Ohm's Law states that the voltage (V) across a resistor is equal to the current (I) through it multiplied by its resistance (R): V = I x R. This fundamental relationship allows you to calculate any one of the three quantities if you know the other two.

How do I calculate power using Ohm's Law?

Power (P) in watts can be calculated three ways: P = V x I, P = I squared x R, or P = V squared / R. All three formulas are equivalent and derived from P = V x I combined with Ohm's Law V = I x R.

How do I calculate series and parallel resistor combinations?

For series resistors, total resistance is the sum: Rtotal = R1 + R2 + R3. For parallel resistors use the reciprocal formula: 1/Rtotal = 1/R1 + 1/R2 + 1/R3. For two resistors in parallel: Rtotal = (R1 x R2) / (R1 + R2).

How does wire resistance affect a circuit?

Wire resistance is calculated as R = rho x L / A, where rho is the resistivity of the conductor material (copper = 1.72 x 10^-8 ohm-m), L is wire length in meters, and A is the cross-sectional area in m squared.

What is the difference between watts, VA, and VAR?

In DC circuits, power in watts (W) = V x I. In AC circuits, apparent power in volt-amperes (VA) = Vrms x Irms, real power in watts = VA x power factor (cos phi), and reactive power in VAR = VA x sin phi.

Ohm’s Law Calculator — Voltage, Current, Resistance & Power | CalcEngines

Electronics Calculators · CalcEngines

Ohm’s Law & Power Calculator

Solve for voltage (V), current (I), resistance (R), and power (P) from any two known values. Includes series & parallel resistor analysis and wire resistance calculator.

V · I · R · P Unit conversion Live circuit Series & Parallel Wire resistance

Solve For

Input Method

Known Values

Enter two known values above — results update instantly.

Formula Reference

Voltage

V = I × R
V = P / I
V = √(P × R)

Volts (V). EMF drives current through resistance.

Current

I = V / R
I = P / V
I = √(P / R)

Amperes (A). Flow of charge through the circuit.

Resistance

R = V / I
R = V² / P
R = P / I²

Ohms (Ω). Opposition to current flow.

Power

P = V × I
P = I² × R
P = V² / R

Watts (W). Energy dissipated per second.

Live Circuit Diagram

Series Circuit — Updates in Real-Time

Results

Result

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Voltage V

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Current I

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Resistance R

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Power P

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Power Dissipated

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Watts

Energy / Hour

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Joules

Energy / Day

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Joules

Resistor Power Rating Guide

Package	Typical Rating	Derated (70%)	Common Use
0402 SMD	1/16 W (63 mW)	44 mW	Signal, low-power
0603 SMD	1/10 W (100 mW)	70 mW	Signal circuits
0805 SMD	1/8 W (125 mW)	88 mW	General purpose
1206 SMD	1/4 W (250 mW)	175 mW	General purpose
2512 SMD	1 W	700 mW	Power sensing
Through-hole 1/4W	250 mW	175 mW	Prototyping
Through-hole 1/2W	500 mW	350 mW	General purpose
Through-hole 1W	1 W	700 mW	Power circuits
Through-hole 2W	2 W	1.4 W	Power circuits
Wirewound 5W+	5–25 W	3.5–17.5 W	High-power loads

Ohm’s Law Wheel

Series Resistors

Apply Voltage / Current

Supply Voltage

Supply Current

Series rule: R_total = R₁ + R₂ + R₃ + …
Same current flows through every resistor. Voltage divides proportionally to resistance.

Results

Total Resistance

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Total Power

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Circuit Current

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Resistor Count

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Total Voltage

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Per-Resistor Breakdown

#	Value	Voltage Drop	Current	Power	% of Total

Parallel Resistors

Apply Voltage

Voltage Across Combination

Parallel rule: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + …
Same voltage across every resistor. Total current is the sum of individual branch currents.

Results

Total Resistance

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Total Power

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Total Current

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Conductance G

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Resistor Count

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Per-Branch Breakdown

#	Value	Voltage	Branch Current	Power	% of Total I

Wire Parameters

Wire Length (one-way)

Wire Diameter

Load Current

Source Voltage

Conductor Material

AWG Quick Reference

Common AWG wire gauges with cross-sectional areas, resistance per metre (copper), and typical current capacity.

