5.0 A of current and consumes 60 W of electrical power from the 12V supply.
A 9V battery powers a standard blue LED requiring 20 mA (0.02 A) with a forward voltage drop of 2.0V across the diode. Calculate the resistance and wattage required for the series resistor.
A 350 Ω (or standard 360 Ω) resistor is required. A standard 0.25 W resistor easily handles the 0.14 W heat dissipation without thermal degradation.
An electric space heater is rated for 2,000 W at 230 V AC. Determine the full-load running current and internal element resistance.
The 2.0 kW space heater draws 8.70 A and presents an effective resistance of 26.45 Ω when operating at full heat.
A microcontroller current sensing circuit uses a 0.05 Ω shunt resistor in series with a 2.5 A DC motor. Find the differential voltage drop and power lost in the shunt.
The shunt produces a 125 mV analog signal for the microcontroller ADC while dissipating only 312.5 mW of heat.
The foundational equations are: Voltage V = I × R (volts = amps × ohms), Current I = V / R (amps = volts / ohms), Resistance R = V / I (ohms = volts / amps), and Power P = V × I (watts = volts × amps). Combining these yields P = I² × R and P = V² / R.
To find resistance, divide the circuit voltage by the electric current flowing through it: R = V / I. For example, if a 12V supply produces a current of 3 amperes through a heating element, the resistance is 12 / 3 = 4 ohms (Ω).
Calculate the actual power dissipation using P = I² × R or P = V² / R, then apply an engineering safety margin of at least 1.5× to 2.0×. For example, if a resistor dissipates 0.14 W, choose a standard ¼-Watt (0.25 W) or ½-Watt (0.5 W) rated resistor to prevent excessive overheating and thermal drift.
Yes. For purely resistive AC loads (like heaters and incandescent lights), Ohm's law applies directly using RMS voltage and current. In AC circuits containing inductors and capacitors, Ohm's law is generalized using impedance (Z): V = I × Z, where impedance accounts for both resistance and frequency-dependent reactance.
Voltage (measured in Volts) is the electrical potential difference or pressure pushing charges between two points. Current (measured in Amperes) is the actual physical flow rate of electric charge passing through a conductor (1 ampere = 1 coulomb per second).
Because power depends on voltage squared: P = V² / R. If you double the voltage (2×), the numerator increases by 2² = 4, resulting in four times more power dissipation and thermal heat generation.
UtilityBase offers a complete suite of engineering tools: launch our Electrical Power Calculator (/engineering-physics/electrical-power-calculator/) for energy consumption and electricity bill calculations, or visit our Amps to kW Calculator (/engineering-physics/amps-to-kw-calculator/) for single-phase and 3-phase industrial power analysis.