Ohm's Law Calculator

Solve voltage, current, resistance, or power

Target Variable

Required Parameters

Current

Resistance

Ohm

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Quick Answer

Use Ohm's Law (V = I × R) to calculate voltage, current, resistance, or power. It is the fundamental equation for all DC circuit analysis and resistive loads.

Documentation

Ohm's Law Calculator — Complete Engineering Reference

Use this interactive Ohm's Law calculator to instantly solve for voltage (V), current (I), resistance (R), or power (P) in any DC circuit. Enter any two known values, and the tool will compute the remaining parameters.

The Ohm's Law Formulas

The fundamental equation is V = I × R. From this base equation, three primary formulas can be derived:

Solve For	Formula	Use Case
Voltage	V = I × R	Find voltage drop across a known resistor
Current	I = V / R	Determine current draw from a power supply
Resistance	R = V / I	Size a resistor for a target current

Power Formulas (Joule's Law Extension)

Power dissipation is directly linked to Ohm's Law through P = V × I. By substituting Ohm's Law, additional power formulas emerge:

Formula	Best Used When You Know
P = V × I	Voltage and Current
P = I² × R	Current and Resistance
P = V² / R	Voltage and Resistance

Engineering Note (per IEC 60083): Always select a resistor with a power rating at least 50% higher than the calculated dissipation to ensure reliability and prevent thermal runaway.

The Ohm's Law Wheel (Magic Triangle)

The Ohm's Law wheel is a circular chart that shows all 12 possible formulas relating V, I, R, and P. It is divided into four quadrants:

Voltage (V): V = I×R, V = P/I, V = √(P×R)
Current (I): I = V/R, I = P/V, I = √(P/R)
Resistance (R): R = V/I, R = P/I², R = V²/P
Power (P): P = V×I, P = I²×R, P = V²/R

To use it: find the variable you want to solve in the center, then use any formula in that quadrant based on the two variables you know.

Practical Applications

Sizing a Current-Limiting Resistor

To limit current through an LED:

Determine the supply voltage (e.g., 5V)
Find the LED forward voltage (e.g., 2V for red)
Choose the desired current (e.g., 20 mA)
Calculate: R = (5V − 2V) / 0.02A = 150 Ω
Round up to the nearest standard value: 220 Ω → Use the nearest E24 value

Reading a Sensor

A 4–20 mA current loop sensor with a 250 Ω sense resistor:

At 4 mA: V = 0.004 × 250 = 1.0V
At 20 mA: V = 0.020 × 250 = 5.0V
This maps the sensor range perfectly to a 1–5V ADC input

Verifying a Ground Resistor

When measuring a ground resistor with a multimeter, the expected voltage drop is V = I × R. If the measured voltage doesn't match, suspect a cold solder joint or a wrong value resistor — a standard 1/4W resistor can carry up to 25 mA at full wattage.

Impedance vs. Resistance

For AC circuits, Ohm's Law extends to impedance:

Z = √(R² + X²) where X is reactance
V = I × Z replaces V = I × R for AC
Use our Reactance Calculator for capacitive (Xc) and inductive (XL) calculations

Common Pitfalls

Unit Mismatch — The most frequent error. Always convert to base units (V, A, Ω) before calculating. 1 kΩ = 1000 Ω, 1 mA = 0.001 A.
Exceeding Power Ratings — A mathematically correct resistor value can still fail if the power dissipation exceeds the rating. Always verify P = I²R.
Non-Ohmic Devices — Ohm's Law does not apply directly to diodes, LEDs, transistors, or other semiconductor devices.
Temperature Effects — All resistors change value with temperature. Metal film resistors (±50 ppm/°C) are more stable than carbon composition (±500 ppm/°C).
Internal Resistance of Sources — Real batteries and power supplies have internal resistance, causing voltage drop under load.

