Ohm's Law Calculator
This ohms law calculator works out voltage, current, resistance, and power the moment you enter any two of them. Ohm's law is V = I × R, and paired with the power formula P = I × V it lets you solve for whatever's missing. Pick what you know, type the numbers, and you'll see all four values at once with the exact equation used. There's a live circuit drawing that updates as you type, so you can watch the relationship between volts, amps, ohms, and watts instead of just trusting the math.
- Six input modes
- Solves all four
- Live circuit view
- Formula shown
- V = IR & P = IV
Last updated June 18, 2026 Method: V = I × R, P = I × V Reviewed by the Calcowa math team
Enter two valid, non-zero values to see the result.
Solved from voltage and resistance.
I = V / R = 12 / 6 = 2 A | P = V² / R = 24 W
What is Ohm's law?
Ohm's law says voltage equals current times resistance, written V = I × R. Georg Simon Ohm worked it out in 1827, and it's still the first rule you'll meet in any electronics class. If you know any two of those three quantities, you can always find the third. That's what makes it the backbone of circuit work.
It holds for ohmic parts, where resistance doesn't budge as the current changes. Most resistors, wires, and plain conductors act this way at a steady temperature. Once you add the power formula P = I × V, you've got a complete system where any two values pin down the other two, and that's exactly what this ohms law calculator solves.
How do you use the Ohm's law formula?
To use Ohm's law, pick the two values you already know and apply the matching rearrangement. Here's the full sequence:
- 1
Pick what you knowDecide which two of voltage, current, resistance, or power you've got, then choose that mode above.
- 2
Apply the matching formFor voltage use V = I × R, for current use I = V / R, and for resistance use R = V / I.
- 3
Bring in power if neededPower is P = I × V, or P = I² × R when you only know current and resistance.
- 4
Square or root when power leadsFrom power and resistance, current is I = √(P/R) and voltage is V = √(P × R).
- 5
Read all four resultsThe calculator shows every value at once, so you don't have to chase one number at a time.
An Ohm's law example, step by step
Say you've got a 12 V battery feeding a 6 Ω resistor. Divide voltage by resistance to find the current, then multiply current by voltage to find the power the resistor dissipates.
I = 12 / 6 = 2 A, then P = 2 × 12 = 24 W
two amps of current and twenty-four watts of power
Type those same numbers into the calculator above and you'll get the matching current, power, and the formula without doing a thing by hand.
What are volts, amps, ohms, and watts?
Voltage (V, volts) is the electrical pressure pushing charge through a circuit. Current (I, amps) is the flow rate of that charge, how many coulombs pass a point each second. Resistance (R, ohms) is the opposition to the flow, set by the material and its shape. Power (P, watts) is how fast energy gets used, where one watt is one joule per second.
They're all tied together. One volt is the pressure it takes to push one amp through one ohm, and one watt is one volt times one amp. If a single conversion is all you're after, the watts to amps calculator handles the P = IV rearrangement directly.
Where is Ohm's law used?
Ohm's law is everywhere in electrical work. When you're sizing a resistor, checking whether a fuse rating fits, finding the load on a power supply, or working out how much current an LED needs, you're leaning on V = IR. Electricians use it to confirm a wire gauge can carry the current without overheating, and hobbyists use it on almost every breadboard build.
It also feeds straight into running costs. The electricity cost calculator starts from power in watts, and Ohm's law is how you reach watts from a circuit's voltage and resistance. Find the power draw here first, then drop it into that tool. You'll find both in the energy calculators hub.
Ohm's law calculator vs the power formula
Ohm's law strictly covers V, I, and R, while the power formula P = IV pulls power into the system. Together they let you solve for any one of the four quantities from any other two, which is the twelve formulas you'll see in the table below. You don't have to memorize them all, since you can derive each one by mixing V = IR with P = IV, but having them in a single place saves real time when you're moving fast. The key idea is that V, I, R, and P aren't independent: fix any two and the other two are locked in.
The twelve Ohm's law formulas
Here are all twelve rearrangements of Ohm's law and the power formula, grouped by the value you're solving for and the pair you already know. The calculator picks the right one for you, but it's handy to keep the whole sheet in view.
| Find | Formula | Know |
|---|---|---|
| Voltage (V) | V = I × R | I, R |
| Voltage (V) | V = P / I | P, I |
| Voltage (V) | V = √(P × R) | P, R |
| Current (I) | I = V / R | V, R |
| Current (I) | I = P / V | P, V |
| Current (I) | I = √(P / R) | P, R |
| Resistance (R) | R = V / I | V, I |
| Resistance (R) | R = V² / P | V, P |
| Resistance (R) | R = P / I² | P, I |
| Power (P) | P = I × V | I, V |
| Power (P) | P = I² × R | I, R |
| Power (P) | P = V² / R | V, R |
Frequently asked questions
Is Ohm's law the same as the power formula?
Not quite. Ohm's law on its own covers voltage, current, and resistance through V = IR. The power formula P = IV brings power into the picture. Put them together and you can solve for any one of the four quantities from any other two, which is why this tool shows all four results at once.
The three core forms are V = I × R for voltage, I = V / R for current, and R = V / I for resistance. Add the power formula P = I × V and you've got the full set. From there you can derive P = I² × R, P = V² / R, I = √(P/R), and V = √(P × R), which is exactly what this ohms law calculator does for you.
When you know power and resistance, voltage is V = √(P × R). Say you've got 24 W across 6 Ω: multiply them to get 144, then take the square root for 12 V. If instead you know power and current, it's simpler still, since V = P / I.
Voltage is in volts (V), current is in amperes or amps (A), resistance is in ohms (Ω), and power is in watts (W). They link up cleanly: 1 volt equals 1 amp times 1 ohm, and 1 watt equals 1 volt times 1 amp. Once you've got the units straight, the formulas are just bookkeeping.
No, it only holds for ohmic parts, where resistance stays put no matter the voltage or current. Most resistors, wires, and metal conductors at a steady temperature behave that way. It won't work on diodes, LEDs, or transistors, because their resistance shifts with current, so you can't treat them as a fixed R.
Use P = I² × R. If you know the current and the resistance, square the current, multiply by the resistance, and you've got the power in watts. There's no need to find voltage first. This form shows up constantly when you're working out how much heat a resistor will throw off.
Voltage is the electrical pressure that pushes charge through a circuit, while current is the rate that charge actually flows. A handy picture is water in a pipe: voltage is the pressure behind it and current is how fast the water moves. Resistance is the pipe's narrowness, and that's where Ohm's law ties them together.
Related calculators
Working with circuits? These energy tools pair well with the ohms law calculator.
Convert power in watts to current in amps.
Electricity costRunning cost from the watts an appliance uses.
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Need a fast Ohm's law answer?
Use the calculator above, or browse every tool in the energy hub.