Ohm's Law Calculator

Solve any two-variable electrical circuit problem using Ohm's Law (V = IR) and Watt's Law (P = VI). Select which quantity you want to find, enter any two known values, and instantly see all four quantities: Voltage, Current, Resistance, and Power.

Select what you want to find:

V Voltage (V) A Current (I) Ω Resistance (R) W Power (P)

What is Ohm's Law?

Ohm's Law is a fundamental principle of electrical engineering that describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. Formulated by German physicist Georg Simon Ohm in 1827, it states that the current flowing through a conductor between two points is directly proportional to the voltage across those two points, provided the temperature and physical conditions remain constant. The simple relationship V = I × R underpins virtually every circuit analysis task in electronics, electrical engineering, and physics.

Beyond the basic V = IR relationship, Ohm's Law extends naturally to power calculations through Watt's Law (P = V × I). Combining these two laws yields four interrelated equations that allow any one of the four electrical quantities — voltage, current, resistance, or power — to be calculated from any two known quantities. These derived formulas are used daily by electricians, electronics engineers, hobbyists, and students designing circuits, sizing components, and troubleshooting electrical faults.

The law applies to ohmic (linear) conductors — resistors, wires, and simple passive components — where resistance remains constant regardless of applied voltage. Non-ohmic components such as diodes, LEDs, and transistors do not follow Ohm's Law in the strict sense, as their effective resistance changes with voltage and current. Nonetheless, Ohm's Law remains the essential starting point for any electrical analysis, providing the foundation for more advanced techniques like Kirchhoff's Voltage Law and Kirchhoff's Current Law.

Ohm's Law & Watt's Law Formulas

V = I × R Voltage = Current × Resistance

I = V / R Current = Voltage ÷ Resistance

R = V / I Resistance = Voltage ÷ Current

P = V × I Power = Voltage × Current

P = I² × R Power = Current² × Resistance

P = V² / R Power = Voltage² ÷ Resistance

How to Use the Ohm's Law Calculator

1

Choose What to Find

Select the quantity you want to solve for: Voltage (V), Current (I), Resistance (R), or Power (P).
2

Enter Two Known Values

Input any two known electrical values. The calculator accepts any valid pair that uniquely determines the circuit.
3

Click Calculate

All four quantities — Voltage, Current, Resistance, and Power — are calculated and displayed simultaneously.
4

Review All Results

Results are shown in a 2×2 grid. Copy them with one click for use in reports or documentation.

Example Calculation

A 12V battery drives a current of 2A through a resistor:

Voltage (V) = 12 V | Current (I) = 2 A

Resistance R = V / I = 12 / 2 = 6 Ω

Power P = V × I = 12 × 2 = 24 W

How the Ohm's Law Calculator Works

Formula, assumptions, and calculation steps for this engineering tool.

Formula Used

V = I * R, P = V * I

Methodology

Engineering calculators apply standard unit conversions and formula relationships after normalizing measurements to compatible units.

Calculation Steps

Enter dimensions, loads, rates, or electrical values.
Convert the inputs into the formula unit system.
Apply the engineering equation or conversion factor.
Return the result with units and supporting context.

Assumptions and Limits

Material behavior is assumed ideal unless fields specify otherwise.
Code checks, safety factors, and site conditions may require professional review.
Use a qualified engineer for design-critical decisions.

Frequently Asked Questions

What is Ohm's Law?

Ohm's Law states that the voltage (V) across a conductor equals the current (I) through it multiplied by its resistance (R): V = IR. It was formulated by German physicist Georg Simon Ohm in 1827 and is fundamental to all electrical circuit analysis.

Does Ohm's Law apply to AC circuits?

Ohm's Law applies to purely resistive AC circuits in exactly the same way as DC. In circuits with capacitors or inductors, the relationship becomes V = IZ, where Z is impedance — a complex quantity that includes both resistance and reactance. The basic principle remains the same but requires complex number arithmetic.

What are real-world applications of Ohm's Law?

Ohm's Law is used everywhere in electronics: calculating the correct resistor value to limit LED current, designing voltage dividers, sizing fuses and circuit breakers, troubleshooting short or open circuits, and computing heat dissipation in resistive components.