Industry Standards

IEC 60083 — Plugs and socket-outlets for domestic and similar general use
IEC 60115-1 — Fixed resistors for use in electronic equipment
MIL-PRF-55342 — Fixed resistors, chip, established reliability
EIA Publication 330 — The International Electrotechnical Commission standard for resistor marking

Design Notes

Ohm's Law applies precisely to Ohmic materials where resistance remains constant regardless of current or voltage. In practical engineering, resistors heat up when dissipating power (P = I²R), which can alter their resistance (Temperature Coefficient of Resistance). Always verify that your calculated power dissipation is below the resistor's actual wattage rating (typically leave a 50% margin).

Common Mistakes

1
Mixing up units: Forgetting to convert milli-amps (mA) to Amps (A) before calculating.
2
Ignoring power dissipation: A 1/4W resistor will burn up if your calculation yields 0.5W, even if the resistance is correct.
3
Applying it to non-linear components: Ohm's law does not directly apply to diodes, LEDs, or transistors which have dynamic resistance.

Engineering Handbox

1. Identify knowns: V = 12, R = 470 2. Use Formula: I = V / R 3. Calculate: I = 12 / 470 = 0.02553 A (25.53 mA) 4. Calculate Power: P = 12 × 0.02553 = 0.306 W

VerificationCurrent is 25.53 mA and Power is 0.306 W (Requires a 1/2W resistor).

Knowledge Base

What are the 3 main formulas for Ohm's Law?

V = I × R (Voltage), I = V / R (Current), and R = V / I (Resistance). These three formulas are algebraic rearrangements of the same fundamental relationship discovered by Georg Ohm in 1827.

How do I use the Ohm's Law magic triangle?

Place V at the top, I at the bottom-left, and R at the bottom-right. Cover the variable you want to solve: if V is covered, multiply I × R. If I is covered, divide V ÷ R. If R is covered, divide V ÷ I.

Can Ohm's Law be used for AC circuits?

Only for purely resistive AC loads (like heating elements). For circuits with capacitors or inductors, replace Resistance (R) with Impedance (Z), where Z = √(R² + X²). The modified form becomes V = I × Z.

How do I calculate power using Ohm's Law?

Power (P) in watts can be calculated three ways: P = V × I (voltage × current), P = I² × R (current squared × resistance), or P = V² / R (voltage squared ÷ resistance). Choose the formula based on your two known values.

What is 1 kiloohm in ohms?

1 kΩ equals 1,000 ohms. Common prefixes: 1 MΩ = 1,000,000 Ω, 1 kΩ = 1,000 Ω, 1 mΩ = 0.001 Ω. This calculator handles engineering notation like '4k7' (4,700 Ω) and '10u' automatically.

Why doesn't Ohm's Law work for LEDs and diodes?

LEDs and diodes are non-ohmic (non-linear) devices — their resistance changes with voltage and current. They have a fixed forward voltage drop (e.g., ~2V for red LEDs) that doesn't follow V = IR. Use our LED Resistor Calculator for proper LED circuit design.

What happens if I exceed a resistor's power rating?

The resistor overheats, potentially catching fire or failing open-circuit. Always select a resistor with a wattage rating at least 50% above the calculated power dissipation (P = I²R). For example, if P = 0.3W, use a 1/2W resistor minimum.

How do I convert milliamps (mA) to amps (A) for Ohm's Law?

Divide by 1,000. For example, 250 mA = 0.250 A. Similarly, microamps (µA) divide by 1,000,000. Always convert to base units (Volts, Amps, Ohms) before plugging values into V = IR.

What is the difference between resistance and impedance?

Resistance (R) opposes current flow in DC circuits and is purely real (measured in Ohms). Impedance (Z) is the AC equivalent and includes both resistance and reactance (from capacitors/inductors). Impedance is a complex number: Z = R + jX.

Who discovered Ohm's Law and when?

Georg Simon Ohm, a German physicist, published the law in 1827 in his treatise 'Die galvanische Kette, mathematisch bearbeitet'. The unit of electrical resistance, the Ohm (Ω), is named in his honor and was adopted as an SI unit in 1881.

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