Ohm's Law Calculator
Enter any two of voltage, current, resistance or power and get the other two instantly.
Enter any two values — the other two are worked out instantly.
- Voltage
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- Current
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- Resistance
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- Power
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Formulas used
- Enter any two values to see the working.
How it works
Ohm's law says voltage equals current times resistance (V = I × R), and electrical
power is voltage times current (P = V × I). Between those two relationships, any
two of the four quantities are enough to solve for the other two — so this calculator watches
the last two boxes you typed in and rearranges the formula for you. Type 12 V and 4 Ω and you
instantly get 3 A and 36 W; type 5 W and 20 Ω instead and it uses
V = √(P × R) and I = √(P ÷ R).
Each box has its own unit selector — µV/mV/V/kV, µA/mA/A/kA, mΩ/Ω/kΩ/MΩ and µW/mW/W/kW — so you can mix 9 V with 470 Ω and read the answer back as 19.149 mA without juggling decimal points. The summary line always shows the value with the tidiest engineering prefix, and the "Formulas used" list shows exactly which rearrangement produced each answer, which makes it easy to check your own working. Everything is computed in your browser with plain JavaScript — no upload, no account, no ads.
Frequently asked questions
How do you calculate current from voltage and resistance?
Divide the voltage by the resistance: I = V ÷ R. A 12 V supply across a 4 Ω load draws 12 ÷ 4 = 3 A. Type 12 in the Voltage box and 4 in the Resistance box and this calculator fills in 3 A straight away, along with the power (P = V² ÷ R = 36 W) and the exact formulas it used.
What is the formula for power in Ohm's law?
Power is voltage times current, P = V × I. Combined with V = I × R that gives two more forms: P = I² × R when you know the current and resistance, and P = V² ÷ R when you know the voltage and resistance. Going the other way, V = √(P × R) and I = √(P ÷ R) — this tool picks whichever rearrangement fits the two values you entered.
Can I enter milliamps, kilohms or millivolts?
Yes. Every box has its own unit selector — µV/mV/V/kV, µA/mA/A/kA, mΩ/Ω/kΩ/MΩ and µW/mW/W/kW — so you can enter 9 V across 470 Ω and read back 19.1489 mA without converting anything by hand. The summary row always shows each result with the tidiest engineering prefix, and everything is calculated in your browser, so nothing is uploaded.