AWG	Dia. (mm)	Area (mm²)	Ω/m (Cu)	Max Current
30	0.255	0.051	0.339	0.52 A
28	0.321	0.081	0.213	0.83 A
26	0.405	0.129	0.134	1.3 A
24	0.511	0.205	0.0842	2.1 A
22	0.644	0.325	0.0531	3.3 A
20	0.812	0.518	0.0334	5.3 A
18	1.024	0.823	0.0210	8.4 A
16	1.291	1.309	0.0132	13 A
14	1.628	2.081	0.00829	20 A
12	2.053	3.309	0.00521	32 A
10	2.588	5.261	0.00328	50 A

Results

Wire Resistance (return loop)

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One-Way Resistance

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Resistivity ρ

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Voltage Drop

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Power Loss

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Voltage at Load

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Volts

Efficiency

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V Drop %

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Resistivity Reference

Material	Resistivity (Ω·m)	vs Copper	Notes
Silver (Ag)	1.59 × 10⁻&sup8;	−7%	Best conductor
Copper (Cu)	1.72 × 10⁻&sup8;	Baseline	Standard wiring
Gold (Au)	2.44 × 10⁻&sup8;	+42%	Corrosion-resistant
Aluminium (Al)	2.82 × 10⁻&sup8;	+64%	Lightweight

What Is Ohm’s Law?

Ohm’s Law is the foundational relationship in electronics: V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. Formulated by Georg Simon Ohm in 1827, it states that the current through a conductor between two points is directly proportional to the voltage across those points and inversely proportional to the resistance.

From V = I × R, two more forms follow directly: I = V / R and R = V / I. Combined with the power formula P = V × I, you get 12 total relationships that allow you to calculate any of the four quantities — V, I, R, or P — from any two known values.

Practical tip: Always work in base SI units — volts (V), amperes (A), ohms (Ω), and watts (W). Convert mA to A by dividing by 1,000; kΩ to Ω by multiplying by 1,000. Mixing mA with kΩ will give incorrect results unless you account for unit scaling carefully.

Series and Parallel Resistor Combinations

When multiple resistors share the same current path, they are in series and their resistances add directly: R_total = R₁ + R₂ + R₃. The same current flows through every element; voltage divides in proportion to resistance.

When multiple resistors share the same two nodes, they are in parallel and the total resistance is lower than any individual resistor: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃. For just two resistors: R_total = (R₁ × R₂) / (R₁ + R₂).

Design rule: When adding resistors in parallel to reduce total resistance, the result is always lower than the smallest individual value. For equal resistors in parallel, R_total = R / N, where N is the count.

Power Dissipation and Resistor Ratings

Power dissipated in a resistor is P = I² × R = V² / R = V × I. A common mistake is selecting a resistor by value alone without checking power rating. A 100 Ω resistor passing 100 mA dissipates I² × R = 0.01 × 100 = 1 watt — a standard 1/4W through-hole resistor would overheat and fail. The standard derating rule is to use a resistor rated at at least 2× the calculated power for long-term reliability.

Safety warning: Resistors that exceed their power rating can become extremely hot, damaging nearby components or causing fire. Always verify power dissipation at maximum operating conditions, not just typical values.

Frequently Asked Questions

What is Ohm’s Law and what are the three formulas?

Ohm’s Law states that voltage, current, and resistance are related by V = I × R. The three equivalent forms are: V = I × R (voltage), I = V / R (current), and R = V / I (resistance). Combined with power P = V × I, you can derive 12 total formulas.

How do I calculate a resistor value for an LED?

Use R = (V_supply − V_f) / I_f. Example: 5V supply, 2.0V red LED, 20 mA → R = (5 − 2.0) / 0.020 = 150 Ω. Always verify the resistor’s power rating: P = (V_supply − V_f) × I_f.

How do I calculate two resistors in parallel?

For two resistors in parallel: R_total = (R1 × R2) / (R1 + R2). Example: 1 kΩ in parallel with 1 kΩ = 500 Ω. The result is always lower than the smallest individual resistor.

Why does power increase with the square of current?

From P = V × I and V = I × R, substituting gives P = I² × R. Doubling the current quadruples the heat generated. This is why wire sizing and fuse ratings are critical — undersized conductors overheat rapidly under excess current.

Does Ohm’s Law apply to AC circuits?

Ohm’s Law applies to AC circuits for purely resistive loads using RMS values. For reactive circuits (capacitors or inductors), resistance becomes impedance Z, a complex number where |Z| = √(R² + X²). This calculator applies to purely resistive DC or AC circuits.