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Ohm's Law Calculator

Calculate voltage, current, and resistance with the fundamental V=IR equation

Current (I in Amps)

Resistance (R in Ohms)

Voltage (V)

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Volts

Reviewed by Dr. Michael Torres, Ph.D.

Electrical Engineering Specialist | Circuit Analysis Expert

Last Updated: November 24, 2025

Understanding Ohm's Law

Ohm's Law is the most fundamental relationship in electrical engineering and electronics - if you understand nothing else about circuits, master this. Named after German physicist Georg Ohm who discovered it in 1827, this deceptively simple equation underpins virtually every electronic device you use daily.

V = I × R
where V = Voltage (volts), I = Current (amperes), R = Resistance (ohms)

The Three Variables

Voltage (V) is electrical pressure - the force that pushes electrons through a circuit. Think of it like water pressure in a pipe. Measured in volts (V), it represents the potential energy difference between two points. A 9V battery provides 9 volts of electrical pressure.

Current (I) is the flow rate of electric charge. Like gallons per minute for water, current measures how many electrons flow through a point per second. Measured in amperes or amps (A), 1 ampere equals 6.24×10¹⁸ electrons passing a point each second. A typical phone charger provides 1-2.4 amps.

Resistance (R) opposes current flow. Like friction in a pipe, resistance determines how hard voltage must "push" to achieve a given current. Measured in ohms (Ω), higher resistance means less current for the same voltage. A resistor's value determines how much it restricts flow.

The Three Forms of Ohm's Law

Ohm's Law can be rearranged to solve for any unknown variable:

V = I × R - Find voltage when you know current and resistance
I = V / R - Find current when you know voltage and resistance
R = V / I - Find resistance when you know voltage and current

These aren't three different laws - they're three sides of the same fundamental relationship. Many students memorize the "magic triangle" with V at top, I and R at bottom. Cover what you're solving for, and the remaining variables show the equation.

Real-World Example: LED Current Limiting

You want to power a red LED (requires 20mA at 2V) from a 9V battery. What resistor do you need?

Step-by-step calculation:

Voltage drop across resistor = 9V - 2V = 7V
Desired current = 20mA = 0.02A
R = V / I = 7V / 0.02A = 350Ω
Use standard 330Ω or 390Ω resistor

This is one of the most common Ohm's Law applications - every LED circuit uses this calculation to prevent burning out the LED by limiting current.

Power Calculations

Combine Ohm's Law with power equations to analyze energy consumption:

P = V × I (power equals voltage times current)
P = I² × R (substitute V=IR → P=I×I×R)
P = V² / R (substitute I=V/R → P=V×V/R)

Example: A 100Ω resistor with 10V across it dissipates P = 10²/100 = 1 watt of heat. If your resistor is rated for 0.25W, it will burn up - you need at least a 2W resistor for safety margin.

💡 Expert Tips from Dr. Torres

Temperature Changes Everything: A rookie mistake is forgetting that resistance changes with temperature for most materials. A tungsten lightbulb filament at room temperature might be 12Ω, but when hot and glowing, it's 144Ω - a 12-fold increase! This is why lightbulbs draw huge inrush current when first switched on. For precision work, use temperature coefficients (ppm/°C) to compensate.

AC Requires Impedance, Not Resistance: Ohm's Law in its pure V=IR form only applies to DC circuits. For AC, you must use V=IZ where Z is impedance (combining resistance, capacitive reactance, and inductive reactance). Treating AC impedance as pure resistance leads to completely wrong calculations in circuits with capacitors or inductors.

Parallel Resistance Trap: When resistors are in parallel, total resistance is NOT R₁+R₂. It's 1/Rₜₒₜₐₗ = 1/R₁ + 1/R₂. For example, two 100Ω resistors in parallel give 50Ω total, not 200Ω. I see experienced engineers make this error under time pressure.

