EngineeringFormulaElectrical
How to Calculate Ohm's Law
Ohm's Law connects three basic circuit quantities: voltage, current, and resistance. In an ideal resistive circuit, any two of those values determine the third.
Final answer from the example
Current
2 A
Given value
Resistance
6 Ω
Given value
Voltage
12 V
Calculated with V = I × R
Key formulas
Solve for voltage
V = I × R
Use when current and resistance are known.
Solve for current
I = V / R
Use when voltage and resistance are known.
Solve for resistance
R = V / I
Use when voltage and current are known. Current must be nonzero.
Related power relationship
P = V × I
Ohm's Law is often paired with electrical power calculations, but power is a separate quantity.
Variables and units
| Symbol | Meaning | Typical units |
|---|---|---|
| V | Voltage / potential difference | V, mV, kV |
| I | Current | A, mA |
| R | Resistance | Ω, kΩ, MΩ |
| P | Electrical power | W |
Quick reference conversions
Voltage form
V = I × R
Current times resistance.
Current form
I = V / R
Voltage divided by resistance.
Resistance form
R = V / I
Voltage divided by current.
1 kilo-ohm
1 kΩ = 1,000 Ω
Common resistor unit conversion.
1 milliamp
1 mA = 0.001 A
Convert mA to A before calculating.
Ohmic behavior
linear V-I
Current is proportional to voltage for constant resistance.
Step-by-step solved example
Example problem
A resistor has R = 6 Ω and the measured current is I = 2 A. Find the voltage across the resistor.
1. Identify the known values
Current is known: I = 2 A. Resistance is known: R = 6 Ω. Voltage is the unknown.
2. Choose the correct Ohm's Law form
Because voltage is unknown and current and resistance are known, use V = I × R.
3. Substitute the numbers
V = 2 A × 6 Ω.
4. Calculate the voltage
V = 12 V. The resistor has a 12-volt potential difference across it.
5. Check the answer another way
If V = 12 V and R = 6 Ω, then I = V / R = 12 / 6 = 2 A, which matches the given current.
Practical field notes
Units matter more than the algebra
Ohm's Law is simple, but unit mistakes are common. Convert milliamps to amps and kilo-ohms to ohms before multiplying or dividing.
The relationship is linear only for ohmic loads
A fixed resistor is close to ideal. Diodes, LEDs, lamps, transistors, motors, and many electronic loads can have resistance that changes with voltage, current, temperature, or operating point.
AC circuits may need impedance
For purely resistive AC loads, Ohm's Law still works with RMS values. For inductors and capacitors, use impedance rather than plain resistance.
Power and heating are separate checks
After finding voltage or current, check power if component heating matters. For example, P = V × I = 12 × 2 = 24 W in this worked example.
Common mistakes to avoid
- Entering milliamps as amps without converting first, such as treating 20 mA as 20 A instead of 0.020 A.
- Using kΩ as Ω without multiplying by 1,000, such as treating 4.7 kΩ as 4.7 Ω instead of 4,700 Ω.
- Trying to solve R = V / I when current is zero.
- Applying plain resistance formulas to reactive AC circuits that need impedance.
- Assuming non-ohmic components like LEDs obey V = I × R across all operating points.
When to use the calculator instead
Use the calculator when you want to solve any of the three forms quickly, compare units such as mA vs A or kΩ vs Ω, or verify a resistor/current/voltage relationship without hand rearranging the equation.
Calculation FAQs
What is Ohm's Law?
Ohm's Law states that voltage equals current times resistance: V = I × R. Rearranging gives I = V / R and R = V / I.
How do I calculate current using Ohm's Law?
Divide voltage by resistance. For example, 12 V across 6 Ω gives I = 12 / 6 = 2 A.
How do I calculate resistance?
Divide voltage by current. For example, 9 V and 0.5 A gives R = 9 / 0.5 = 18 Ω.
Does Ohm's Law work for LEDs?
Not as a simple fixed-resistance model. LEDs are non-ohmic devices, so their current changes nonlinearly with voltage and usually needs a resistor or driver model.
Does Ohm's Law apply to AC circuits?
It applies directly to purely resistive AC loads using RMS voltage and current. Circuits with inductance or capacitance require impedance.
References
- Ohm's law
Khan Academy
Explains I = V / R and the linear current-voltage relationship for ohmic elements.
- Resistance and Ohm's Law
OpenStax Physics
Describes resistance and the experimental relationship V = IR.
- Ohm's Law - How Voltage, Current, and Resistance Relate
All About Circuits
Shows the Ohm's Law equation and its rearranged forms for voltage, current, and resistance.