When does Ohm's Law not apply?

Non-linear / non-ohmic devices: diodes, LEDs, transistors, incandescent bulbs (resistance changes with temperature). For those, use the device's I-V curve or datasheet rather than V = IR.

Calculate dropping resistor for an LED?

Use R + P known. R = (V_supply − V_LED) / I_LED. For a 3.2V/20mA LED from 5V: R = (5 − 3.2) / 0.020 = 90 Ω. Power dissipated in the resistor = (V_supply − V_LED) × I_LED = 36 mW, so a 1/4 W resistor is fine.

DC, or AC with purely resistive loads. AC circuits with capacitors/inductors involve impedance (Z) and phase — that's a separate calculation with complex numbers.

Why is one of my fields disabled?

It's part of the 'unknown' pair. The calculator works from two knowns to two unknowns. Switch the radio buttons at top to change which pair you're solving for.

Ohm's Law Calculator

Ohm's Law (V = I × R) combined with the power equation (P = V × I) lets you solve any electrical circuit value from any two others. This calculator handles all six combinations: V+I, V+R, V+P, I+R, I+P, R+P. Pick the pair you know, enter values with sensible unit prefixes (mV, A, kΩ, mW, etc), and the other two appear with the formula used. Useful for resistor sizing, LED current limiting, power supply calculations, and basic electronics homework.

—

Known pair

Voltage (V)

Current (I)

Resistance (R)

Power (P)

12 V

0.12 A

100 Ω

1.44 W

Formula: I = V / R, P = V² / R

All four values are computed in base SI units (V, A, Ω, W). Display uses the unit prefix you picked. Highlighted outputs are the computed pair.

How to use

Pick which two values you know (the highlighted pair).
Enter their magnitudes and units.
Read the computed pair and the formula used. Copy the full V/I/R/P summary.

Frequently asked questions

When does Ohm's Law not apply?: Non-linear / non-ohmic devices: diodes, LEDs, transistors, incandescent bulbs (resistance changes with temperature). For those, use the device's I-V curve or datasheet rather than V = IR.
Calculate dropping resistor for an LED?: Use R + P known. R = (V_supply − V_LED) / I_LED. For a 3.2V/20mA LED from 5V: R = (5 − 3.2) / 0.020 = 90 Ω. Power dissipated in the resistor = (V_supply − V_LED) × I_LED = 36 mW, so a 1/4 W resistor is fine.
Is this AC or DC?: DC, or AC with purely resistive loads. AC circuits with capacitors/inductors involve impedance (Z) and phase — that's a separate calculation with complex numbers.
Why is one of my fields disabled?: It's part of the 'unknown' pair. The calculator works from two knowns to two unknowns. Switch the radio buttons at top to change which pair you're solving for.

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