You're Designing a Circuit and Need to Know What Voltage Drop to Expect
Whether you're building an LED circuit, troubleshooting a power supply, or learning electronics for the first time, you'll constantly ask yourself: "What voltage do I need?" "How much current flows here?" "What size resistor should go in?" Ohm's Law answers all three questions. It's the foundation of every electrical calculation you'll ever make.
What This Calculator Does
Ohm's Law (V = I × R) describes the relationship between voltage, current, and resistance in any electrical circuit. You provide any two values, and the calculator instantly solves for the third. Need to find the resistance when you know voltage and current? Done. What about power consumption? The calculator also computes P = V × I, P = I² × R, and P = V²/R so you understand exactly how much energy your circuit dissipates.
How to Use This Calculator
Enter any two of the three values: voltage (volts), current (amps), or resistance (ohms). The calculator will instantly compute the missing value and also calculate power in watts.
For voltage: Enter the potential difference across the component. For a battery-powered circuit, this is often your supply voltage or the voltage drop across a single component.
For current: Enter the amperage flowing through the circuit. In a series circuit, current is the same everywhere. In parallel circuits, current splits across branches.
For resistance: Enter the opposition to current flow, measured in ohms (Ω). This could be a resistor, an LED's internal resistance, or a wire's resistance.
For power: The calculator shows watts consumed. This tells you heat dissipation, battery drain, or energy consumption. A 1-watt resistor will typically physically tolerate up to 1W continuous; a 0.25-watt resistor is common in hobbyist circuits but should only handle 0.25W.
Leave fields blank if they're unknown. The calculator will fill them in. If you enter all three, it will verify they're consistent.
The Formula Behind the Math
Ohm's Law is elegantly simple:
V = I × R
Where:
From this one equation, we can rearrange to solve for any variable:
Power is related by:
Worked Example:
You have a 12-volt power supply, a 470-ohm resistor, and want to know the current and power.
So your 470Ω resistor draws about 25.5 milliamps and dissipates about 0.3 watts. This resistor must be rated for at least 0.5W to be safe.
Our calculator does all of this instantly, but now you understand exactly what it's computing.
LED and LED Driver Circuits
LEDs are one of the most common applications of Ohm's Law. An LED has a forward voltage (typically 2V for red, 3.2V for white or blue) and a rated current (usually 20mA). To protect the LED and set its brightness, you place a current-limiting resistor in series.
If you have a 5V supply and a red LED rated for 20mA, Ohm's Law tells you the resistor value: R = (5V - 2V) / 0.02A = 150Ω. Use a 150Ω resistor, and your LED operates safely. Forget the resistor or use one that's too large, and your LED either burns out instantly or dims too much.
Battery and Power Supply Circuits
When you're powering a circuit from a battery, Ohm's Law helps you predict current draw and battery life. A circuit drawing 100mA from a 2000mAh battery will run for about 20 hours (2000mAh / 100mA). But first, you need to know the current, and Ohm's Law gives you that.
If your circuit has multiple components in series (a voltage divider, a resistor chain), the total voltage drop across all components equals your supply voltage. Ohm's Law applied to each segment tells you the voltage at any point in the circuit.
Troubleshooting and Failure Analysis
A component fails, and you don't know why. Measure the voltage across it and the current through it, apply Ohm's Law, and you can determine whether it's behaving as expected. If a 1kΩ resistor shows 5V across it and draws 10mA, the law says it's only 500Ω, it's damaged and should be replaced.
Tips and Things to Watch Out For
Watch the units. Always use volts, amps, and ohms. If you measure voltage in millivolts or current in milliamps, convert first (divide by 1000) before using the formula. Power in watts requires V in volts and I in amps.
Consider temperature. Resistance changes slightly with temperature. A copper wire at 100°C has slightly higher resistance than at room temperature. In precision circuits, this matters. In hobby projects, it usually doesn't.
Don't exceed power ratings. A 0.25W resistor rated for 250mW cannot safely dissipate 0.5W continuously. It will overheat, change resistance, and eventually fail. Always pick a resistor rated for at least twice the calculated power, and preferably more.
Current direction matters in circuits with diodes or LEDs. Ohm's Law tells you magnitude, but polarity determines whether current actually flows. LED resistors protect in one direction only.
Real-world resistance changes with frequency. At DC (direct current), resistance is straightforward. At AC (alternating current), inductance and capacitance introduce reactance, a frequency-dependent opposition. Ohm's Law still applies, but "resistance" becomes "impedance."
Frequently Asked Questions
What is Ohm's Law used for?
Ohm's Law (V = I × R) is the foundation of electrical circuit analysis. Engineers use it to design circuits, choose resistor values, predict current draw, and troubleshoot failures. If you work with electricity, you use Ohm's Law directly or indirectly every day.
Can I use Ohm's Law for AC circuits?
Yes, but you must account for reactance. Inductors and capacitors introduce frequency-dependent opposition called reactance. The combined effect (resistance + reactance) is impedance (Z). The formula becomes V = I × Z, where Z includes both resistive and reactive components.
What happens if I don't use a resistor with an LED?
The LED will likely burn out instantly. Without the resistor to limit current, the LED's internal resistance cannot limit the full supply voltage. Too much current flows, the LED heats up rapidly, and the junction fails. Always use a current-limiting resistor unless the LED is already integrated into a driver circuit.
How do I choose a resistor color code?
Use our Resistor Color Code calculator. The bands tell you the first two significant digits, the multiplier, and tolerance. For a 470Ω resistor: Yellow (4), Violet (7), Yellow (×10²), Gold (±5% tolerance).
Why does my component get hot?
Ohm's Law explains why: P = I² × R. If current is high and resistance is low (or vice versa), power dissipation is high. The component heats up. Choose a resistor rated for the calculated power, or reduce current by increasing resistance or decreasing voltage.
What's the difference between DC and AC in Ohm's Law?
DC (direct current) is constant, so Ohm's Law applies directly: V = I × R. AC (alternating current) oscillates, and inductors and capacitors store energy, introducing reactance. You must use impedance: V = I × Z.
Related Calculators
Use our Resistor Color Code Calculator to decode the bands and confirm the resistor value you need. The LED Resistor Calculator automates the process of finding the right current-limiting resistor for LEDs. For more complex circuits with capacitors and inductors, explore our Capacitor and Inductor Calculators.