Ohms Law Calculator: Voltage, Current, and Resistance

I Blew a Fuse Because I Didn't Do the Math

True story. I was wiring up a 12V LED strip in my shop — nothing fancy, just some under-cabinet lighting so I could actually see what I was doing at the workbench. I grabbed a power supply I had lying around, connected everything, and within about 45 seconds the whole thing went dark. Blew the inline fuse. I stood there holding the wire like an idiot for a good ten seconds before I realized I'd never actually checked how much current the strip was going to draw. I just assumed a little LED strip couldn't pull that much. It could.

That's the thing about electricity — it doesn't care about your assumptions.

So I sat down, did the Ohm's Law calculation (on my phone, obviously), figured out the current draw was about 2.5 amps when the fuse was only rated for 1 amp, and swapped in the right fuse. Took me five minutes of math to fix what took me an hour of frustration to diagnose. And honestly, that's the whole pitch for knowing Ohm's Law — it saves you from yourself.

Ohm's Law in Plain English (and One Formula)

Ohm's Law is one of those things that sounds more intimidating than it actually is. It's basically just a relationship between three things: voltage, current, and resistance. You know two of them, you can figure out the third. That's it — three values, one formula, and you're done.

And because algebra works both ways, you can rearrange it depending on what you're solving for:

• Current: I = V ÷ R
• Resistance: R = V ÷ I
• Voltage: V = I × R

I remember the first time someone showed me the "Ohm's Law triangle" — you cover the variable you want to find, and what's left tells you the formula. I nodded like I understood. I didn't. It took me actually plugging in real numbers a few times before it clicked.

So let's do that.

Say you've got a 24V power supply and a heating element with a resistance of 8 ohms. How much current flows through it? You take 24 and divide by 8, and you get 3 amps. That's it. Now you know you need wiring and a fuse rated for at least 3 amps (honestly, you'd want some headroom — maybe a 5 amp fuse). If you'd skipped this step, you might end up like me, standing in the dark holding a wire.

Try it yourself with our

— punch in any two values and it spits out the third.

Where This Actually Matters (A Bunch of Real Scenarios)

Textbooks make Ohm's Law feel like a classroom exercise, but I've used it in genuinely practical situations more times than I can count. Here's a quick reference table for some common scenarios I've run into or seen other people run into:

Scenario	Known Values	What You're Solving For	Result
LED strip on 12V supply, 4.8Ω resistance	V = 12, R = 4.8	Current (I)	2.5 amps
Motor drawing 6A on a 120V circuit	V = 120, I = 6	Resistance (R)	20 ohms
Resistor of 330Ω with 0.02A flowing	I = 0.02, R = 330	Voltage (V)	6.6 volts
Space heater on 240V, 16Ω element	V = 240, R = 16	Current (I)	15 amps
Car headlight bulb, 0.96Ω at 12V	V = 12, R = 0.96	Current (I)	12.5 amps

That last one — the car headlight — actually surprised me when I first calculated it. 12.5 amps for a single bulb! That's a lot of current for something that fits in the palm of your hand. But that's exactly why automotive wiring uses those chunky fuses and relatively thick gauge wire. The numbers don't lie, even when they feel wrong.

If you're also trying to figure out what wire gauge to use after you've calculated your current, our

is the natural next step. And if power (watts) is what you're really after, there's a related formula — P = V × I — which you can work through with our

.

The Part People Get Wrong

So why does everyone mess this up at some point?

Usually it's units. Someone measures resistance in kilohms (kΩ) but plugs it into the formula as if it's ohms. A 4.7kΩ resistor is 4,700 ohms, not 4.7. I've seen people get answers off by a factor of a thousand because of this, and then they blame the formula. The formula is fine. Your units aren't.

The other common mistake is assuming Ohm's Law works perfectly for everything. It doesn't. It's great for simple resistive loads — heaters, incandescent bulbs, basic resistors. But things like LEDs, transistors, and diodes are nonlinear, meaning their resistance changes depending on the voltage and current. Ohm's Law still gives you a useful approximation (especially if you're calculating a current-limiting resistor for an LED), but it's not the whole picture for those components. I had no idea what that meant at first, honestly, until I tried to calculate the resistance of an LED and got a different number every time I measured it.

For more complex circuit work, you might also want to look at our

for series and parallel combinations, or the

if you're stepping down voltage with a pair of resistors.

Quick Worked Example: Sizing a Current-Limiting Resistor

This comes up constantly if you're doing anything with LEDs. Say you've got a 9V battery and an LED that has a forward voltage of about 2V and you want roughly 20mA (that's 0.02 amps) flowing through it. What resistor do you need?

First, figure out the voltage drop across the resistor: 9V minus 2V = 7V. Then use Ohm's Law:

R = V ÷ I = 7 ÷ 0.02 = 350 ohms

The nearest standard resistor value is 330Ω, which would give you slightly more than 20mA — about 21.2mA, which is totally fine for most LEDs. Or you go up to 390Ω and get about 18mA. Either works. This is one of those places where "close enough" is genuinely close enough.

For related electrical calculations — especially if you're working with AC circuits or need to factor in

— we've got tools for that too. And if you're doing any kind of energy cost estimation, the

pairs nicely with these power and current calculations.

Also worth bookmarking: our

if you keep getting tripped up by milliamps vs. amps or kilohms vs. ohms. No shame in it — I still double-check myself.

Can I use Ohm's Law for AC circuits?

Sort of. For purely resistive AC loads (like a space heater), Ohm's Law works just fine. But once you introduce capacitors or inductors, you're dealing with impedance instead of simple resistance, and the math gets more involved. For basic calculations though — figuring out current draw on a resistive load at a known voltage — yeah, it works.

What's the difference between ohms and kilohms?

1 kilohm (kΩ) = 1,000 ohms. That's it. Just make sure you convert before plugging into the formula. A 4.7kΩ resistor is 4,700Ω. Miss that conversion and your answer will be off by a factor of 1,000.

Why does my measured resistance differ from the calculated value?

A few reasons. Component tolerances are a big one — a resistor marked 330Ω might actually be anywhere from 313Ω to 347Ω if it's a 5% tolerance part. Temperature also changes resistance in most materials. And if you're measuring a component that's still in a circuit, parallel paths can throw off your reading. Basically, real life is messier than the formula, but the formula still gets you in the ballpark.