Ohm's Law Calculator Formula
Understand the math behind the ohm's law calculator. Each variable explained with a worked example.
Formulas Used
Resistance
resistance_ohm = voltage_v / current_aPower
power_w = voltage_v * current_aVariables
| Variable | Description | Default |
|---|---|---|
voltage_v | Voltage(V) | 12 |
current_a | Current(A) | 0.5 |
How It Works
How Ohm's Law Works
Ohm's Law links voltage, current, and resistance.
Formula
V = I x R
Where:
Rearranging gives R = V / I and I = V / R. Power is P = V x I.
Worked Example
A 12 V battery drives 0.5 A through a resistor.
- 01Resistance R = V / I = 12 / 0.5 = 24 ohms
- 02Power P = V x I = 12 x 0.5 = 6 W
When to Use This Formula
- Selecting the correct resistor value for an LED circuit so the current stays within the LED's rated maximum.
- Determining the wire gauge needed for a circuit by calculating the current that will flow at a given voltage and load resistance.
- Troubleshooting a circuit board where a component is overheating — calculating expected vs. actual current draw reveals whether resistance has changed due to a fault.
- Sizing a power supply by calculating total current demand from known voltage and resistance values across all connected loads.
- Calculating power dissipation (P = IV or P = I²R) in a resistor to ensure it won't exceed its wattage rating and burn out.
Common Mistakes to Avoid
- Applying Ohm's law to non-ohmic components like diodes or transistors — these have non-linear voltage-current relationships, so V = IR only applies to resistive elements.
- Confusing series and parallel resistance — in series, resistances add directly (R_total = R1 + R2), but in parallel, you must use the reciprocal formula (1/R_total = 1/R1 + 1/R2).
- Forgetting to account for internal resistance of the power source, which causes the actual terminal voltage to be lower than the rated voltage under load.
- Using peak voltage instead of RMS voltage in AC circuits — Ohm's law with AC uses RMS values, and using peak values will overestimate current by a factor of about 1.41.
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that current is proportional to voltage and inversely proportional to resistance: V = I x R.
When does Ohm's Law not apply?
It does not apply to nonlinear devices like diodes and transistors where resistance changes with voltage or current.
How do I find current if I know voltage and resistance?
Rearrange to I = V / R. For example, 12 V across 24 ohms gives 0.5 A.
Learn More
Guide
Ohm's Law Guide
A complete guide to Ohm's Law. Learn the relationship between voltage, current, and resistance, with practical examples, the power triangle, and circuit analysis tips.
Ready to run the numbers?
Open Ohm's Law Calculator