Please provide any 2 values and click "Calculate" to get the other values in the ohm's law equations V = I × R and P = V × I.
Modify the values and click the calculate button to use
DC Voltage Source (V): The circle with + and − symbols on the left represents a DC voltage source (like a battery).
Resistor (R): The zigzag line on the right represents the load or resistor in the circuit.
Current (I): The green arrow at the top shows the conventional direction of current flow — from positive to negative.
Relationship: Voltage drives Current against Resistance, expressed as V = I × R.
Choose the variable to solve for in the middle, then use the relationship for the two known variables within each quadrant.
Ohm's Law is probably the most important thing you'll learn about electricity — and honestly, it's way simpler than it sounds. At its core, it's just a relationship between three things: voltage (the electrical push), current (how much electricity flows), and resistance (what slows it down).
Here's the easiest way to think about it. Imagine a garden hose. Voltage is the water pressure — how hard the water pushes. Current is how much water actually comes out the end. And resistance is how narrow the hose is. Crank up the pressure, more water flows. Pinch the hose, less water comes out. That's literally it. Ohm's Law just puts numbers to that idea.
V = I × R
Voltage (volts) = Current (amps) × Resistance (ohms)
This one little formula is behind everything electrical in your life. Your phone charger? Ohm's Law. Your car's headlights? Ohm's Law. That LED project you're building? You guessed it. Once you understand V = I × R, you can figure out pretty much any basic circuit problem. Not bad for something that fits on a sticky note.
Using our calculator is dead simple. Here's what you do:
Pick any two values you know (like voltage and current)
Enter them in the right boxes
Hit calculate
Get all four values instantly
Say you have an LED that needs 20mA of current with a 9V battery. You need to find the right resistor. Step 1: Voltage = 9V, current = 0.02A (20mA). Step 2: Plug into calculator. Step 3: Resistance = 450 ohms. Grab a 470-ohm resistor (standard value) and your LED works perfectly. No smoke. No magic. Just math.
You'll see this triangle everywhere. Imagine a triangle split into three sections — V is on top, I and R are on the bottom.
Cover V
You see I × R
V = I × R
Cover I
You see V ÷ R
I = V ÷ R
Cover R
You see V ÷ I
R = V ÷ I
That's it. Three formulas from one triangle. You don't even need to memorize them if you remember the triangle.
Ohm's Law is great, but most circuits also care about power. Power is how much energy the circuit uses or produces. It's measured in watts.
Power = Voltage × Current | P = V × I
Our calculator handles this too. So if you know voltage and current, you also get power. Or if you know power and resistance, you get everything else.
Say you're building a speaker. 12V battery, speaker needs 50 watts. P = V × I → 50 = 12 × I → I = 4.17 amps. Your battery needs to supply at least 4.17A. If it can only do 2A, the speaker won't work right. You just saved yourself from buying the wrong battery.
Mistake #1: Mixing Up Units
The #1 error. People use milliamps (mA) instead of amps (A). 1A = 1000mA. So 20mA = 0.02A. Plugging in 20 instead of 0.02 gives completely wrong answers. Always convert to base units first — volts, amps, ohms, watts.
Mistake #2: Forgetting About Series vs Parallel
Ohm's Law works for individual components. In series, resistances add up (two 100Ω = 200Ω total). In parallel, it's trickier (two 100Ω = 50Ω total). Calculate total resistance first, then use Ohm's Law.
Mistake #3: Using the Wrong Formula
People use V = I × R when they should use I = V ÷ R. Write down what you know and what you want to find. Then pick the right formula from the triangle.
The most common beginner project. LED needs ~20mA, drops ~2V. With a 9V battery: voltage across resistor = 9V - 2V = 7V. Resistance = 7V ÷ 0.02A = 350Ω. Grab a 330Ω or 390Ω resistor and you're good.
Robot needs 5V at 2A. Your supply says 5V, 1A. Robot resistance = 5V ÷ 2A = 2.5Ω. With 1A supply, voltage would drop. You need a supply that delivers at least 2A.
Car radio draws 5A at 12V = 2.4Ω resistance. If something shorts and resistance drops to 0.1Ω, current jumps to 120A — that blows the fuse. Now you know there's a short somewhere.
