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 × R. This fundamental relationship allows you to calculate any one of the three quantities if you know the other two. It applies to resistive components in both DC and AC circuits (using RMS values for AC).

How do I calculate power using Ohm's Law?

Power (P) in watts can be calculated three ways: P = V × I (voltage times current), P = I² × R (current squared times resistance), or P = V² / R (voltage squared divided by resistance). All three formulas are equivalent and derived from P = V × I combined with Ohm's Law V = I × R.

How do I calculate series and parallel resistor combinations?

For series resistors, total resistance is simply 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, the simplified formula is Rtotal = (R1 × R2) / (R1 + R2).

How does wire resistance affect a circuit?

Wire resistance is calculated as R = ρ × L / A, where ρ is the resistivity of the conductor material (copper = 1.72×10⁻⁸ Ω·m), L is wire length in meters, and A is the cross-sectional area in m². Longer and thinner wires have higher resistance, causing voltage drop and power loss. This is why power cables use thick conductors and why long low-voltage runs need to be carefully engineered.

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

In DC circuits, power in watts (W) = V × I. In AC circuits, apparent power in volt-amperes (VA) = Vrms × Irms, real power in watts (W) = VA × power factor (cos φ), and reactive power in VAR = VA × sin φ. For purely resistive loads, power factor = 1 and watts = VA. Reactive components (capacitors and inductors) store and release energy without consuming it, resulting in reactive power.

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

Electronics Calculators

Ohm’s Law 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 = V × I · Series & Parallel · Wire Resistance

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Solve For

Enter Two Known Values

Current (I)

Resistance (R)

Power (P) — optional

Enter Two Known Values

Voltage (V)

Resistance (R)

Power (P) — optional

Enter Two Known Values

Voltage (V)

Current (I)

Power (P) — optional

Enter Two Known Values

Voltage (V)

Current (I)

Resistance (R) — optional

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 ¼W	250 mW	175 mW	Prototyping
Through-hole ½W	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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Resistors 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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Resistors 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 and their 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 (current equals voltage divided by resistance) and R = V / I (resistance equals voltage divided by current). 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 the 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. Series combinations increase total resistance and are used for current limiting, voltage dropping, and snubber circuits.

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). The same voltage appears across every branch; current divides inversely to resistance.

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. This technique is commonly used to achieve non-standard resistance values and to distribute power dissipation across multiple components.

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 ¼W 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 in normal conditions, or 4× for elevated-temperature environments.

Surface-mount resistors (0402, 0603, 0805) have lower power ratings than through-hole types but perform well when properly mounted on a PCB with adequate copper pours for heat spreading. Wirewound resistors offer the highest power ratings (2–100W+) but introduce inductance — avoid them in high-frequency signal paths.

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 to solve for any unknown from any two knowns.

How do I calculate resistor value for an LED?

Use R = (V_supply − V_f) / I_f. Substitute your supply voltage, the LED’s forward voltage (typically 2.0V red, 3.2V blue/white), and your desired current (usually 10–20 mA). Example: 5V supply, 2.0V red LED, 20 mA → R = (5 − 2.0) / 0.020 = 150 Ω. Use the nearest standard E12 value (150 Ω is already E12). Always check 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, the shortcut formula is R_total = (R1 × R2) / (R1 + R2). For example: 1 kΩ ∥ 1 kΩ = (1000 × 1000) / (1000 + 1000) = 500 Ω. For three or more, use: 1/R_total = 1/R1 + 1/R2 + 1/R3 + … The result is always lower than the smallest individual resistor.

How do I calculate power in an electrical circuit?

Power (watts) can be found three ways: P = V × I (most fundamental), P = I² × R (when you know current and resistance), or P = V² / R (when you know voltage and resistance). For AC circuits, use RMS values. Remember to check that components can handle the calculated power — resistors must be derated to at least 50% of their rated power for reliability.

What is a voltage divider and how does it work?

A voltage divider produces a fraction of the input voltage using two series resistors: V_out = V_in × R2 / (R1 + R2). The output is taken between R2 and ground. For example, with R1 = 10 kΩ, R2 = 4.7 kΩ, and V_in = 12V: V_out = 12 × 4700 / (10000 + 4700) = 3.84V. Voltage dividers are load-sensitive — always ensure your load resistance is much higher than R2.

How do I calculate wire resistance and voltage drop?

Wire resistance is R = ρ × L / A, where ρ is resistivity (copper = 1.72×10⁻⁸ Ω·m), L is total wire length (both conductors — there and back), and A is cross-sectional area in m². For a 10-metre 1mm diameter copper cable: A = π(0.0005)² = 7.85×10⁻⁷ m², R = 1.72×10⁻⁸ × 20 / 7.85×10⁻⁷ = 0.44 Ω. At 5A load: voltage drop = 5 × 0.44 = 2.2V.

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 (with capacitors or inductors), the relationship becomes V = I × Z, where Z is impedance (in ohms) — a complex number combining resistance and reactance. The magnitude |Z| = √(R² + X²), where X is the net reactance. Power in AC circuits is split into real power (watts), reactive power (VAR), and apparent power (VA).

Calculations are theoretical estimates based on ideal component models. Actual circuit performance depends on component tolerances, temperature, parasitic elements, PCB layout, and operating conditions. Always verify designs with measurements and apply appropriate safety margins.
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