⚠️ Common Mistakes to Avoid

Unit Conversion Errors: Forgetting to convert milliamps (mA) to amps (A) or kilohms (kΩ) to ohms (Ω) before calculations is the #1 error I see. 20mA is 0.02A, not 20A. Always convert to base units (V, A, Ω) first.
Assuming Ohm's Law Applies to Everything: Diodes, LEDs, transistors, and other non-linear components do NOT obey Ohm's Law. Current doesn't scale linearly with voltage for these devices. You must use their specific I-V characteristic curves.
Misapplying to Entire Circuits: Ohm's Law applies to individual components, not whole circuits unless they're purely resistive. You can't measure the total voltage and current of a complex circuit and divide to get "the resistance." You must analyze each element.
Ignoring Power Ratings: Calculating the right resistance is useless if the resistor burns up. Always check power dissipation (P=I²R or P=V²/R) against the component's wattage rating. Use at least 50% safety margin - a 0.5W calculation needs a 1W resistor minimum.

When Ohm's Law Doesn't Apply

Ohm's Law is a physical law for ohmic conductors, but many materials and devices are non-ohmic:

Diodes & LEDs: Exponential I-V relationship, not linear
Transistors: Controlled current sources, complex behavior
Thermistors: Resistance changes dramatically with temperature
Arc lamps: Negative resistance region where current increases decrease voltage
Superconductors: Zero resistance below critical temperature

Always verify a component is ohmic before applying V=IR. Check the datasheet's I-V curve - if it's a straight line through the origin, it's ohmic. Any curve indicates non-ohmic behavior requiring different analysis.

Practical Circuit Troubleshooting

Ohm's Law is invaluable for diagnosing circuit problems. If a circuit isn't working:

Measure voltage with a multimeter - should match calculated V=IR
Measure current - if much higher than expected, check for shorts (low resistance path)
Measure current - if much lower than expected, check for opens (infinite resistance)
Measure resistance - compare to theoretical value to find damaged components

Example: A 12V fan motor drawing 0.5A should have R=12/0.5=24Ω. If you measure 6Ω, internal windings are partially shorted. If you measure 100Ω, there's high resistance due to corrosion or damage.

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Frequently Asked Questions

What is Ohm's Law and what does V=IR mean?

Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R), written as V=IR. This fundamental electrical relationship means that the voltage across a conductor is directly proportional to the current flowing through it, with resistance as the constant of proportionality. Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω). For example, if 2A flows through a 10Ω resistor, the voltage is 2×10=20V.

How do you calculate voltage, current, or resistance using Ohm's Law?

Ohm's Law can be rearranged to solve for any variable: V=IR (voltage), I=V/R (current), or R=V/I (resistance). To find voltage, multiply current by resistance. To find current, divide voltage by resistance. To find resistance, divide voltage by current. For instance, if you have 12V and 4Ω, current=12/4=3A. Always ensure units are consistent: voltage in volts, current in amperes, resistance in ohms.

When does Ohm's Law not apply?

Ohm's Law applies to ohmic materials (conductors with constant resistance), but fails for non-ohmic devices like diodes, transistors, lightbulbs (whose resistance changes with temperature), and superconductors. It also doesn't work for AC circuits with reactive components (capacitors/inductors) without considering impedance. Additionally, at very high frequencies or in non-linear materials, the simple V=IR relationship breaks down. Always verify whether your component follows Ohm's Law before applying it.

What's the difference between resistance and impedance?

Resistance (R) is opposition to current flow in DC circuits and applies to pure resistors. Impedance (Z) is the more general term for AC circuits, combining resistance with reactance from capacitors and inductors. While Ohm's Law uses V=IR for DC, AC circuits use V=IZ where Z is impedance measured in ohms. Impedance includes both magnitude and phase angle, whereas resistance is purely real. In DC or at very low frequencies, impedance equals resistance.

How is Ohm's Law used in real-world applications?

Ohm's Law is essential in electronics design, circuit troubleshooting, power calculations, and electrical safety. Engineers use it to select resistors for LED current limiting, calculate wire gauge for safe current capacity, design voltage dividers, analyze circuit behavior, and size power supplies. Electricians use it to determine safe load currents, diagnose wiring problems, and calculate voltage drops. It's fundamental to designing everything from simple flashlight circuits to complex computer motherboards.

📚 Expert References & Further Reading

Horowitz, P., & Hill, W. (2015). The Art of Electronics (3rd ed.). Cambridge University Press.
Sedra, A. S., & Smith, K. C. (2015). Microelectronic Circuits (7th ed.). Oxford University Press.
All About Circuits - Ohm's Law. https://www.allaboutcircuits.com/
Khan Academy - Electrical Engineering. https://www.khanacademy.org/
IEEE Xplore Digital Library - Circuit Theory. https://ieeexplore.ieee.org/