Free Ohm's Law Calculator Online

Ohm's Law Calculator: Solve for Voltage, Current, Resistance, or Power

An Ohm's law calculator takes whatever two electrical quantities you already know — voltage, current, resistance, or power — and works out the remaining two automatically. Punch in a voltage and a current, and it hands back the resistance the circuit must have and the power it's dissipating, all four numbers updating together the instant you type. This particular calculator runs entirely in your browser, so there's no upload, no server round trip, and no waiting: enter any pair of values and the other two populate immediately, with the solved fields marked so you can tell at a glance which numbers you supplied and which ones the tool derived.

The reason this calculation comes up so often is that real components rarely hand you all four numbers at once. A resistor's package might print its resistance but say nothing about the current it will carry at a given voltage. A power supply's label might state watts and volts but leave amps for you to work out. A datasheet might specify a maximum current without telling you the resistance needed to stay under it. Ohm's law and its companion power formulas tie all four quantities together, so knowing any two is always enough to pin down the rest — this tool just handles the algebra so you don't have to rearrange the formula by hand every time.

How to Use This Ohm's Law Calculator

The interface is four input boxes and a set of result cards that update live:

1. Type a number into any two of the four fields — Voltage (V), Current (A), Resistance (Ω), or Power (W). The order doesn't matter; any pairing works.
2. The moment two valid numbers are present, the other two fields are computed and displayed in result cards below, each one labeled "(solved)" so you can distinguish the values you entered from the values the calculator derived.
3. You can also fill in three or all four fields at once. On this page, the calculator always solves from a fixed priority pair — voltage-and-current first, then voltage-and-resistance, and so on down the list — and recomputes the remaining fields from that pair, silently overwriting anything else you typed that doesn't match. If you need the calculator to actively flag a mismatch instead of quietly overwriting it, that stricter behavior is what the metered API endpoint does, described further down.
4. If fewer than two fields are filled in, or if the combination you've entered would require dividing by zero — voltage with zero current, for instance — the calculator shows an inline message explaining what's missing instead of returning a broken or infinite number.
5. Clear a field and type a new value at any point; the four results recompute instantly without a submit button or a page reload.

Every one of the six possible pairings — voltage-and-current, voltage-and-resistance, voltage-and-power, current-and-resistance, current-and-power, and resistance-and-power — is handled, so it doesn't matter which two numbers you happen to have in front of you.

The Four Formulas Behind the Four Quantities

Ohm's law itself only directly relates three quantities — voltage, current, and resistance — but pairing it with the power formula (power equals voltage times current) produces two additional derived formulas, giving four equivalent ways to reach any missing value depending on which pair you start from:

Known pair	Formula used	What it solves
Voltage & Current	R = V / I, then P = V × I	Resistance and power
Voltage & Resistance	I = V / R, then P = V × I	Current and power
Voltage & Power	I = P / V, then R = V / I	Current and resistance
Current & Resistance	V = I × R, then P = V × I	Voltage and power
Current & Power	V = P / I, then R = V / I	Voltage and resistance
Resistance & Power	V = √(P × R), then I = √(P / R)	Voltage and current

Two shortcut versions of the power formula are worth knowing on their own, since they let you skip a step when resistance or current isn't the quantity you're missing: P = I² × R (power from current and resistance directly, without first finding voltage) and P = V² / R (power from voltage and resistance directly, without first finding current). Both are algebraically identical to P = V × I once you substitute in Ohm's law — they're just rearranged for convenience depending on which two numbers you started with, and the calculator uses whichever path gets to an answer without an unnecessary extra step.

A concrete example ties the whole table together: with a 12-volt source driving 2 amps through a circuit, resistance works out to 12 ÷ 2 = 6 ohms, and power works out to 12 × 2 = 24 watts. Check that same circuit against the shortcut formulas and it holds: 2² × 6 = 24 watts, and 12² ÷ 6 = 24 watts as well — three different routes to the identical figure, which is exactly the kind of internal consistency that makes Ohm's law useful as a check on a reading that looks off.

Why People Reach for an Ohm's Law Calculator

A handful of recurring situations account for most searches for this kind of tool:

• Choosing a resistor value for an LED or other component. A hobbyist wiring an LED to a known supply voltage typically knows the source voltage and the LED's target current from its datasheet, and needs the resistance that limits current to a safe level — plugging voltage and current in returns exactly that resistance, plus the power the resistor needs to dissipate without overheating.
• Checking a power supply or battery's output against a load. Given a battery's voltage and a device's rated current draw, working out the effective resistance and power draw helps confirm the combination is sensible before connecting anything.
• Sanity-checking a multimeter reading. If a meter shows a voltage and a current that don't seem to match the resistor value printed on a component, running the same two numbers through Ohm's law surfaces the discrepancy immediately, which can point to a bad connection, a mislabeled part, or a meter probe placed on the wrong points.
• Working backward from a wattage rating. Appliances and components are frequently labeled in watts rather than amps. Combining that wattage with a known supply voltage yields the current the device actually draws, which matters when sizing a fuse, breaker, or extension cord.
• Coursework and exam problems. Ohm's law is one of the first formulas taught in any electronics or physics curriculum, and working through problems with a calculator that shows all four values at once — rather than one formula rearrangement at a time — makes it easier to see how changing one input ripples through the other three.

