Ohm's Law Calculator
Enter any two known values and the calculator will solve for the other two. Works for voltage (V), current (I), resistance (R), and power (P).
Voltage
—
volts
Current
—
amps
Resistance
—
ohms (Ω)
Power
—
watts
Ohm's Law Formulas
Ohm's Law defines the relationship between voltage, current, and resistance in an electrical circuit:
$$V = I \times R$$
Combined with the power formula $P = V \times I$, you can derive 12 equations from just these two:
| Solve for | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| V (volts) | $V = I \times R$ | $V = P \div I$ | $V = \sqrt{P \times R}$ |
| I (amps) | $I = V \div R$ | $I = P \div V$ | $I = \sqrt{P \div R}$ |
| R (ohms) | $R = V \div I$ | $R = V^2 \div P$ | $R = P \div I^2$ |
| P (watts) | $P = V \times I$ | $P = I^2 \times R$ | $P = V^2 \div R$ |
The Ohm's Law Triangle
A handy memory aid: draw a triangle with V at the top, I on the bottom-left, and R on the bottom-right. Cover the unknown value and the remaining two show the formula — side by side means multiply, one over the other means divide.
Practical Examples
Example 1 — Find current through a resistor:
A 9 V battery is connected to a 470 Ω resistor. What current flows?
I = V ÷ R = 9 ÷ 470 = 0.0191 A (19.1 mA)
Example 2 — Find resistance for a target current:
You have a 12 V supply and want no more than 20 mA (0.020 A) through an LED circuit. What resistance?
R = V ÷ I = 12 ÷ 0.020 = 600 Ω (use 620 Ω standard value)
Example 3 — Find power dissipated in a resistor:
100 mA flows through a 100 Ω resistor. Power = I² × R = 0.1² × 100 = 1 watt
Use a resistor rated for at least 2 W for safety margin.
Important Notes
- Ohm's Law applies to resistive (ohmic) loads — resistors, heating elements, incandescent bulbs
- It does not directly apply to non-linear devices (diodes, LEDs, transistors) or reactive loads (capacitors, inductors)
- In AC circuits, use impedance (Z) instead of resistance for inductive or capacitive loads
- Power calculated here is real power (watts) for purely resistive DC circuits
Frequently Asked Questions
What are the units in Ohm's Law?
Voltage is measured in volts (V), current in amps (A) or milliamps (mA = 0.001 A), resistance in ohms (Ω), and power in watts (W). Always convert milliamps to amps (divide by 1,000) before using the formulas.
Does Ohm's Law work for AC circuits?
For purely resistive AC loads (toasters, electric heaters, incandescent bulbs), yes — Ohm's Law holds with RMS voltage and current values. For reactive loads (motors, transformers, capacitors), you must use impedance in place of resistance, and account for power factor.
What is a milliohm or megohm?
A milliohm (mΩ) is one-thousandth of an ohm — used for very low resistances like wire and contacts. A megohm (MΩ) is one million ohms — used for insulation resistance and high-impedance circuits.