Using the Calculator for AC Circuits
Ohm's Law works for AC too, but with impedance instead of resistance. For simple AC circuits with just resistors, it's identical. For capacitors or inductors, calculate impedance first.
The Power Wheel
There's a bigger version of the triangle — the Ohm's Law Wheel — showing all 12 possible formulas for voltage, current, resistance, and power. Our calculator handles all of them automatically.
What About Temperature?
Resistance changes with temperature. For most metals, resistance goes up as temperature rises. If your circuit gets hot, current will change. Keep this in mind for real-world projects.
You don't need to know this to use the calculator. But if you're curious: electricity is moving electrons. Voltage is the push that makes them move. Resistance is stuff that gets in their way (atoms bumping into each other).
More push = more electrons move = more current. More stuff in the way = fewer electrons move = less current.
That's why V = I × R makes sense. Voltage and current are directly related. Resistance and current are inversely related.
Georg Simon Ohm was a German physicist who published his law in 1827. People didn't believe him at first. They thought his math was too simple to describe electricity.
Turns out, he was right. And now his name is on one of the most used formulas in all of engineering. Not bad for a guy who was initially ignored.
Ohm's Law works for ohmic materials (most metals and resistors). It doesn't work perfectly for:
Diodes and LEDs (they have a voltage drop that doesn't follow Ohm's Law)
Transistors (they're more complex)
Very high frequencies (where things get weird)
Superconductors (where resistance is zero)
For 99% of what you'll do, Ohm's Law works perfectly. Just know that some components are special.
Here's a trick I use all the time. If you calculate something, check it with the power formula too.
Say you calculate current as 2A at 12V. Power should be 24W. If your power calculation gives something different, you made a mistake somewhere. Our calculator does this automatically. But if you're doing it by hand, always double-check with another formula.
Always use the right resistor for your LED
Never touch live wires (even low voltage can hurt)
Use a multimeter to check your work
If something gets hot, turn it off
Electricity is amazing. But it doesn't care about your feelings. Respect it.
Power is the rate at which electrical energy is transferred per unit time, measured in Watts. Joule's Law combined with Ohm's Law gives alternative expressions:
P = V × I | P = V² / R | P = I² × R
Ohm's Law says voltage equals current times resistance. Think of water in a pipe: voltage is pressure, current is water flow, resistance is pipe narrowness. More pressure = more flow. Narrower pipe = less flow. That's it.
Cover the value you want to find. Cover V → I × R (V = I × R). Cover I → V ÷ R (I = V ÷ R). Cover R → V ÷ I (R = V ÷ I). Three formulas from one simple memory trick.
LEDs have very low resistance. Without a resistor, they draw too much current and burn out instantly. The resistor limits current to a safe level. Use Ohm's Law to calculate the right resistor for your LED and battery.
Voltage is the push that makes electrons move. Current is how many electrons actually move. Like water: voltage = pressure, current = flow rate. You can have high pressure but low flow if the pipe is narrow.
Use R = V ÷ I. Example: 12V and 3A → resistance = 12 ÷ 3 = 4 ohms. Our calculator does this instantly for any two known values.
Power is energy used per second, measured in watts. P = V × I. If you know voltage and current, you know power. Alternative formulas: P = V² ÷ R and P = I² × R. Our calculator shows all three.
Yes, but with impedance instead of resistance. For simple AC circuits with only resistors, Ohm's Law works exactly the same. For circuits with capacitors or inductors, calculate impedance first.
Too high resistance = component won't get enough current to work. Too low resistance = too much current, component overheats and burns out. Always calculate the right value first.
Divide by 1000. 20mA = 0.02A. 500mA = 0.5A. 1A = 1000mA. This is the #1 mistake beginners make — plugging 20 into a formula instead of 0.02.
It's a circular chart showing all 12 formulas combining voltage (V), current (I), resistance (R), and power (P). Our calculator handles all these combinations automatically.
Heat makes atoms vibrate more, which gets in the way of electrons trying to flow. More vibration = more resistance. This is why wires get warm and circuits behave differently when hot.
Georg Simon Ohm, a German physicist, published it in 1827. People initially dismissed his work as too simplistic. He was later proven right and the unit of resistance (ohm) is named after him.