Reading the Result: What "Solved" Means

Each result card is labeled to show whether that number came directly from you or was derived. This matters because a solved value inherits any imprecision in the inputs it was built from — if a current reading from a multimeter is only accurate to two significant figures, the resistance and power computed from it inherit that same limited precision, even though the calculator itself displays several decimal places. Treat the extra decimal places as mathematical exactness in the arithmetic, not as extra measurement accuracy that wasn't actually in the original readings.

Supplying three or four values instead of the minimum two is a genuinely useful habit once you have the readings available, because comparing what you typed against what the result card shows turns the page into a manual consistency check. If a supplied value doesn't line up with what the other two imply, that's a signal worth investigating — a component might not be behaving as its label claims, a connection might be adding unexpected resistance, or one of the readings might simply have been mistyped.

A Note on Safety

Every figure this calculator returns is a mathematical estimate based on the numbers you type in — it has no way to know about wire insulation ratings, ambient temperature, component tolerances, or anything else about your actual physical setup. That's fine for choosing a resistor for a low-voltage hobby project or checking your understanding of a textbook problem, but it is not a substitute for measuring a real circuit with a meter, and it is absolutely not authoritative guidance for household wiring, mains-voltage circuits, or any installation governed by an electrical code. If you're working with anything connected to household or building power, that work falls under whatever electrical code applies where you live — in the US, that means the NEC — and belongs in the hands of a licensed electrician for installation and sign-off. Mains electricity can injure or kill; a calculator result is never a reason to skip that step.

Common Mistakes When Applying Ohm's Law

A few errors show up repeatedly enough to flag directly. Mixing up units is the most frequent one — entering milliamps as if they were amps produces a resistance a thousand times too small, and the calculator has no way to know your 15 was meant to be 0.015. Always convert to base units (volts, amps, ohms, watts) before entering a number. Another common slip is treating a component's rated maximum as though it were the operating value; an LED's datasheet current rating is a ceiling, not necessarily the current it will draw at a given voltage without a limiting resistor in place, so plugging in the rated maximum as if it were a measured value can produce a resistance that's technically correct for that scenario but doesn't match reality. Finally, remember that Ohm's law in this simple form assumes a purely resistive, DC circuit — components like capacitors, inductors, and most real-world AC loads introduce reactance and phase shift that this straightforward V, I, R, P relationship doesn't capture, so treat results for those cases as a starting approximation rather than the full picture.

Doing the Math Without a Calculator

Ohm's law is simple enough to compute by hand or with a basic calculator once you remember the rearrangement — dividing voltage by current for resistance, multiplying voltage by current for power, and so on. Where a dedicated tool saves time is in the derived cases: working from power and resistance alone requires a square root (V = √(P×R)), which is easy to mess up by hand, and remembering all six pairings without a reference means either memorizing the whole table or rederiving it from scratch each time. If this needs to run as one step inside an application, a spreadsheet, or an automated script rather than a one-off browser lookup, the same Ohm's law solver is exposed as a metered API endpoint that accepts any two of the four quantities and hands back a JSON response with all four filled in — the API documentation covers the exact request shape and how credits are charged.

What happens if I enter values that contradict each other?

On this page, the calculator solves from a fixed priority pair and recomputes the rest, so a contradictory extra value is silently overwritten in the result cards rather than flagged — comparing what you typed against what the result card shows is how you catch the mismatch yourself. The metered API version of this same solver is stricter: if you supply more than two values and they don't agree with each other within a small tolerance, it rejects the request outright instead of silently picking one and ignoring the rest, which is useful for catching a typo or confirming that a real-world reading matches theory in an automated pipeline.

Can I use this for AC circuits?

The V, I, R, P relationships here describe simple resistive DC behavior. Real AC circuits with inductors, capacitors, or motors involve reactance and a phase angle between voltage and current that this straightforward form of Ohm's law doesn't account for — treat results for AC loads as an approximation rather than an exact answer.

Why does the calculator reject zero current or zero resistance in some fields?

Several of the underlying formulas divide by current or by resistance. A zero in either of those positions would require dividing by zero, which has no finite answer, so the calculator flags that combination instead of returning an undefined or infinite result.

Is the power figure the same as what I'd see on an electricity bill?

Not directly. The power figure here is instantaneous wattage for the specific voltage and current you entered. Turning that into a cost requires knowing how many hours a device runs and your electricity rate — a separate calculation handled by a dedicated energy-cost tool rather than this one.

Related Electrical Tools

Once you have a resistance value in hand, a few companion tools pick up from there. If that resistance corresponds to a physical resistor, the resistor color code calculator translates it into the color bands you'd expect to see printed on the part, or reads bands you already have back into a resistance value. If your circuit involves current flowing over a length of wire rather than through a single component, the voltage drop calculator extends this same current-and-resistance relationship to a real conductor run and reports how many volts are lost along the way. And once you know the wattage a device is pulling, the electricity cost calculator turns that figure into a daily, monthly, or yearly running cost.