Why can't I just enter one value?

Two independent values are always required to uniquely determine a circuit. One value alone is insufficient: knowing only 12V tells you nothing about current or resistance without an additional constraint.

What units does this calculator use?

Voltage is in Volts (V), current in Amperes (A), resistance in Ohms (Ω), and power in Watts (W). Enter values in these base units; the calculator handles all conversions internally.

Real-World Applications

💡

LED Resistor Sizing

Calculate the current-limiting resistor required to protect an LED from overcurrent: R = (Vsupply − Vforward) / Iforward. A 5V supply, 2V LED, and 20mA target current requires a 150Ω resistor.

🔌

Fuse & Circuit Breaker Selection

Determine the maximum current a circuit will draw using I = P / V — a 2,400W appliance on a 120V circuit draws 20A, requiring at minimum a 20A fuse or breaker.

🔋

Battery Internal Resistance

Measure battery internal resistance by comparing open-circuit voltage to loaded voltage: R_internal = (V_open − V_loaded) / I_load — essential for diagnosing aging batteries.

🏗️

Cable & Wire Sizing

Electrical engineers use Ohm's Law to calculate voltage drop across cable runs: V_drop = I × R_cable. Excessive voltage drop degrades motor performance and violates electrical codes.

🎸

Speaker Impedance Matching

Audio amplifier output power depends on speaker impedance (R). An 8Ω speaker connected to an amplifier with 20V RMS output delivers P = V²/R = 50W — matching speaker to amplifier maximises power transfer.

☀️

Solar Panel System Design

Size charge controllers and inverters using I = P / V. A 3,000W solar array at 48V requires a charge controller rated for at least 62.5A — safety margins then increase this to a 70–80A unit.

Common Mistakes

Confusing AC and DC applications

Ohm's Law applies directly to DC resistive circuits. In AC circuits, impedance (Z) replaces resistance (R), and voltage and current are phasors — the simple V = IR relationship holds only for purely resistive AC circuits, not for capacitive or inductive loads.

Using peak voltage instead of RMS in power calculations

Mains AC voltage (120V or 230V) is the RMS value, not the peak. The peak voltage is approximately 1.414× higher (170V and 325V respectively). Using peak voltage in P = V²/R overstates power by a factor of 2.

Ignoring wire and connection resistance

Long cable runs and loose connections add real resistance to a circuit. Ignoring this resistance in calculations leads to underestimated voltage drop, unexpected component behaviour, and potential safety issues in high-current circuits.

Assuming Ohm's Law applies to all components

Diodes, LEDs, transistors, and other semiconductor devices are non-ohmic — their resistance is not constant and varies with operating conditions. Using V = IR directly for a diode gives incorrect results; their I-V characteristics require different analysis.

Not accounting for temperature effects on resistance

Resistance of most metals increases with temperature (positive temperature coefficient). A wire that measures 1Ω at room temperature may measure 1.2Ω when hot. For motor windings and heating elements, this temperature dependence is significant and should be factored into design calculations.

Ohm's Law Formula Quick Reference

Find	Formula	Known Values
Voltage (V)	V = I × R = √(P × R) = P / I	Current & Resistance, or Power & Resistance
Current (I)	I = V / R = P / V = √(P / R)	Voltage & Resistance, or Power & Voltage
Resistance (R)	R = V / I = V² / P = P / I²	Voltage & Current, or Power & Current
Power (P)	P = V × I = I² × R = V² / R	Voltage & Current, or Current & Resistance

References

Ohm, Georg Simon. Die galvanische Kette, mathematisch bearbeitet. T.H. Riemann, 1827.
Hayt, W.H. and Kemmerly, J.E. Engineering Circuit Analysis. McGraw-Hill, 2018.
Nilsson, J.W. and Riedel, S.A. Electric Circuits. Pearson, 2014.
Sedra, A.S. and Smith, K.C. Microelectronic Circuits. Oxford University Press, 2014.
IEEE Standard 141. Recommended Practice for Electric Power Distribution for Industrial Plants. IEEE, 1993.