Ohm's Law Calculator
Calculate voltage, current, resistance, and power using V=IR and P=VI
Calculate voltage from current and resistance.
Quick Reference:
- V = IR: Voltage (V) = Current (A) × Resistance (Ω)
- I = V/R: Current (A) = Voltage (V) / Resistance (Ω)
- R = V/I: Resistance (Ω) = Voltage (V) / Current (A)
- P = VI: Power (W) = Voltage (V) × Current (A)
- P = I²R: Power (W) = Current² × Resistance
- P = V²/R: Power (W) = Voltage² / Resistance
Understanding Ohm's Law
Ohm's Law is the fundamental relationship in electrical circuits, discovered by German physicist Georg Ohm in 1827. It states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance.
The Basic Formula: V = IR
The formula V = IR relates voltage (V in volts), current (I in amperes), and resistance (R in ohms). This can be rearranged to solve for any variable: I = V/R or R = V/I.
Power Calculations
Electrical power can be calculated using several equivalent formulas:
- P = VI - Power equals voltage times current
- P = I²R - Power equals current squared times resistance
- P = V²/R - Power equals voltage squared divided by resistance
The Ohm's Law Wheel
The Ohm's Law wheel (or pie chart) is a useful tool that shows all twelve formulas relating V, I, R, and P. Given any two values, you can calculate the other two using the appropriate formulas.
Practical Applications
- LED circuits: Calculate resistor values for current limiting
- Power consumption: Determine energy usage of devices
- Wire sizing: Calculate voltage drop over wire lengths
- Circuit design: Design voltage dividers and loads
Unit Prefixes
Electrical values often use metric prefixes for convenience:
- Milliamps (mA): 1 mA = 0.001 A
- Microamps (μA): 1 μA = 0.000001 A
- Kilohms (kΩ): 1 kΩ = 1,000 Ω
- Megohms (MΩ): 1 MΩ = 1,000,000 Ω
- Milliwatts (mW): 1 mW = 0.001 W
- Kilowatts (kW): 1 kW = 1,000 W
Common Examples
- USB charger: 5V, 2A = 10W power output
- LED with 220Ω resistor at 5V: ~23mA current
- 100W light bulb at 120V: ~0.83A, 144Ω
Worked Example: Sizing an LED Resistor
Suppose you want to run a red LED (forward voltage 2 V, target current 20 mA) from a 5 V supply. The resistor must drop 5 − 2 = 3 V at 0.02 A, so R = V/I = 3 / 0.02 = 150 Ω. Power dissipated in the resistor is P = I²R = 0.02² × 150 = 0.06 W, well within a standard ¼-watt part. This one calculation uses both the resistance and power modes of the calculator and is the single most common Ohm's Law task in hobby electronics.
Frequently Asked Questions
How do I use Ohm's Law to find voltage, current, or resistance?+
Ohm's Law relates the three quantities as V = I × R. Given any two, the third follows: current is I = V/R and resistance is R = V/I. Pick the matching tab in the calculator, enter the two known values with their units, and the result appears instantly.
How is electrical power calculated from Ohm's Law values?+
Power in watts can be found three equivalent ways: P = V × I, P = I² × R, or P = V² / R. Which formula you use depends on which pair of values you know — for example, a 12 V supply across a 6 Ω load dissipates P = 144/6 = 24 W. The calculator's power tab accepts any valid pair.
What is the Ohm's Law wheel shown in the calculator?+
The wheel is a reference diagram containing all twelve formulas that connect voltage, current, resistance, and power. Given any two of the four quantities, the wheel shows the formula that yields each of the other two — the calculator's Wheel tab computes all of them at once from any two inputs.
Does Ohm's Law apply to every electrical component?+
No. It holds for ohmic components such as resistors and most metallic conductors at constant temperature. Diodes, LEDs, transistors, and devices like incandescent filaments (whose resistance changes with temperature) are non-ohmic, so V = IR only describes them approximately at a single operating point.
Is this Ohm's Law calculator free and usable on mobile?+
Yes. It is completely free with no sign-up, and all math runs in your browser so it responds instantly and sends nothing to a server. The responsive design works on phones, which is handy at the workbench or in the field.
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