Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R): V = I × R. This fundamental relationship in electronics means if you increase voltage while keeping resistance constant, current increases proportionally. If you increase resistance while keeping voltage constant, current decreases. The law was discovered by German physicist Georg Ohm in 1827 and applies to most resistive circuits.

How do I calculate resistance using the ohm law calculator?

To find resistance, select 'Resistance (R)' as the solve-for option, then enter voltage and current. Formula: R = V / I. Example: 12V source with 0.5A current = 12 / 0.5 = 24 ohms. For an LED with 5V supply and 20mA (0.02A) forward current: R = 5 / 0.02 = 250 ohms. Use the nearest standard resistor value (270 or 220 ohms).

How do I calculate power in an electrical circuit?

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). Example: a 12V circuit with 2A draws P = 12 × 2 = 24 watts. A 100-ohm resistor at 10V dissipates P = 100 / 100 = 1 watt. Always check the power rating of resistors to prevent overheating.

What is the difference between voltage, current, and resistance?

Voltage (V, volts) is the electrical pressure or potential difference that drives current through a circuit — like water pressure in a pipe. Current (I, amperes) is the flow rate of electric charge — like the volume of water flowing. Resistance (R, ohms) opposes the flow of current — like pipe friction. Higher resistance means less current for the same voltage. Power (P, watts) = voltage × current = energy consumed per second.

How do I calculate the correct resistor for an LED?

To calculate the series resistor for an LED: R = (Vsupply - Vforward) / Iforward. A typical red LED has Vforward ≈ 2V and needs Iforward = 20mA (0.02A). With a 5V supply: R = (5 - 2) / 0.02 = 150 ohms. Use the nearest standard value (150Ω or 220Ω). Power dissipated in the resistor: P = (Vsupply - Vforward) × Iforward = 3 × 0.02 = 0.06W. A 1/4W resistor is sufficient.

What are the limitations of Ohm's Law?

Ohm's Law applies to linear (ohmic) resistors where resistance is constant regardless of voltage or current. It does not apply to non-linear devices such as diodes (resistance varies with forward voltage), transistors (controlled by base current), LEDs (threshold voltage behavior), batteries (internal resistance changes with state of charge), or components at extreme temperatures. In AC circuits, impedance (Z) replaces resistance to account for capacitive and inductive reactance.

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Ohm's Law Calculator

Ohm's Law: V = I × R (voltage = current × resistance). Power formulas: P = V × I = I² × R = V² / R. To solve for any variable, you need two known values. Example: 12V source, 24Ω resistor → I = 12/24 = 0.5A, P = 12 × 0.5 = 6W. LED example: 5V supply, 20mA current → R = 5/0.02 = 250Ω (use 270Ω standard). Unit prefixes: k = ×1000 (kΩ), m = ÷1000 (mA, mW), μ = ÷1,000,000 (μA).

Calculate voltage, current, resistance, or power using Ohm's Law (V=IR) and the power formula (P=IV). Solve for any variable given the other two. Includes NAND, LED, and common circuit presets.

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Solve For

Presets

Known Values (enter any 2 of the 3 remaining)

Voltage (V) — volts

Current (I) — amperes

Resistance (R) — ohms (Ω)

Power (P) — watts

Select what to solve for, then enter any 2 of the remaining values to calculate.

How to Use

1
Select what to solve for
Click Voltage (V), Current (I), Resistance (R), or Power (P) to choose your unknown. The selected field becomes read-only and will show the calculated result.
2
Enter two known values
Type any 2 of the 3 remaining values. For example, to find resistance: enter voltage (V) and current (I). Values in standard units: volts, amperes, ohms, watts.
3
View all results
The calculator immediately shows all 4 values — voltage, current, resistance, and power — with automatic unit formatting (kΩ for large values, mA for small currents).
4
Copy results
Click Copy to get all 4 values as formatted text. Use presets (LED, light bulb, USB device) for quick common circuit calculations.

Frequently Asked Questions

What is Ohm's Law?: Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R): V = I × R. This fundamental relationship in electronics means if you increase voltage while keeping resistance constant, current increases proportionally. If you increase resistance while keeping voltage constant, current decreases. The law was discovered by German physicist Georg Ohm in 1827 and applies to most resistive circuits.
How do I calculate resistance using the ohm law calculator?: To find resistance, select 'Resistance (R)' as the solve-for option, then enter voltage and current. Formula: R = V / I. Example: 12V source with 0.5A current = 12 / 0.5 = 24 ohms. For an LED with 5V supply and 20mA (0.02A) forward current: R = 5 / 0.02 = 250 ohms. Use the nearest standard resistor value (270 or 220 ohms).
How do I calculate power in an electrical circuit?: 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). Example: a 12V circuit with 2A draws P = 12 × 2 = 24 watts. A 100-ohm resistor at 10V dissipates P = 100 / 100 = 1 watt. Always check the power rating of resistors to prevent overheating.
What is the difference between voltage, current, and resistance?: Voltage (V, volts) is the electrical pressure or potential difference that drives current through a circuit — like water pressure in a pipe. Current (I, amperes) is the flow rate of electric charge — like the volume of water flowing. Resistance (R, ohms) opposes the flow of current — like pipe friction. Higher resistance means less current for the same voltage. Power (P, watts) = voltage × current = energy consumed per second.
How do I calculate the correct resistor for an LED?: To calculate the series resistor for an LED: R = (Vsupply - Vforward) / Iforward. A typical red LED has Vforward ≈ 2V and needs Iforward = 20mA (0.02A). With a 5V supply: R = (5 - 2) / 0.02 = 150 ohms. Use the nearest standard value (150Ω or 220Ω). Power dissipated in the resistor: P = (Vsupply - Vforward) × Iforward = 3 × 0.02 = 0.06W. A 1/4W resistor is sufficient.
What are the limitations of Ohm's Law?: Ohm's Law applies to linear (ohmic) resistors where resistance is constant regardless of voltage or current. It does not apply to non-linear devices such as diodes (resistance varies with forward voltage), transistors (controlled by base current), LEDs (threshold voltage behavior), batteries (internal resistance changes with state of charge), or components at extreme temperatures. In AC circuits, impedance (Z) replaces resistance to account for capacitive and inductive reactance.

Ohm's Law Formula Wheel

Solve For	Formula 1	Formula 2	Formula 3
Voltage (V)	V = I × R	V = P / I	V = sqrt(P × R)
Current (I)	I = V / R	I = P / V	I = sqrt(P / R)
Resistance (R)	R = V / I	R = V² / P	R = P / I²
Power (P)	P = V × I	P = V² / R	P = I² × R

Common Component Voltage and Current Ratings

Component	Typical Voltage	Typical Current	Notes
Red/Yellow LED	1.8–2.2V forward	20mA	Need series resistor
Blue/White/Green LED	3.0–3.5V forward	20mA	Higher forward voltage
USB 2.0	5V	Up to 500mA	2.5W max
USB-C PD	5–20V	Up to 5A	100W max
AA Battery	1.5V	~1A max	Internal resistance ~1Ω
9V Battery	9V	~500mA max	Internal resistance ~2Ω
Arduino 5V Pin	5V	200mA max	Shared with USB
Raspberry Pi 3.3V	3.3V	50mA max	GPIO pin limit