Ohm's Law Calculator

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Ohm's Law Calculator — Enter Any 2 Values

Enter any two known values and the other two are calculated instantly. Lock two fields to keep them fixed, or choose from the 6 combinations below.

Known Values (pick any 2)

Voltage

Current

Resistance

Power

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Voltage Divider Calculator

Calculate the output voltage of a resistive voltage divider. V_out = V_in × R₂ / (R₁ + R₂)

Input Voltage (V_in)

Resistor 1 (R₁) — top

Resistor 2 (R₂) — bottom

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LED Resistor Calculator

Calculate the series resistor needed for an LED. R = (V_supply − V_forward) / I_forward

Supply Voltage

LED Forward Voltage (V_f)

LED Forward Current (I_f)

Typical LED: 10–30 mA (common: 20 mA)

Number of LEDs (series)

LEDs connected in series

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Energy & Cost Calculator

Calculate electrical energy consumption and cost. Energy (Wh) = Power (W) × Time (h)

Power

Time

Electricity Rate (per kWh)

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Ohm's Law Formula Wheel

The complete Ohm's Law wheel showing all 12 formulas relating V, I, R, and P.

All 12 Formulas

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

Ohm's Law Guide

What is Ohm's Law?

Ohm's Law states that the current through a conductor is proportional to the voltage and inversely proportional to the resistance: V = I × R. Named after German physicist Georg Ohm (1827), it is the fundamental law of electrical circuits.

The 4 Key Quantities

V — Voltage (Volts, V): Electrical pressure or potential difference driving current
I — Current (Amps, A): Rate of charge flow (electrons moving through conductor)
R — Resistance (Ohms, Ω): Opposition to current flow
P — Power (Watts, W): Rate of energy transfer = V × I

Common Unit Prefixes

Voltage: 1 kV = 1,000 V | 1 mV = 0.001 V
Current: 1 mA = 0.001 A | 1 μA = 0.000001 A
Resistance: 1 kΩ = 1,000 Ω | 1 MΩ = 1,000,000 Ω
Power: 1 kW = 1,000 W | 1 mW = 0.001 W

Real-World Examples

AA Battery: 1.5V, ~1A max → R = 1.5Ω internal
USB 5V charger: 5V, 2A = 10W power delivery
100W light bulb: 120V, 0.83A, 144Ω resistance
LED (typical): 2V forward, 20mA = 0.04W
Car battery: 12V, 500A cranking = 6,000W peak

Limits of Ohm's Law

Ohm's Law only applies to ohmic conductors (metals at constant temperature). It does NOT apply to:
• Semiconductors (diodes, transistors) — non-linear V-I relationship
• Light bulbs at high temperature — resistance increases with heat
• Electrolytes and ionized gases — complex behavior
• AC circuits — must use impedance (Z) instead of resistance